# The Star Formation Across Cosmic Time (SFACT) Survey. I. Survey Description
and Early Results from a New Narrow-Band Emission-Line Galaxy Survey

John J. Salzer Department of Astronomy, Indiana University, 727 East Third
Street, Bloomington, IN 47405, USA  David J. Carr Department of Astronomy,
Indiana University, 727 East Third Street, Bloomington, IN 47405, USA
Jennifer Sieben Department of Astronomy, Indiana University, 727 East Third
Street, Bloomington, IN 47405, USA  Samantha W. Brunker Department of
Astronomy, Indiana University, 727 East Third Street, Bloomington, IN 47405,
USA  Alec S. Hirschauer Department of Astronomy, Indiana University, 727 East
Third Street, Bloomington, IN 47405, USA  Space Telescope Science Institute,
3700 San Martin Dr., Baltimore, MD 21218, USA

###### Abstract

We introduce the Star Formation Across Cosmic Time (SFACT) survey. SFACT is a
new narrow-band survey for emission-line galaxies (ELGs) and QSOs being
carried out using the wide-field imager on the WIYN 3.5 m telescope. Because
of the superior depth and excellent image quality afforded by WIYN, we
routinely detect ELGs to r = 25.0. Our survey observations are made using
three custom narrow-band filters centered on 6590 Å, 6950 Å, and 7460 Å. Due
to the sensitivity of the survey, we are able to simultaneously detect sources
via a number of different emission lines over a wide range of redshifts. The
principal lines detected in SFACT are H$\alpha$ (redshifts up to 0.144), [O
III]$\lambda$5007 (redshifts up to 0.500) and [O II]$\lambda$3727 (redshifts
up to 1.015). In this paper we detail the properties of the survey as well as
present initial results obtained by analyzing our three pilot-study fields.
These fields have yielded a total of 533 ELG candidates in an area of 1.50
deg2 (surface density of 355 ELGs deg-2). Follow-up spectra for a subset of
the ELG candidates are also presented. One of the key attributes of the SFACT
survey is that the ELGs are detected in discrete redshift windows that will
allow us to robustly quantify the properties of the star-forming and AGN
populations as a function of redshift to z = 1 and beyond. The planned
acquisition of additional narrow-band filters will allow us to expand our
survey to substantially higher redshifts.

††facilities: WIYN:3.5m

##  1 Introduction

Most of what is known about activity in galaxies has been learned by studying
objects cataloged in dedicated surveys that probe for the telltale signs of
that activity. This is true regardless of whether that activity is due to
above-average levels of star formation or is caused by accretion of matter
onto a supermassive black hole. These surveys have been carried out at
wavelengths across the electromagnetic spectrum. Early and extremely
influential examples include the objective-prism survey for UV-excess galaxies
carried out by Markarian and colleagues at the Byurakan Observatory (e.g.,
Markarian et al., 1967, 1981) and the 3C radio continuum survey carried out
with the Cambridge radio interferometer (Edge et al., 1959; Bennett, 1962;
Laing et al., 1983).

One of the most commonly adopted survey approaches for detecting activity has
been to search for galaxies that exhibit strong optical or UV emission lines
in their rest-frame spectra. These emission-line galaxy (ELG) surveys have
utilized a number of different selection techniques. For example, several
early ELG surveys utilized objective-prism spectroscopy to select their
candidates (e.g., Smith, 1975; MacAlpine et al., 1977; MacAlpine & Williams,
1981; Sanduleak & Pesch, 1982; Pesch & Sanduleak, 1983; Wasilewski, 1983;
Markarian et al., 1983; Zamorano et al., 1994, 1996; Ugryumov et al., 1999;
Hopp et al., 2000; Salzer et al., 2000, 2001, 2002). Alternatively, several
surveys have been carried out using narrow-band (NB) imaging data (e.g.,
Boroson et al., 1993; Ryan-Weber et al., 2004; Kakazu et al., 2007; Werk et
al., 2010; Ly et al., 2011; Kellar et al., 2012; Sobral et al., 2012, 2013;
Stroe & Sobral, 2015; Cook et al., 2019; Salzer et al., 2020; Khostovan et
al., 2020; Watkins et al., 2021; Martínez-Solaeche et al., 2022). Very strong-
lined ELGs can be selected using standard broad-band (BB) colors when the line
equivalent widths are very high (e.g., Rosenwasser et al., 2022). This latter
approach has been successful in detecting extreme objects like the Green Peas
(Cardamone et al., 2009) and Blueberries (Yang et al., 2017). Finally, the
HETDEX survey (Gebhardt et al., 2021) utilizes multiple integral field units
to carry out a filled, non-targeted, wide-area ELG survey.

We have initiated a new NB survey for emission-line objects called SFACT: Star
Formation Across Cosmic Time. SFACT takes advantage of the instrumentation and
excellent image quality of the WIYN 3.5 m telescope to deliver a rich and
diverse catalog of star-forming galaxies from the local universe to z = 1, as
well as AGN and QSOs to z $>$ 5.

SFACT attempts to build upon the legacy of these previous ELG surveys. It is
being carried out using the NB imaging technique, and employs a unique set of
custom filters that allows the survey to extend to high redshifts. The high
sensitivity of the telescope and camera combination allows us to be sensitive
to multiple emission lines in each image, which provides a natural
multiplexing component that makes the survey method more efficient.

This paper presents a full description of the new NB ELG survey. It also gives
preliminary results from our pilot study, including BB photometry, NB fluxes,
and spectroscopic data for the sources detected in the first three survey
fields. In Section 2 we describe the motivations for carrying out the survey,
while Section 3 lays out the overall survey design. The results of our pilot
study are given in §4, which includes example imaging and spectral data,
summaries of the photometric properties of the SFACT candidates, and the
presentation of their redshift, luminosity and star-formation rate (SFR)
distributions. We provide a comparison between SFACT and a number of recent NB
surveys in §5 in order to highlight the similarities and differences of our
new sample of ELGs relative to existing surveys. Section 6 describes a number
of planned applications for the survey, and includes additional example
spectra to illustrate the utility of the survey for addressing a number of
science questions. The current status of the survey and future plans are
presented in Section 7, and we summarize our study in §8.

Two companion data papers present the survey results for our initial pilot
study. The first (Sieben et al., 2023, hereafter SFACT2) presents the complete
list of newly discovered SFACT candidates from our three pilot-study fields.
SFACT2 also includes more complete details of our observational methodology
and data processing, as well as a full description of how our BB photometry
and NB line-flux measurements are carried out. The second (Carr et al., 2023a,
hereafter SFACT3) tabulates results from our initial follow-up spectroscopy of
the pilot-study ELG candidates. This paper describes in detail how our spectra
are obtained and processed, and presents redshifts, line fluxes, and key
emission-line ratios. Taken together, the three initial SFACT papers will
serve to provide a complete description of our survey design, motivation, and
methodology. Subsequent papers in this series will present additional data
sets, as well as results from science applications that are described below.

A standard $\Lambda$CDM cosmology with $\Omega_{m}=0.27$,
$\Omega_{\Lambda}=0.73$, and $H_{0}=70$ kms-1 Mpc-1 is assumed in this paper.

##  2 Motivation for the Survey

Several factors have driven the development of the SFACT survey, both
scientific and practical. In this section we briefly describe the factors that
motivated us to carry out this project, since they serve to shape the survey
design and methodology.

The combination of WIYN 3.5 m telescope111The WIYN Observatory is a joint
partnership of the University of Wisconsin-Madison, Indiana University,
Pennsylvania State University, Purdue University, University of California -
Irvine, and the NSF’s NOIRLab. and the wide field One Degree Imager (ODI)
camera offers an almost unique opportunity to carry out the SFACT survey. As
described in the next section, WIYN and its instrumentation suite allow us to
execute the SFACT survey in a way that plays to both the strengths of the
facility and to the strengths of the science goals of the project. In
particular, our ability to use the same telecsope with a multi-object
spectroscopic instrument efficiently provides for essential follow-up
spectroscopy of our candidate ELGs.

Naturally, a key motivation for carrying out SFACT revolves around the science
goals of the project. The planned science applications are broad, covering a
range of topics in extragalactic astronomy. The namesake project of the survey
is to carefully measure the star-formation rate density of the universe from z
= 0 to z = 1 and beyond. In addition, we also plan to evaluate the evolution
of the metal abundances of galaxies over a similar redshift range. We will use
SFACT to detect and characterize the population of AGNs over the redshift
range covered by the survey, and to probe the evolution of their volume
densities and metallicities with lookback time. We will explore the
environments of all of our ELG populations, both star-forming and AGN, by
utilizing deep redshift survey information carried out in each SFACT field. We
will discover numerous examples of extreme but rare objects at a wide range of
redshifts: Green Peas (Cardamone et al., 2009; Brunker et al., 2020),
Blueberries (Yang et al., 2017), and extremely metal-poor dwarf galaxies
(e.g., Hirschauer et al., 2016; McQuinn et al., 2020). Finally, the
comprehensive nature of our survey will allow us to place these extreme types
of galaxies into context with the overall population of star-forming galaxies
at each redshift covered by the survey. Section 6 gives a more comprehensive
discussion of each of these science applications that are planned for SFACT.

The SFACT survey is, in many ways, an outgrowth of the H$\alpha$ Dot survey
(Kellar et al., 2012; Salzer et al., 2020; Watkins et al., 2021). As an
additional motivation, we mention the desire to build upon (and improve upon)
this and other previous NB surveys. The SFACT program adds a number of new
aspects that makes it fairly unique. Based on the observed depth of the
H$\alpha$ Dot survey presented in (Watkins et al., 2021), we are able to
predict the depth of the putative SFACT survey. Accounting for the differences
in the telescope apertures and filter widths, we arrive at an estimate for the
new survey to reach a median NB emission-line flux of $\sim$2 $\times$ 10-16
erg s-1 cm-2. Furthermore, we predict a typical surface density of
$\sim$80-120 emission-line objects per filter per deg2. By utilizing multiple
NB filters on the same fields, one naturally achieves a multiplexing
advantage. With the current set of three NB filters, we expect to discover
240-360 ELGs deg-2, or between 120 and 180 objects per field. Since this large
number of ELGs are detected in a volume limited by the NB filters, the
projected volume densities of star-forming galaxies and AGN will be quite
high. The promise of achieving these large volume densities was a strong
motivator for carrying out the SFACT survey.

##  3 Design of the Survey

###  3.1 Telescope and Instrumentation

The concept for the SFACT survey was developed around the WIYN 3.5 m telescope
and the ODI camera. Several important factors drove the survey design.

(i) The large aperture and superior image quality delivered by the WIYN
telescope will allow the survey to reach an excellent depth. Based on our
previous experiences with NB imaging on smaller telescopes with the H$\alpha$
Dot survey (see §2), we projected that we would be able to achieve a median
r-band magnitude of approximately 22.7 for the survey, and to detect faint
emission-line objects to r $\sim$ 25.0.

(ii) The f/6.3 beam of the Nasmyth focus on WIYN is slow enough to allow for
use with NB filters. Typically, wide-field imagers are fed by fast beams
which, when used with NB filters, result in strong spatial variations of the
effective transmitted bandpass on the detecter. The slower convergence of the
WIYN Nasmyth focus provides for a more uniform bandpass across the detector.

(iii) The advent of the ODI camera. The camera has an image scale of 0.11″
pixel-1, which allows it to take advantage of the excellent image quality
delivered by the WIYN telescope. ODI was originally designed to deliver a full
1∘ $\times$ 1∘ FOV, However, the final commissioned version of ODI has a FOV
of 48′ $\times$ 40′, for a survey area of $\sim$0.53 sq. deg.. The loss of
nearly half of the expected detector area is unfortunate, as it necessitated a
longer time frame for carrying out the program, essentially doubling the
project timescale.

(iv) The availability of the Hydra multi-fiber positioner on WIYN allows for a
direct path for acquiring follow-up spectra of our candidates. These spectra
play a central role in the use of the SFACT ELG sample for carrying out the
various science applications planned for the survey constituents (e.g., §6).
The emission-line nature of the SFACT candidates translates into our ability
to use WIYN for both the imaging selection and the spectroscopic confirmation.

(v) The recognition that, at the photometric depth achievable with WIYN, we
would be sensitive to multiple emission lines from ELGs and QSOs present in
each survey field. That is, a NB filter at a fixed wavelength would
simultaneously be sensitive to H$\alpha$ emission from low-redshift galaxies,
to the [O III]$\lambda$5007 line at intermediate redshifts, to [O
II]$\lambda$3727 at higher redshifts, and to the various UV lines from QSOs
(e.g., Mg II $\lambda$2798, C III] $\lambda$1909, C IV $\lambda$1548,
Ly$\alpha$ $\lambda$1215) at high redshifts. The concept from the start was to
utilize a limited number of NB filters and to think of the survey in terms of
probing specific redshift windows, with different emission lines being
detected in different windows. The concept of redshift windows will be a
central theme throughout the rest of this paper.

###  3.2 Narrow-Band Filters

The specific choice of the SFACT NB filters represents a combination of
compromise and scientific opportunity. Among the key drivers for the selection
of the filters were their size and cost, plus the fact that ODI filter
assembly is only capable of holding nine filters at a time. Five of these
filter slots are permanently designated to hold the standard ugriz BB filters.
This leaves only four slots available for any additional filters. The full-
field ODI filters are 42 $\times$ 42 cm, and cost in the neighborhood of
$70-$80k each. These two factors immediately suggest that creating a large set
of NB filters with contiguous and overlapping wavelength coverage is not a
realistic goal.

Due to the high cost and uncertainties concerning the fabrication of such
large NB filters, we opted to initially purchase a single NB filter. After
much internal debate, we settled on a central wavelength of 6950 Å. The width
of the filter was set at $\sim$90 Å, driven largely by the concerns of the
potential vendors over meeting design specifications if the bandpass were too
narrow. For the purpose of our survey, this bandwidth represents a compromise.
In NB surveys such as ours, a smaller bandwidth will result in higher
sensitivity for emission lines, since it reduces the diluting effect of the
underlying continuum within the bandpass (i.e., results in a higher contrast
for the line). For this reason, most extragalactic H$\alpha$ filters sets are
designed with bandwidths of 50 – 60 Å or smaller (e.g., Van Sistine et al.,
2016). For example, the filters used to identify planetary nebulae in nearby
galaxies (e.g., Jacoby et al., 1990) had bandwidths of $\sim$30 Å. The use of
$\sim$90 Å wide filters will result is a somewhat lower sensitivity for SFACT.
On the other hand, the larger bandwidth of the SFACT filters results in a
correspondingly larger redshift range over which ELGs can be detected. This
increases the survey volume and hence the number of objects that are detected.

Table 1: Properties of the SFACT NB Filters

Filter | $\lambda_{cent}$ | $\Delta\lambda$ | z range - H$\alpha$ | z range - [O III] | z range - [O II] | z range - Ly$\alpha$
---|---|---|---|---|---|---
| Å | Å |  |  |  |
(1) | (2) | (3) | (4) | (5) | (6) | (7)
NB659 = NB2 | 6590 | 81.1 | -0.002 – 0.011 | 0.308 – 0.325 | 0.757 – 0.780 | 4.389 – 4.458
NB695 = NB1 | 6950 | 91.0 |  0.052 – 0.066 | 0.378 – 0.397 | 0.852 – 0.877 | 4.680 – 4.759
NB746 = NB3 | 7460 | 96.7 |  0.129 – 0.144 | 0.480 – 0.500 | 0.988 – 1.015 | 5.099 – 5.181
NB812* | 8120 | 90 |  0.230 – 0.244 | 0.613 – 0.631 | 1.166 – 1.191 | 5.646 – 5.720
NB912* | 9120 | 90 |  0.383 – 0.397 | 0.812 – 0.830 | 1.435 – 1.459 | 6.469 – 6.543

Note. — * Proposed additional SFACT filters

After the delivery of the first survey filter in June 2016 (NB695, designated
as NB1 within the SFACT survey), we carried out a series of test observations.
The results of these preliminary observations verified the validity of our
survey method. Once this was established, we proceeded to order two additional
NB filters for ODI. The first of these was selected to have a central
wavelength of 6590 Å (NB659, designated as NB2), and could serve as a zero
redshift H$\alpha$ filter for general purpose use in addition to being useful
for SFACT. The second has a central wavelength of 7460 Å (NB746, designated as
NB3). Table 1 lists the key features for all three ODI NB filters, including
the central wavelength ($\lambda_{cent}$), the filter width ($\Delta\lambda$,
defined as the full width at half the maximum transmission level), and the
redshift ranges covered by each filter for several strong emission lines.
These new filters were delivered in August 2018 and were put into service
immediately. For all observing runs from Fall 2018 onward, the full suite of
three NB filters have been used to observe all survey fields.

Figure 1: Left: Filter tracings of the three NB filters currently in use with
SFACT. Right: Plot showing the wavelength coverage of the SFACT NB filters as
well as the redshift ranges associated with the detection of strong optical
nebular emission lines. Each vertical column represents a specific NB filter,
while the vertical location of each rectangular box represents the redshift
range for the emission line indicated. We show the redshift windows only for
H$\alpha$, [O III]$\lambda$5007, and [O II]$\lambda$3727; the majority of the
SFACT candidates are detected via one of these three lines. Also shown are the
two additional filters that we plan to add to the survey. The plot on the
right-hand side compresses the wavelength scale in order to better illustrate
the distribution of the redshift windows covered by the survey.

Figure 1 shows the transmission profiles of the three SFACT NB filters, as
well as the redshift coverage for the key optical lines of H$\alpha$, [O
III]$\lambda$5007, and [O II]$\lambda$3727\. In particular, the righthand
portion of the figure illustrates the redshift windows surveyed by our method.
The subplot on the far right suppresses the wavelength range and shows the
spacing of the redshift windows. As can be seen, the wavelengths of the
filters were chosen to yield a fairly uniform redshift coverage for these
windows. The figure includes two additional filters that are planned for as
part of a future expansion for the survey. These latter two filters are
located in the well known gaps of the telluric OH spectral lines at $\sim$8120
Å and $\sim$9120 Å. These two new filters will help to fill in the gaps in the
current distribution of redshift windows, as well as extend the overall
redshift coverage to redshifts approaching 1.5 for the strong optical nebular
lines. Their proposed properties are also listed in Table 1.

We point out two additional features regarding the SFACT NB filters. First,
the location of the NB695 filter was selected such that galaxies detected with
it via their [O III]$\lambda$5007 emission would have their H$\alpha$ \+ [N
II] lines redshifted into the night sky OH gap at $\sim$9120 Å. Hence objects
detected via [O III] emission in NB695 will have H$\alpha$ emission falling in
NB912 (see Table 1). Second, the wavelength range of NB659 is such that only
extremely low-redshift systems will be detected via their H$\alpha$ line (z
$<$ 0.010). This in turns means that the volume surveyed with this filter for
H$\alpha$ emitters will be very small. Hence, very few H$\alpha$-detected
galaxies are expected to be found with this filter. There is only one such
object in the three pilot study fields presented in the current paper, a very
low-luminosity dwarf star-forming galaxy.

###  3.3 Planned Survey Size and Location of Survey Fields

One of the goals of the SFACT survey is to deliver large enough samples of
ELGs to be able to carry out robust derivations of astrophysical interest
within each redshift window. Examples of these derivable products include
emission-line selected luminosity functions, star-formation rate densities,
and mass-metallicity/luminosity-metallicity relations. Hence, we established a
goal of detecting 800-1200 ELGs within each redshift window when summed over
all survey fields.

As indicated in §2, the expectation for the typical number of ELGs detected
per filter per field is 40-60 objects. This will of course vary substantially
from field-to-field due to cosmic variance. Using this number as an average
per field, and assuming that the three survey filters will yield similar
numbers of detections when averaged over all of the survey fields, one arrives
at an estimate of the number of fields that would be required to complete the
survey: 50 to 60 fields. This represents 25 - 30 deg2 of sky coverage. When
completed, the SFACT survey will catalog between 6000 and 11,000 ELGs using
the current three survey filters, and proportionally more when the two
additional filters (NB812 and NB912) are added.

The selection of the SFACT survey field locations follows a number of
criteria. Fields need to be located within the footprint of the Sloan Digital
Sky Survey (SDSS; York et al., 2000) to facilitate the photometric calibration
of our images (see SFACT2). Most fields are located at high Galactic latitude
to minimize foreground extinction, and have declinations between roughly 10∘
and 50∘ so that they transit within 20∘ of the zenith from Kitt Peak. Since
the total survey area is fairly small, we opted to observe a series of widely-
scattered fields in both the Spring and Fall observing seasons to help ensure
that the overall survey is not subject to cosmic variance (i.e., highs and
lows in the galaxy density). This latter criterion is especially important for
SFACT, since the NB filters only cover limited swaths of redshift space. It is
easy to imagine that in some fields one or more of the filters may be
surveying a low density void at the redshift of one of the key emission lines
and yield very few detections. Conversely, in some fields the same filters may
hit a rich galaxy filament or cluster environment, resulting is an excess of
detections. By averaging over several dozen widely-spaced fields, we expect to
completely wash out the effects of cosmic variance.

Many of the SFACT field locations observed during the early stages of the
project are centered on known Green Pea galaxies (e.g., Cardamone et al.,
2009; Brunker et al., 2020). These extreme star-forming galaxies were selected
from [O III]-detected ELGs in either the KISS (Salzer et al., 2000, 2001;
Gronwall et al., 2004; Jangren et al., 2005) or H$\alpha$ Dot (Kellar et al.,
2012; Salzer et al., 2020) surveys. The reasons for this are two-fold. First,
the locations of these Green Peas are distributed fairly randomly across broad
areas of both Galactic caps. This satisfies one of the criteria specified
above. Second, these fields have been the subject of a focused redshift survey
(e.g., Brunker et al., 2022). The data from this redshift survey will provide
a deep comparison sample for future studies that look into the impact of the
local environment on the properties of the SFACT ELGs.

Additional SFACT field locations have been selected based simply on the
availability of observing time, the desire to observe fields that are widely
spaced across the sky, and the need to observe within $\sim$3 hours of the
meridian. If no suitable Green Pea target is available within a range of right
ascension that is up during a scheduled observing run, we select an
appropriate field location that is devoid of bright stars that allowed us to
fill our observing schedule.

##  4 Preliminary Results from the Pilot Study

The first observing run during which the two newer NB filters (NB2 and NB3)
were available took place in September 2018. During this run we completed the
observations for three fields with all six filters (gri broad-band plus NB1,
NB2 and NB3 narrow-band; see SFACT2) for the first time. These three fields
were designated as the SFACT pilot-study fields. The data obtained from our
pilot-study observations were used to test our analysis methods and to fine-
tune our selection software. They are also the first fields for which
substantial follow-up spectroscopy was obtained. In this section we present
initial results from our analysis of these three fields.

Basic information about the three pilot-study fields is given in Table 2. This
includes the field designation, where we adopt the nomenclature SFF##. Here
SFF stands for SFACT Fall (for Fall fields); Spring fields are designated
SFS##. The number is a running number that designates each field within a
given season. For the SFF fields the first fifteen field locations are listed
in ascending RA order, while subsequent fields are numbered in the order in
which the imaging observations are completed. The remaining information
presented in the table includes the name of the Green Pea galaxy that the
field is centered on (if appropriate), the celestial coordinates of the field
center, the number of ELG candidates detected in each of the three NB filters,
the total number of SFACT candidates in each field, and the number for which
follow-up spectroscopy currently exists.

###  4.1 Results from the Imaging Survey

In this section we illustrate the observationally derived properties of the
SFACT ELG candidates utilizing the results from our pilot study. The details
of the observations, object selection, and photometric analysis are presented
in the SFACT2 companion paper. Here we give a brief description of how our ELG
candidates are selected, to help give context to the subsequent presentation
describing the properties of our survey constituents.

The procedure for selecting SFACT objects follows common practice (e.g.,
Kellar et al., 2012; Sobral et al., 2012, 2013; Stroe & Sobral, 2015; Cook et
al., 2019; Salzer et al., 2020; Khostovan et al., 2020; Watkins et al., 2021;
Martínez-Solaeche et al., 2022). In particular, we follow the methodology
developed for the H$\alpha$ Dots survey (Kellar et al., 2012; Salzer et al.,
2020; Watkins et al., 2021). An automated object-finding routine catalogs
every source located within each of our survey fields. Next we measure the
fluxes for every object in each of the three NB images as well as each of the
corresponding “continuum” images (constructed from the BB images). We then
identify the objects that possess a statistically significant excess of NB
flux, and flag them as ELG candidates. Specifically, we consider objects as
ELG candidates if they display an excess of flux in the NB image that is 0.4
magnitudes brighter than the flux in the continuum image and if the detected
excess is at or above the 5$\sigma$ level. See SFACT2 for details.

####  4.1.1 Number of Detected Emission-Line Objects

The total number of emission-line detections for the pilot study fields is 533
(see Table 2). Since the area covered by these three fields is 1.5025 deg2,
the surface density of objects detected is 355 ELGs deg-2. This number is
consistent with the projected surface density estimate derived in §2. As
alluded to in §3, the number of ELGs detected in a given field through a given
filter is highly variable, being dependent on the large-scale structure
present in each field (i.e., cosmic variance). For example, the number of ELGs
detected in SFF10 varies by a factor of $\sim$5 (22 objects in NB2, and 110 in
NB3). The implication is that the NB2 filter is sampling mostly low-density
regions in this direction, while both NB1 and NB3 are intersecting some high-
density portions of the universe. The other two fields show substantially
lower variations between filters, but there is significant variation between
the fields (e.g., 132 ELGs for SFF01, and 216 for SFF10).

Table 2: The SFACT Pilot-Study Fields | Central |  |  |  | # Detections |  | Total # | # with
---|---|---|---|---|---|---|---|---
Field | Object |  $\alpha$(J2000) |  $\delta$(J2000) | NB1 | NB2 | NB3 | Candidates | Spectra
(1) | (2) | (3) | (4) | (5) | (6) | (7) | (8) | (9)
SFF01 | — | 21:43:00.0 | 20:00:00.0 |  41 |  47 |  44 | 132 | 110
SFF10 | H$\alpha$ Dot 55 |  1:44:40.3 | 27:54:35.5 |  84 |  22 | 110 | 216 | 201
SFF15 | H$\alpha$ Dot 22 |  2:39:12.6 | 27:52:02.3 |  67 |  71 |  47 | 185 | 146

The average number of objects detected per filter per field are: NB1 = 64.0,
NB2 = 46.7, NB3 = 67.0. While these averages are still subject to significant
uncertainty due to cosmic variance between the three survey fields, they
reflect the expectation indicated in §3 that the number of objects detected in
NB2 will be lower than the other two filters due to the fact that the survey
volume for low-redshift objects (H$\alpha$ detections) will be extremely
small. A naive expectation would be that there will be essentially zero
H$\alpha$ detections with NB2, leading to a typical detection rate with that
filter that is $\sim$2/3rds the value for NB1 and NB3. Our preliminary numbers
are approximately consistent with that expectation.

As alluded to in SFACT2, our object selection for the pilot-study fields was
meant to be inclusive, in the sense that we tended to include a number of more
questionable sources in the final catalogs. This is not to say that we relaxed
the quantitative selection criteria for these fields. Rather, we were more
inclusive during the final visual inspection of the candidates where we
identify and reject false detections such as image artifacts. The motivation
for doing this was to include a number of the more questionable objects in our
spectroscopic follow-up lists, with the plan to let the spectra verify or
reject these more dubious sources. By using the follow-up spectra as a guide,
we better trained ourselves to identify the false detections in the subsequent
survey fields. For this reason, the number of cataloged sources in these three
fields is likely to be higher than we might typically expect in future survey
lists, and the fraction of true emission-line objects is likely to be lower.

####  4.1.2 Categories of Emission-Line Objects Cataloged

The SFACT survey is designed to detect emission lines in a broad range of
extragalactic sources. The varied nature of the sources selected require us to
differentiate between classes of objects, exclusively for the purpose of
expediting the photometric measurements and the follow-up spectroscopy. We
currently recognize three categories of objects: (1) compact objects with a
centralized emission region, (2) extended galaxies with one or more H II
regions, and (3) bright H II regions. We stress that these three classes are
used only to differentiate how the objects are treated by the SFACT analysis
and measurement software; they do not represent distinct types of ELGs (e.g.,
starburst nuclei, Seyferts, QSOs, etc.). The three categories of SFACT objects
are illustrated in Figure 4 below.

The vast majority of the SFACT emission-line objects fall into the first group
of compact objects. This includes essentially all of the objects that are
detected via their [O III] line (redshifts of 0.30 - 0.50), all of the [O II]
detections (redshifts 0.75 - 1.02), and QSOs. It also includes the smaller,
more compact H$\alpha$ detections. Objects in this class have the bulk of
their emission emanating from the central region of the galaxy. This allows us
to use the photometric center of the object as the location for measuring both
the BB magnitudes as well as the emission-line flux. The same location is used
for follow-up spectroscopy.

The survey often detects individual H II regions in nearby galaxies (H$\alpha$
detections, redshifts of 0.00 - 0.15). In some cases, large, face-on spirals
yield dozens of H II regions. While we retain the full set of individual H II
regions in our internal lists, for the purposes of the survey we do not
include all of the H II regions in our final catalog. Rather, we record the
location of the galaxy center, so that the BB magnitudes and total emission-
line flux from the galaxy can be accurately measured. These are the category 2
objects.

In some cases, there is no detectable emission associated with the galaxy
center of the category 2 objects. For this reason, we typically also catalog
the single brightest H II region in the extended galaxies and target it for
follow-up spectroscopy. These are the category 3 objects. This scheme ensures
that the brightest emission region in each SFACT detection is observed
spectroscopically. Objects in this group are always a sub-unit of an object in
category 2, although not all objects in category 2 have a corresponding H II
region in our final catalog. The latter circumstance will occur when, for
example, the center of the galaxy possesses a strong emission source.

The number of detections listed in Table 2 do not include all of the
individual H II regions detected by our software. Rather, each detected galaxy
appears only once, unless there is a bright H II region (category 3)
associated with it. In our pilot-study catalogs, there are 40 extended
galaxies (category 2) and 19 H II regions contained in the three fields.

####  4.1.3 Broad-band Photometry of the Candidates

Figure 2: Histograms showing the BB magnitudes and colors for the full sample
of SFACT emission-line candidates from the three pilot-study fields (N = 533).
upper left: r-band magnitude distribution, with a median value of r = 22.51
and a faint limit of r $\sim$ 25.8. upper right: i-band magnitude
distribution, with a median value of i = 22.15. lower left: g-band magnitude
distribution, with a median value of r = 23.18 and a faint limit of g $\sim$
26.3. lower right: g$-$r color distribution, with a median value of 0.65.
While the bulk of the galaxies have observed colors between 0.2 $<$ g$-$r $<$
1.2, many objects with extreme colors are present in the sample.

Figure 2 presents the results of our BB photometry for the 533 SFACT
candidates in the pilot-study fields. The details of our measurement and
calibration procedures are given in SFACT2. Here we show the cumulative
histograms of the three BB magnitudes measured by the survey (gri), as well as
the g$-$r color distribution. The median apparent magnitudes are listed in
each panel of the figure, and the vertical dashed lines indicate the location
of the median in each histogram.

The apparent magnitude distributions shown in Figure 2 reflect the depth of
our sample. Focusing on the r-band histogram (upper left), it is seen that the
majority of SFACT ELG candidates have r-band magnitudes in the range 21-24
mag, with a tail to both brighter and fainter magnitudes. The faintest object
detected has r = 25.85, while the brightest has r = 15.60. Nearly all of the
objects with magnitudes brighter than r = 19.0 represent low redshift extended
galaxies detected via their disk H II regions (referred to as category 2
objects above). The median value of r = 22.51 is consistent with faintest
objects that are reliably detected by SDSS. The g-band and i-band magnitude
distributions are similarly deep, with median values of g = 23.18 and i =
22.15.

The histogram of g$-$r colors shown in Figure 2 (lower right) exhibits a
surprisingly broad range. The median color of 0.65 is consistent with the
colors of early-type spirals, and the color range that encompasses the bulk of
the sample, 0.2 $<$ g$-$r $<$ 1.2, includes many very red systems. Typically
one would expect that a sample of galaxies that is dominated by actively star-
forming systems would exhibit bluer colors. Two factors appear to be creating
this counter-intuitive result. First, the colors presented here are observed
values; we have not applied any Galactic reddening corrections (expected to be
small for these survey fields) or redshift-dependent corrections (K
corrections). Since the galaxy sample includes many systems at redshifts of up
to 1.0, one should expect that the relevant K corrections will be significant.
Second, for a sample of galaxies detected via their strong emission lines, it
would not be surprising if the emission lines themselves impacted the observed
colors in a redshift-dependent fashion. For example, the presence of strong [O
III]$\lambda$5007 emission in the SDSS r filter is what leads to the detection
of Green Pea galaxies using SDSS BB photometry (Cardamone et al., 2009). For
the SFACT galaxies with spectroscopic follow-up, the [O III]-selected galaxies
(n = 179) have a median g$-$r color of 0.82, compared with g$-$r = 0.51 for
the lower-redshift H$\alpha$-selected subset (n = 107). Hence, we conclude
that the presence of strong [O III] emission is skewing the observed g$-$r
colors redward for a significant fraction of the SFACT galaxies.

####  4.1.4 Emission-Line Fluxes of the Candidates

Figure 3: Histogram of the NB fluxes measured for all 533 SFACT candidates
from the three pilot-study fields. We make no distinction between objects
detected in the different NB filters. The median emission-line flux (denoted
by the dashed vertical line) is 2.97 $\times$ 10-16 erg s-1 cm-2, while the
sample appears to be substantially complete to $\sim$1.9 $\times$ 10-16 erg
s-1 cm-2. The lower (black) histogram shows the distribution of NB fluxes for
the 38 SFACT candidates that were found to be false detections in our follow-
up spectroscopy (see Section 4.2)

While the BB magnitudes are a relevant and convenient way to parameterize the
depth of the survey, SFACT is inherently an emission-line selected sample of
galaxies. Therefore, it is the emission-line flux distribution that truly
defines the sensitivity limits of the survey. Figure 3 plots the measured NB
emission-line fluxes for all 533 SFACT objects in the pilot-study fields. Once
again, we refer the reader to the SFACT2 companion paper for details on the
measurement and calibration procedures used to obtain the flux values shown in
Figure 3.

The histogram of emission-line fluxes reveals a strongly peaked distribution,
with most sources exhibiting fluxes between 10-15 and 10-16 erg s-1 cm-2. The
median flux value is 2.97 $\times$ 10-16 erg s-1 cm-2, while the minimum
detected flux is 1.01 $\times$ 10-16 erg s-1 cm-2. The histogram of fluxes
rises to a peak at $\sim$1.9 $\times$ 10-16 erg s-1 cm-2, after which the
number of detections begins to fall off. We take this latter quantity to be
representative of the completeness limit of the survey.

At the bright end of the distribution, all of the objects with fluxes above
10-14 erg s-1 cm-2 (n = 16) are bright extended galaxies. They are all nearby,
detected via their H$\alpha$ emission in either NB1 or NB3. In most cases, the
line emission from these objects is a combination of disk H II regions and
nuclear emission from a central starburst. The SFACT objects in the next
decade of line flux (10-14 to 10-15 erg s-1 cm-2) represent a mixture of
sources. Of the 73 objects in this flux range, 30 are H$\alpha$-detected
extended galaxies, similar in nature to the very brightest sources. However,
the remaining 43 include many compact [O III]-detected objects, plus one
luminous [O II]-selected source and two bright QSOs.

####  4.1.5 Example SFACT Objects

$\bigcirc$ $\bigcirc$ $\bigcirc$

$\bigcirc$ $\bigcirc$ $\bigcirc$

$\bigcirc$ $\bigcirc$ $\bigcirc$

$\bigcirc$ $\bigcirc$ $\bigcirc$

Figure 4: Example emission-line objects from the SFACT survey. Each row shows
a trio of images: left is the BB continuum image, center is the NB image
before continuum subtraction, and right is the continuum-subtracted NB image.
Each image cutout shows a field-of-view of 50 $\times$ 50 arcsec. Top: The
spiral galaxy SFF10-NB1-C22247, which is marked by the lower set of red
circles. Also indicated is the H II region SFF10-NB1-C22168. Middle: The faint
(g = 24.98) source SFF15-NB2-D2938. Note how the object is dramatically
brighter in the NB images. Bottom: The ELG SFF10-NB1-D11121. Spectra of all
four SFACT sources illustrated in this figure are presented in Figure 5

Figure 4 presents examples of newly discovered ELGs found by SFACT in the
pilot-study fields. We present these images to illustrate the survey method as
well as to give an indication of the data quality. Each row in the figure
presents three 50 $\times$ 50 arcsec cutouts. The left image in each row shows
the BB image of the field that was used in the continuum subtraction. In all
cases shown, this would be the sum of the r-band and i-band images. The middle
figure is that of the NB image within which the emission-line object was
detected, shown before any continuum subtraction. The rightmost figure is that
of the continuum-subtracted NB image. The red circles displayed in each panel
identify the SFACT objects.

The top row of images illustrate an extended galaxy as detected by SFACT. The
galaxy shown is SFF10-NB1-C22247; this designation refers to the center of the
galaxy. This particular object was detected via H$\alpha$ emission in the NB1
filter. It has a total g-band magnitude of 16.59 and an integrated NB flux of
2.14 $\times$ 10-14 erg s-1 cm-2. These characteristics make SFF10-NB1-C22247
one of the very brightest sources cataloged in the three pilot-study fields in
both BB and NB flux. As described in §4.1.2, we often include a bright H II
region located in the disk of an extended galaxy such as this in the catalog.
We do this in cases where the H II region exhibits substantial emission and/or
in cases where the emission from the galaxy center is weak or nonexistent. In
the case shown here, the H II region (designated SFF10-NB1-C22168) is the
brightest knot of emission within the galaxy.

The second row of Figure 4 presents the survey images for one of the faintest
sources in the pilot study. This galaxy, SFF15-NB2-D2938, is barely visible in
the continuum image, but is readily evident in the NB image. This source,
which was detected in NB2, has a g-band magnitude of only 24.98 and a NB flux
of 3.26 $\times$ 10-16 erg s-1 cm-2. This object provides a wonderful
illustration of one of the aspects of our data analysis process: due to the
manner with which we carry out our searches for emission-line objects, SFACT
is capable of detecting ELGs with little or no continuum emission (see
SFACT2).

The final object shown in Figure 4 is less extreme in its appearance than the
other two, making it perhaps more representative of a typical SFACT detection.
This source, SFF10-NB1-D11121, was detected as an emission-line object in
filter NB1. Its observed properties include a g-band magnitude of 23.28 and a
NB flux of 4.76 $\times$ 10-16 erg s-1 cm-2. The line emission is clearly
spatially extended. While the emission is strong, it does not exhibit the high
equivalent width evident in SFF15-NB2-D2938.

The spectra for all of the SFACT detections shown here are presented in Figure
5. Many additional example images of SFACT ELGs are given in SFACT2.

###  4.2 Results from the Spectroscopic Follow-Up Observations

Once an SFACT field has been processed and searched for ELG candidates, it
must still be observed spectroscopically in order to make it useful for
science. Because SFACT is sensitive to faint sources via a variety of emission
lines, spectroscopic confirmation is required to ascertain which line a given
object was detected with. Lower redshift objects are often, but not always,
sufficiently resolved in our images to allow us to identify them as
H$\alpha$-detections. However, for most of the higher redshift sources,
including [O III] and [O II] detections, the sources are unresolved or only
marginally resolved in our images. Hence, we typically cannot identify the
emission line present in our survey filter without the aid of spectra. In
addition, spectroscopy is required to identify the activity class of each
source (e.g., star forming vs. AGN).

As alluded to in §3.1, follow-up spectroscopy of the SFACT candidates was
planned for from the start of the project. The predicted density of ELG
candidates as well as the footprint of the ODI camera was well matched for the
Hydra fiber positioner available on the WIYN 3.5 m telescope. Despite the
faint nature of the SFACT objects, the fact that their spectra are dominated
by emission lines makes it possible to acquire adequate spectra for even the
faintest sources. Details of the spectroscopic observations, data processing,
and measurement are presented in a companion paper (SFACT3). Here we present a
brief overview of the essential results of the spectroscopic observations of
the three pilot-study fields.

####  4.2.1 Number of Spectroscopically Observed SFACT Objects

Naturally, the follow-up spectroscopy of our candidates lags the imaging
observations. The earliest opportunity to acquire spectra of the SFACT
candidates occurs one year after the full set of imaging data for a given
field have been obtained. However, in practice it has usually taken
substantially longer to get complete spectroscopic data for all of the
candidates. The reasons for this are varied (e.g., loss of telescope access
for two semesters due to the Covid-19 pandemic, weather losses, etc.). Hence,
the follow-up spectroscopy of the three pilot-study fields is not yet
complete. Nonetheless, it is sufficiently far along to allow us to present a
reasonably robust picture of the nature of the SFACT objects.

Table 2 includes the number of objects for which follow-up spectra have been
obtained. Overall, 453 out of 533 objects have been observed spectroscopically
(85.0%). Each of the individual fields has a substantial fraction of its ELG
candidates observed to date: 83.3% for SFF01, 93.1% for SFF10, and 78.9% for
SFF15.

For the SFACT objects with spectral observations in our pilot-study fields,
415 out of 453 (91.6%) are found to be true emission-line objects. That is,
91.6% of our sources exhibited moderate-to-strong line emission located within
the NB filter where the excess flux signal was present in our survey images.
The 8.4% false-detection rate is in keeping with expectations, given that we
chose to be inclusive in terms of the quality of the sources that were passed
during our final object inspection (see SFACT2). In most cases, the false
detections represent faint sources with weaker putative emission that exhibit
properties that locate them near the thresholds of our selection criteria (see
Figure 3).

It is worth noting that only a small fraction of the SFACT objects have
existing spectra in SDSS. This is expected, given that the SDSS galaxy
redshift survey only observed galaxies brighter than r $\sim$ 18 (Strauss et
al., 2002), while only a small fraction of the SFACT galaxies are brighter
than r = 18 (see Figure 2). Since the SDSS galaxy redshift survey was not
carried out in the Fall sky, only 2 out of 533 SFACT candidates found in the
pilot-study fields also possess SDSS spectra (0.4%). For 13 Spring SFACT
fields where the catalog construction is complete, 61 out of 1659 objects
(3.7%) have spectra in SDSS. Of these, 14 are QSOs, while 47 are galaxies.
Among the latter group, 100% are detected by SFACT via their H$\alpha$ line
(redshifts below z = 0.15), and nearly all are brighter than r = 18.

####  4.2.2 Example Spectra

Figure 5 presents example spectra of objects discovered in our survey. The
spectra shown correspond to the SFACT candidates illustrated in Figure 4. Our
SFACT spectra typically cover the wavelength range between $\sim$4700 and 7600
Å. The objects whose images and spectra are shown in Figures 4 and 5 were
chosen primarily to illustrate SFACT ELGs detected by the three principal
emission lines selected for in the survey: H$\alpha$, [O III]$\lambda$5007,
and [O II]$\lambda$3727.

The top row of Figure 5 shows the spectrum of the central region of the spiral
galaxy shown in Figure 4, as well as a spectrum of one of the outlying H II
regions detected in our imaging data. As indicated in §4.1.2, the SFACT survey
always includes the center of any extended galaxy for which disk H II regions
are identified in our imaging data. We also often include the brightest H II
region in our catalogs, particularly if the central region has only weak or
nonexistent line emission indicated in our NB images. In this case, the
central region of the galaxy includes a moderately bright, high-surface-
brightness nucleus which harbors a Low-Ionization Nuclear Emission Region
(LINER; Heckman, 1980). The spectrum of the H II region reveals a normal,
relatively metal-rich star-forming knot. The galaxy has a redshift of z =
0.0515 (distance = 232 Mpc), and has an absolute magnitude of Mg = $-$20.2.

Figure 5: Spectral plots of the objects illustrated in Figure 4. The y-axis
plots the flux in erg s-1 cm-2 Å-1, while the x-axis shows the wavelength in
Å. The vertical red dashed lines found in all spectral plots presented in this
paper indicate the FWHM bandwidth of the NB filter used to detect each object
whose spectrum is shown. The upper row shows the spectra of both the central
region (left) and an outlying H II region (right) for the spiral galaxy shown
in the top image of Figure 4. The nuclear spectrum indicates that this object
is a LINER. The lower row includes the spectrum of an [O III]-detected star-
forming galaxy found in the NB2 filter (left), and a higher-redshift [O
II]-detected system (right).

The [O III]-detected object whose spectrum is shown in the lower left portion
of Figure 5 is a low-luminosity star-forming galaxy with a redshift z =
0.3107. The strong [O III]$\lambda$5007 emission line was detected in the NB2
filter. We measure a g-band magnitude of 24.98, from which we derive an
absolute magnitude of Mg = $-$16.1. It is seen to be essentially unresolved in
the survey imaging data shown in Figure 4. This object has physical and
spectral properties that are reminiscent of blue compact dwarfs (BCDs) in the
local universe (e.g., Janowiecki & Salzer, 2014; Janowiecki et al., 2017).
However, SFACT has detected it at a distance of over 1600 Mpc.

The final example spectrum presented in Figure 5 is that of an [O II]-detected
object (lower right). As is clear from the figure, the spectra of the [O
II]-selected SFACT objects typically do not provide access to many of the
nebular emission lines that both aid in the line identification process and
provide vital diagnostics regarding the nature of the source. We sometimes
detect the [Ne III]$\lambda$3869 line, as is the case with the object shown in
Figure 5. But in many cases we only detect the [O II] doublet itself. The
identification of the detected emission line is seldom in doubt, however,
because the doublet is typically marginally resolved at our spectral
resolution. Due to the lack of meaningful emission-line diagnostics, we
typically cannot assign the [O II]-detected ELGs into an activity class. This
step will need to await the acquisition of longer-wavelength spectral data
that cover the rest-frame spectra up to $\sim$5000 Å. Hence, at the current
time, we can only partially assess the nature of this object. With a redshift
of z = 0.8753 and an absolute magnitude Mg = $-$20.5, we know that this galaxy
is quite luminous. Our guess in that this is a starburst galaxy, as opposed to
an AGN, based on the high [O II]/[Ne III] flux ratio, the narrowness of the
emission lines, and the fact that the [Ne V]$\lambda\lambda$3426,3346 lines
are not present.

Additional example images and spectra of both SFACT galaxies and QSOs can be
found in SFACT2 and SFACT3.

####  4.2.3 Redshift Distribution

The redshifts of the SFACT galaxies detected in our pilot-study fields are
shown in Figure 6, presented in quasi-histogram form. Only objects detected
via one of the strong optical nebular lines are included; higher redshift QSOs
are not shown in this figure. The plot is designed to graphically illustrate
the redshift windows sampled by the survey filters for the principal emission
lines. We note that the width of each bin corresponds to the width of each
redshift window, getting broader with increasing redshift. Specifically, these
widths correspond to the redshift range of objects contained within the half-
height width of the relevant filter (see Table 1).

The redshift windows shown in the histogram break down into three primary
groups. The lowest-redshift objects (0.0 $<$ z $<$ 0.15) are the
H$\alpha$-detected galaxies. Many of these represent large, extended galaxies
with multiple H II regions in their disks. As suggested earlier, we expect the
number of H$\alpha$ detections in NB2 to be extremely small. Only one such
object exists in the three pilot-study fields: SFF01-NB2-B19198 is an H II
region in a nearby dwarf irregular galaxy (z = 0.0034). The other two NB
filters have a significant number of H$\alpha$ detections. In particular, the
NB3 H$\alpha$ detections represent the single largest sample of ELGs in the
current sample (n = 65).

Figure 6: Histogram showing the redshift distribution of the SFACT ELGs. Only
galaxies detected via their H$\alpha$, [O III], H$\beta$, [O II] and [Ne III]
lines are included in the figure; higher redshift QSOs are excluded. Unlike a
typical magnitude-limited redshift survey, the SFACT survey samples the
universe in discrete redshift windows

The next grouping in Figure 6 exhibits a more complex structure in the
histogram because the galaxies are being detected via three different lines.
The tallest bins represent the galaxies detected via their [O
III]$\lambda$5007 line. Unlike the situation for the H$\alpha$-detected ELGs,
the effective volumes covered by the three NB filters for the [O III] line do
not differ by large amounts. Hence, the numbers of objects detected in the
three filters should be comparable (although modulated somewhat by cosmic
variance). This is in fact what we observe. The number of [O III]$\lambda$5007
detections are NB2 = 62, NB1 = 47, and NB3 = 53. The total number of [O
III]$\lambda$5007-detected galaxies (n = 162) represents the largest number
for a specific line in the sample.

The second set of galaxies in this region of the histogram represent objects
that are detected via the [O III]$\lambda$4959 line. These are galaxies where
the $\lambda$5007 line has redshifted out of the NB filter, but the
$\lambda$4959 line is still present. Since the latter line is roughly a factor
of 2.9$\times$ weaker than the former (e.g., Osterbrock & Ferland, 2006), we
naturally expect far fewer detections via this line. This is what is observed:
a total of 16 ELGs are detected via their [O III]$\lambda$4959 line, only
$\sim$10% the number of [O III]$\lambda$5007 detections. Finally, there are
three galaxies that are detected by their H$\beta$ emission (two in NB1 and
one in NB3). The H$\beta$-detected objects represent only a small contribution
to the overall survey.

When the need to distinguish between which of the three lines was detected in
the survey is of less relevance, the objects in this grouping will be lumped
together and simply referred to as being [O III]-detected. However, for many
applications the distinction as to the proper identification of the detected
line is essential (e.g., calculating star-formation rate densities). Whenever
relevant, we will separate the galaxies detected by these three lines into
distinct subsamples.

The final grouping in Figure 6 represents the highest redshift group, nearly
all of which are detected via the [O II]$\lambda$3727 doublet. ELGs with
redshifts in the range 0.75 to 1.02 are included within this group. At these
distances, all of the detected objects are unresolved in our images.
Naturally, the [O II]-detected component of the survey are predominantly among
the faintest objects in the sample. As was the case with the [O III]-detected
component of the survey, the [O II] ELGs are fairly evenly distributed between
the three filters: NB2 = 26, NB1 = 42, and NB3 = 28. While the total number of
[O II] detections (n = 96) is lower than both the H$\alpha$ and [O III]
subsamples, it nonetheless represents a sizable fraction of the total number
of SFACT objects ($\sim$25% of the ELGs included in Figure 6). We also note
that a single object was detected due to the presence of a strong [Ne
III]$\lambda$3869 line falling in the NB1 filter.

There are 11 spectroscopically confirmed QSOs detected in the SFACT pilot-
study fields. Of these, one was detected via the [O II] doublet (z = 0.756),
five were selected when Mg II $\lambda$2798 fell in one of the survey filters
(redshifts between 1.34 and 1.68), two were found through their C III]
$\lambda$1908 emission (redshifts between 2.43 and 2.94), and three were
detected via the C IV $\lambda$1549 line (redshifts between 3.23 and 3.85).
The number of detected QSOs is small compared to the number of ELGs discovered
by SFACT, but this is to be expected given the relatively small volumes
surveyed by our three pilot-study fields combined with the small volume
densities of QSOs. We project that we will detect over 200 QSOs in the full
survey.

###  4.3 Properties of the SFACT Emission-Line Objects: A Preliminary View

Despite the incomplete nature of the spectroscopic follow-up of the SFACT
galaxies detected in our pilot-study fields, we can nonetheless obtain a
fairly good understanding of the nature of the sources detected in our sample.
In this section we present a preliminary look at the properties of the SFACT
galaxies. A more complete assessment of the nature of the sources detected in
our survey will be presented in future papers in this series.

####  4.3.1 Absolute Magnitude Distributions

Figure 7: Histograms showing the g-band absolute magnitude distributions for
the SFACT objects located in our pilot-study fields. The upper panel shows the
Mg distribution for the lower-redshift H$\alpha$-detected galaxies, while the
middle and lower panels show the same distributions for the intermediate-
redshift [O III]-selected galaxies and the [O II]-selected galaxies,
respectively. In all three panels the black-lined histogram plots the
luminosity distribution for the full sample. The latter includes the higher-
luminosity QSOs.

In Figure 7 we present histograms of the g-band absolute magnitudes of the
SFACT galaxies with redshifts in the three pilot-study fields. The galaxies
are separated into three groups based on the emission line responsible for
their detection. Each panel of the figure also shows the luminosity
distribution for the full sample (black-lined histogram). Objects detected as
H II regions are excluded from Figure 7. Rather, we show the luminosities of
the entire galaxy within which the H II regions reside.

The overall distribution of absolute magnitudes is remarkably broad, with the
range $-$16 $>$ Mg $>$ $-$21 being well represented and exhibiting a fairly
flat array of values. This is characteristic of emission-line-selected galaxy
samples in general (e.g., Salzer et al., 1989, 2020), since the detection of
lower-luminosity ELGs is typically enhanced in such surveys. This is in
contrast to the luminosity distributions of traditional magnitude-limited
galaxy surveys, which tend to be strongly peaked with the majority of galaxies
falling within $\pm$1 magnitude of M∗ (for reference, M${}^{*}_{g}$ = $-$20.1;
Blanton et al. (2003)). We conclude that SFACT, like previous emission-line
surveys, is quite sensitive to low-luminosity dwarf systems, particularly for
the low- and intermediate-redshift detections.

The upper panel of Figure 7 highlights the Mg distribution of the
H$\alpha$-detected SFACT galaxies. These galaxies are found at lower redshifts
(see Figure 6), and represent the most diverse subset of the overall survey.
Many of the galaxies are larger spirals or irregulars with multiple H II
regions detected in our images. These represent the more luminous galaxies
shown in the upper panel (Mg $<$ $-$19). The lower luminosity galaxies in the
histogram are typically compact star-forming systems of the type that are
commonly labeled as BCDs in the nearly universe. With one exception (see
below), the H$\alpha$-selected ELGs are all detected in NB1 (distances of
230–300 Mpc) and NB3 (distances of 615–685 Mpc). It is a testament to the
depth of the SFACT survey method that it can routinely detect dwarf star-
forming galaxies with such low luminosities at these distances. For example,
the second-lowest luminosity galaxy in this subsample was detected in NB3. It
has an absolute magnitude of Mg = -13.97 and a distance of 628 Mpc.

The single exception mentioned in the previous paragraph is SFF01-NB2-B19198,
the one NB2 H$\alpha$-detection in the three pilot-study fields. This galaxy
is a fairly low-surface-brightness dwarf with a single H II region. Based on
its redshift, it has a distance of 18.9 Mpc and a g-band absolute magnitude of
$-$12.38. This makes it the lowest luminosity system in the current study.

The luminosity histogram of the [O III]-selected SFACT galaxies is shown in
the middle panel of Figure 7. The distribution of values is very symmetric,
with a median Mg of $-$18.1. This relatively low-luminosity median value (2
magnitudes below M${}^{*}_{g}$) is perhaps surprising at first glance, given
that the distances to the galaxies in this subsample range between 1610 and
2970 Mpc. However, it is important to recognize that any [O III]-selected
galaxy sample will be subject to a well understood metallicity effect: the [O
III] emission lines are weak in high-metallicity, high-luminosity systems and
become stronger in lower metallicity systems. The strength of the [O III]
doublet peaks at metal abundances of $\sim$10% solar, abundances typically
found in galaxies with $-$16 $>$ Mg $>$ $-$19\. This is precisely the
luminosity range occupied by the bulk of the [O III]-detected subsample. Most
of these systems will be intermediate- and low-luminosity star-forming
galaxies, including some BCDs at values of Mg $>$ $-$17\. There is a modest-
sized tail of higher luminosity galaxies among the [O III]-selected SFACT
galaxies. These include a few Seyfert 2 galaxies as well a number of putative
Green Pea-like galaxies with intermediate and high luminosities and large [O
III] equivalent widths ($>$ 200 Å).

The bottom panel in Figure 7 highlights the [O II]-selected galaxies. Not
surprisingly, these objects dominate the high luminosity end of the
distribution. In fact, unlike the case for the other two emission lines
illustrated in the figure, the luminosity histogram of the [O II]-detected
SFACT objects is reminiscent of the distributions seen in magnitude-limited
samples. The median value of Mg is $-$20.3. The distribution of absolute
magnitudes can be understood as a combination of the extreme distances
involved for the [O II] subsample (4.7 to 6.9 Gpc) plus the fact that the
strength of the [O II] doublet does not exhibit as strong a metallicity
dependence as [O III]$\lambda\lambda$5007,4959. The observed distribution has
tails to both low- and high-luminosities. The high-luminosity systems most
likely include several AGN. At the present time our follow-up spectra do not
include sufficient spectral-line diagnostics to cleanly distinguish between
star-forming galaxies and AGN for the [O II]-detected SFACT objects. Survey
plans include a second round of spectroscopy for the [O II]-selected galaxies
that will include wavelength coverage redward of our initial follow-up spectra
(see §7). On the low-luminosity side of the distribution, we observe that
SFACT is sensitive to some fairly low-luminosity systems at these large
distances, albeit in small numbers. The lowest luminosity galaxy detected via
the [O II] doublet has Mg = $-$17.5.

####  4.3.2 Emission-Line Diagnostic Diagrams

The ratios of strong emission lines have long been used as diagnostics for the
physical conditions present in star-forming galaxies and AGN (e.g., Baldwin et
al., 1981; Veilleux & Osterbrock, 1987). In particular, commonly used
diagnostic diagrams allow astronomers to distinguish between the two primary
ionization sources present in ELGs (hot-star photo-ionization and black hole
accretion disks) and provide estimates of the metal abundance of the hot gas.

The specific redshift ranges present in our catalog, combined with the choice
of spectral coverage for our follow-up spectra, results in the need to use
different diagnostic diagrams for the different emission lines responsible for
the detection of the sources. For the H$\alpha$-selected SFACT galaxies, our
spectral wavelength coverage includes the [N II]/H$\alpha$ and [O
III]/H$\beta$ ratios, but excludes the [O II] doublet (as well as the [S
II]$\lambda\lambda$6731,6716 doublet for NB3 detections). Hence for the
H$\alpha$-detected sources we utilize what is commonly referred to as the BPT
diagram (Baldwin et al., 1981): [O III]/H$\beta$ vs. [N II]/H$\alpha$. For all
of our [O III]-selected SFACT galaxies, the lines in the vicinity of H$\alpha$
are redshifted beyond the high-wavelength end of our spectral coverage. In
this case, the only viable option is to use the [O III]/H$\beta$ vs. [O II]/[O
III] diagram. As alluded to above, our current spectral coverage does not
provide for any useful diagnostics for the [O II]-detected galaxies; further
assessment of the nature of these galaxies will need to await additional
spectral data that cover redder wavelengths (see §7).

Figure 8: Emission-line diagnostic diagram for the H$\alpha$-selected SFACT
galaxies presented in the current paper. These objects are found in the
redshift range z = 0.00 – 0.15. The different categories of emission-line
sources are indicated by different symbols, as specified in the legend. The
solid curve represents photo-ionization models from Dopita & Evans (1986), and
the dashed curve shows the empirical demarkation line between AGN and star-
forming galaxies from Kauffmann et al. (2003).  Figure 9: Emission-line
diagnostic diagram for the [O III]-selected SFACT galaxies presented in the
current paper. These galaxies are found in the redshift range z = 0.30 – 0.52.
The different types of emission-line sources are indicated by different
symbols, as specified in the legend. The dashed line represents the trend line
shown in Figure 1 of Baldwin et al. (1981).

Figure 8 shows the emission-line diagnostics for the H$\alpha$-detected SFACT
objects. This subsample includes a mixture of extended galaxies where the
fiber was placed on the center of the galaxy (magenta triangles), H II regions
located within these galaxies (typically the brightest emission knot; red
circles), and the less extended, more compact ELGs (blue squares). The solid
curve represents a locus of high-excitation stellar photo-ionization models
from Dopita & Evans (1986), while the dashed line is the empirical demarkation
line between AGN and star-forming galaxies proposed by Kauffmann et al.
(2003). Only 56 of the 108 H$\alpha$-selected objects had all four of the
lines necessary to form these two line ratios detected above the sensitivity
threshold of the automated measurement software used by SFACT (see SFACT3).

The blue squares plotted in Figure 8 include a fairly diverse set of objects.
They include many low-luminosity, low-metallicity systems plotted in the upper
left portion of the diagram. The objects in the lower right are mainly the
more metal rich, luminous star-forming galaxies and their associated H II
regions (magenta triangles and red circles). Two of the ELGs are located just
above the Kauffmann et al. (2003) demarkation line, and may well be low-
ionization nuclear emission region (LINER) AGNs. Based on the combination of
Figures 7 and 8, we conclude that the ELGs represented by the
H$\alpha$-selected portion of SFACT appear to span the full range of star-
forming galaxies.

The spectral-line diagnostic diagram for the [O III]-selected ELGs is shown in
Figure 9, which plots the logarithm of [O III]/H$\beta$ against the logarithm
of [O II]/[O III]. As is the case with Figure 8, many of the SFACT galaxies
detected via their [O III] lines are not included in the figure because one or
more of the necessary lines was not measured. Only 70 of the 178 galaxies in
this subsample have both line ratios available. The dashed line represents the
trend line shown in Figure 1 of Baldwin et al. (1981), which is fit to
emission-line ratios of approximately solar metallicity H II regions and
planetary nebulae.

The [O III]-selected star-forming galaxies are located well above the BPT
trend line. This is to be expected, since their lower luminosities (Figure 7)
coupled with the well-known trends from luminosity-metallicity relations
(LZRs, e.g., Hirschauer et al., 2018) imply that these ELGs have, on average,
substantially sub-solar abundances. These lower metal abundances result in
higher [O III]/H$\beta$ ratios for a given value of the excitation ([O II]/[O
III]). If these galaxies could be plotted in Figure 8 they would tend to be
found in the upper left portion of the star-forming galaxy sequence.

A single Seyfert 2 galaxy is also plotted in Figure 9. It is located well
above the BPT trend line and is clearly separated from the star-forming
galaxies, consistent with expectations. A total of three SFACT objects in the
three pilot-study fields were provisionally classified at Seyfert 2s based on
the appearance of their spectra (i.e., high [O III]/H$\beta$ ratios). However,
the other two currently lack measurements of the [O II] doublet, preventing us
from including them in the figure.

####  4.3.3 Star-Formation Rates

Figure 10: Histogram showing the distribution of the star-formation rates for
the H$\alpha$-detected SFACT ELGs in the three pilot-study fields (n = 108).
The object that stands out as having a low SFR is the sole H$\alpha$ detection
in the NB2 filter.

We computed H$\alpha$ star-formation rates (SFRs) for the galaxies in the
lowest redshift windows that are detected by H$\alpha$. We use the NB fluxes
measured from our survey images for this purpose, since they are more
accurately determined than the spectroscopic line fluxes and capture 100% of
the emission-line flux for our extended objects (which applies to essentially
all of the H$\alpha$-detected sample). For galaxies with multiple H II regions
(e.g., Figure 4, top row), we compute the global SFR using the integrated line
emission from the full galaxy. The H$\alpha$ fluxes are corrected for the
presence of [N II] emission within the filter bandpass. They are also
corrected for absorption using the Balmer decrement (f(H$\alpha$)/f(H$\beta$))
when this quantity is measured in our spectra (as is the case for most
H$\alpha$-detected sources). We use the corrected H$\alpha$ flux and the
redshift-determined distance to determine the H$\alpha$ luminosity, then
utilize the standard Kennicutt (1998) relation (SFR [M⊙ yr-1] = 7.9 $\times$
10-42$\cdot$L(H$\alpha$)) to estimate the SFR.

The distribution of our derived SFRs is shown in Figure 10. Despite the small
size of the current sample and the limited survey volume of the three pilot-
study fields, the SFACT sample is seen to be providing a robust measurement of
the SFR distribution at the high end (log(SFR) $>$ 0.5). The bulk of the
sample is found to have log(SFR) values between $-$1.5 and 1.5 (0.03 $<$ SFR
$<$ 30 M⊙ yr-1), with a median SFR value of 0.32 M⊙ yr-1. The lack of
detections at lower SFRs can be readily understood by referring to Figure 6.
Essentially all of the H$\alpha$ detections are made in the NB1 and NB3
filters. Once again, the one exception is SFF01-NB2-B19198, the sole H$\alpha$
detection in NB2. This galaxy stands out in Figure 10 as having by far the
lowest SFR of any object in the sample.

The current sample of SFRs is presented to illustrate the sensitivity of the
SFACT survey and to establish the range over which the SFR measurements can be
robustly determined. Future papers in this series will explore the star-
formation characteristics of much larger samples of SFACT galaxies, including
the ELGs detected in the higher redshift windows.

##  5 Comparison with Previous Narrowband Surveys

We compare the characteristics of the SFACT survey galaxies with a number of
representative and recent NB surveys with the goal of helping to place the
current survey into context. We focus this comparison on three surveys that
sample the ELG population in the same redshift range covered by SFACT: Stroe &
Sobral (2015), miniJPAS (Martínez-Solaeche et al., 2022), and LAGER (Khostovan
et al., 2020). We stress that each of the surveys being discussed was designed
and carried out with a specific goal (or set of goals) in mind. Each of the
survey methodologies has obvious merit, and the resulting galaxy samples
accomplished the goals set for the projects. The aim of this comparison is not
to rank the surveys in any way. Rather, the purpose of this section is to
allow the reader to better visualize the utility of the SFACT survey by
comparing it directly to these other successful programs.

The Stroe & Sobral (2015) study presents a NB survey selecting galaxies via
their H$\alpha$ emission. The primary goals of this project were to securely
measure the luminous end of the LHα luminosity function at low redshift (z
$\sim$ 0.20) and to quantify the level of cosmic variance present in small-
area surveys. In order to do this, the survey needed to cover a larger area on
the sky, but it did not need to be particularly deep. It utilized two NB
filters that detected H$\alpha$ emitters in the redshift ranges 0.186$-$0.203
and 0.217$-$0.233. The imaging was carried out on the 2.5 m Isaac Newton
Telescope and used relatively short exposures (5 $\times$ 600 s). Hence, the
resulting galaxy sample is relatively shallow when compared to other surveys
(e.g., Sobral et al., 2012, 2013): the characteristic 50% line flux
completeness limit is $\sim$2 $\times$ 10-15 erg s-1 cm-2. The areal coverage
of 12.8 deg2 allowed the survey to detect sufficient numbers of lower redshift
H$\alpha$-emitting galaxies to carry out the planned study. The final sample
of H$\alpha$-detected galaxies yielded 7.4 and 9.8 objects deg-2 in the two NB
filters.

The miniJPAS survey (Martínez-Solaeche et al., 2022) is carried out in a
manner which is quite different from most traditional NB surveys. Rather than
using one, or a few, NB filters to survey for emission lines at specific
redshifts, miniJPAS utilizes a set of 54 filters with bandwidths of
$\Delta\lambda$ $\sim$ 145 Å. These filters overlap each other so as to
provide continuous wavelength coverage from 3800 $-$ 9100 Å. The miniJPAS
survey, which covers 1 deg2 overlapping the AEGIS (Davis et al., 2007) field,
is a fore-runner of the J-PAS survey, which will eventually cover $\sim$8000
deg2. While no emission-line flux completeness limits are provided, the sample
appears to be 50% complete for objects with a S/N of 5 at r $\sim$ 21.0. The
survey detected a total of 2154 potential ELGs, most of which are expected to
be H$\alpha$-detections in the redshift range z = 0.00 $-$ 0.35. Of these, 255
had sufficient S/N to allow for the determination of the emission line ratios
[O III]/H$\beta$ and [N II]/H$\alpha$ with an uncertainty of 0.2 dex. These
detection rates are for the full redshift range specified above. In order to
directly compare these numbers with the NB surveys discussed here, we need to
adjust for the limited bandpasses employed by the other surveys. Adopting a
characteristic filter width of $\Delta\lambda$ = 100 Å results in a redshift
coverage of $\Delta$z $\sim$ 0.016 near the middle of the miniJPAS redshift
range. This results in approximate detection rates of 98 ELGs deg-2 per NB
filter for all candidates, and 12 ELGs deg-2 per NB filter for the high-
quality subsample.

The Lyman Alpha Galaxies at Epoch of Reionization (LAGER) (Khostovan et al.,
2020) survey is a deep NB program being carried out with DECam on the CTIO
Blanco 4.0 m telescope. As the name implies, a key focus of the project is to
detect Ly$\alpha$-emitting galaxies at high redshift (z $\sim$ 6.93). In
addition, the survey detected large numbers of ELGs via their H$\alpha$ (z =
0.47), [O III] (z = 0.93), and [O II] (z = 1.59) emission lines. The total
integration time allotted to the single DECam field (FOV = 3.0 deg2) with the
LAGER NB filter ($\lambda_{cent}$ = 9640 Å, $\Delta\lambda$ = 92 Å) was 47.25
h, which resulted in extremely deep coverage. Focusing on the H$\alpha$
detections to allow the best comparison with the other surveys, LAGER
catalogued 1577 candidate ELGs with a 50% line flux completeness limit of
$\sim$2.5 $\times$ 10-17 erg s-1 cm-2. The detection rate of 526
H$\alpha$-detected ELGs deg-2 in the single NB filter is extremely impressive.
The single field presented in Khostovan et al. (2020) overlaps the COSMOS
field; the full LAGER survey proposes to cover a total of 8 fields and 24
deg2.

As presented earlier in this paper, the SFACT survey has an approximate 50%
line flux completeness limit of $\sim$2 $\times$ 10-16 erg s-1 cm-2, based on
the three pilot-study fields. This places SFACT midway between the Stroe &
Sobral (2015) and Khostovan et al. (2020) surveys: the SFACT line flux
completeness limit is approximately 10 times fainter than the Stroe & Sobral
(2015) survey but is only 10% as faint as the corresponding limit for the
Khostovan et al. (2020) survey. Based on the detection of 533 ELG candidates,
the surface density of SFACT-detected galaxies is 355.3 ELGs deg-2 across all
filters, and 132.8 ELGs deg-2 per NB filter. This latter number accounts for
the fact that there are essentially no H$\alpha$ detections for SFACT in the
NB2 filter. To better compare with the other surveys, we utilize our spectral
data to derive detection rates of 50.5 deg-2 for the H$\alpha$-detected ELGs
in NB3 (z range of 0.129 $-$ 0.144) and an average of 42.0 ELGs deg-2 for each
of the three [O III] redshift windows. The SFACT detection rates are seen to
be intermediate between those of the Stroe & Sobral (2015) and miniJPAS
surveys on the one hand, and the LAGER survey on the other.

A key difference between SFACT and all of the surveys described above is the
fact that SFACT has been designed from the start to include spectroscopic
follow-up of the sources detected. The others rely on the use of photometric
redshifts to indicate which emission line is detected in the NB filter. While
this approach works well on average, it is certainly not fool proof.
Furthermore, the lack of confirming spectra requires one to adopt the
assumption that 100% of the NB detections are in fact real ELGs. Experience
would suggest that no astronomical survey is perfect, and that some fraction
of the objects selected as ELGs will be spurious. Of course, having follow-up
spectroscopy not only provides an absolute verification of the survey
constituents, but it also allows for additional science applications. While
all of the comparison samples have been used successfully to estimate the
star-formation rate density at the redshifts covered by their filter(s), the
existence of spectroscopic follow-up will allow SFACT to also probe the
evolution of the galaxy metal abundance with large, robust samples of ELGs and
unambiguously identify rare objects like AGN and Green Pea galaxies (see §6).

##  6 Applications of the SFACT Survey

In this section we describe a number of proposed applications for the SFACT
survey. This list is limited mainly by the interests of the current SFACT team
members. It is not meant to be exhaustive. Rather, we hope to convey a glimpse
of some of the exciting science outcomes expected from SFACT in the next
several years. Naturally, most of these projects will need to wait until we
have obtained a more complete set of follow-up spectra. As mentioned in the
following section, however, this point is not necessarily that far off. We are
already working toward deriving preliminary results for a number of these
applications based on the partial ELG samples currently in hand.

###  6.1 Evolution of the Star-Formation Rate Density to z = 1 and Beyond

The measurement of the star-formation rate density (SFRD) from the local
universe to z =1 and beyond represents one of the major planned applications
of SFACT, and the survey name was chosen as an epithetical reference to it.
The design of the SFACT survey should yield optimal results for the
measurement of the SFRD across this redshift range. As specified in §3.3, we
expect to have between 800 and 1200 emission-line-selected galaxies in each of
our redshift windows, and these numbers are consistent with our results from
the pilot-study fields presented in Figure 6, at least for the H$\alpha$ and
[O III]-detected objects. These large samples will provide for robust
estimates of the SFRD in each redshift window, and our field selection method
will naturally account for cosmic variance.

It is worth stressing that the sensitivity of the SFACT survey results in a
comprehensive sample of star-forming galaxies within each redshift window.
This point is made evident by reference to Figure 7, which shows that all
three primary lines detect galaxy samples that peak at or below the knee in
the galaxy luminosity function (M${}^{*}_{g}$ = $-$20.1; Blanton et al.
(2003)). For example, the upper panel of Figure 7 shows that the distribution
of H$\alpha$-detected ELGs in the three pilot-study fields includes galaxies
with g-band absolute magnitudes from Mg = $-$21 (i.e., more luminous than
M${}^{*}_{g}$) to Mg = $-$12\. The histogram suggests that the survey is
fairly complete to Mg = $-$16 for the H$\alpha$ detections. Similarly, the
middle histogram in Figure 7 indicates the [O III]-selected sample is fairly
complete to Mg = $-$17\. Even the galaxies detected via the [O II] doublet
probe galaxies with luminosities well below M${}^{*}_{g}$. This point is
further solidified by examination of the SFR histogram shown in Figure 10. The
peak in the distribution at SFR $\sim$ 0.1 M⊙ yr-1 corresponds to an H$\alpha$
luminosity of $\sim$1.3 $\times$ 1040 erg s-1, more than an order of magnitude
below L${}_{H\alpha}^{*}$ at these redshifts (Stroe & Sobral, 2015). Hence,
the determination of the SFRD using SFACT galaxies will be based on large and
comprehensive samples.

Our current set of NB filters allows us to probe the star-forming galaxy
population to z = 1. In the near future we hope to add two additional filters
(see Table 1) that will detect galaxies via H$\alpha$ to z = 0.40, via [O III]
to z = 0.83, and via [O II] to z = 1.46. When the survey is complete, we
expect SFACT to yield accurate SFRDs for each of redshift windows specified in
Table 1. These measurements should provide “hard points” in the distribution
of SFRDs out to z $\sim$ 1.5.

###  6.2 Characterization of the Strong-Lined AGN population

While the detection of star-forming galaxies is a particular strength of the
SFACT survey method, our sample of ELGs will also include many AGNs. These
include Seyfert galaxies, LINERS, and QSOs. While the number of AGN detected
in our pilot-study fields is small relative to the star-forming galaxy
population, when integrated over all survey fields and redshift windows we
expect to detect them in substantial numbers, particularly in the H$\alpha$
and [O III]-selected portions of the sample.

Two specific areas of research that are of interest to the SFACT team members
and that can be effectively explored with the survey data are the demographics
of AGN at intermediate redshifts (z = 0.2 to 0.9) and the evolution of AGN
metallicities with redshift. Both topics are relatively under-explored but
will be ripe for further study using SFACT. For example, the survey will be
able to be used to test whether the number density of AGNs increases in lock
step with the density of star-formation activity with look-back time by
measuring both using the same survey and within the same volumes of space.

There has been renewed interest in the determination of the metal abundances
of AGN (e.g., Dors et al., 2020; Flury & Moran, 2020; Carvalho et al., 2020).
Most of this work has focused on estimating the abundances of Seyfert 2
galaxies in the local universe. However, a recent study that includes Seyfert
2 galaxies out to z = 0.4 suggests that higher redshift AGN possess lower
metallicities than their low-z counterparts (Carr et al., 2023b). We expect to
be able to probe the metal abundances of Seyfert 2 galaxies to redshifts
approaching z = 0.9, to extend this result to higher redshifts and with larger
samples of AGN.

###  6.3 Demographics of Dwarf Star-forming Galaxies to z = 0.5

It is clear from Figure 7 that the SFACT selection method strongly favors the
detection of intermediate- and low-luminosity galaxies, particularly within
the H$\alpha$ and [O III]-detected portions of the survey. This result comes
about for a number of reasons, including the metallicity-related effect on the
strength of the [O III] doublet mentioned in §4.3.1. Other factors include the
sensitivity of the survey to faint sources, the increased contrast between
strong star-forming knots and the underlying continuum in low-luminosity
systems, and the simple fact that dwarf galaxies – star-forming or otherwise –
are more common in the universe than more luminous galaxies. SFACT can readily
detect galaxies with absolute magnitudes of Mg = $-$15 to z $\sim$ 0.15 via
the H$\alpha$ line and Mg = $-$16 out to z $\sim$ 0.50 via the [O III] line.

These characteristics of the survey will allow us to probe the properties and
demographics of dwarf star-forming systems to substantial redshifts. In
particular, SFACT will detect large samples of BCDs in all of the redshift
windows below z = 0.50. This will allow for the unprecedented opportunity to
study this important class of star-forming galaxy to cosmologically
significant distances. With sufficient follow-up spectral data, it will be
possible to probe for redshift dependences in the metal abundances of the
BCDs. We also expect to be able to constrain the evolution of the dwarf star-
forming galaxy population from z = 0.5 to today.

###  6.4 Environments of Star-Forming Galaxies and AGNs

As described in §3.3, many of the SFACT survey fields coincide with fields
that contain a previously known Green Pea galaxy. Brunker et al. (2022) has
carried out a redshift survey in these fields in order to study the galactic
environments that Green Peas are located in. We plan to build upon this
previous work by continuing to obtain redshifts of galaxies in these fields as
part of the SFACT follow-up spectroscopy campaign. Every multi-fiber
configuration observed with Hydra includes non-SFACT galaxies selected from
SDSS in any extra fibers. Since most of the SFACT fields require 3 to 5 Hydra
configurations to observe all of the ELG candidates, these extra fibers will
yield 50 to 100 additional field-galaxy redshifts in each field. In addition,
we have plans to carry out a more focused redshift survey of field galaxies in
several of the SFACT fields, with the goal of acquiring a fairly complete
redshift sample for galaxies to g $\sim$ 21\. These extra redshifts for faint
SDSS galaxies located within our survey fields will be in addition to the
existing SDSS redshift survey data (Strauss et al., 2002). This will allow us
to probe in detail the distribution of galaxies out to z $\sim$ 0.5.

The science driver for obtaining these redshifts is to allow us to study the
environments of the SFACT star-forming galaxies and AGN. With a typical yield
of $\sim$150 ELG candidates per field, the SFACT survey provides an excellent
opportunity for studying the effect that environment plays on driving activity
in galaxies. The planned redshift survey of field galaxies located in and
around the SFACT fields will be deep enough to probe environmental impact
using all of the H$\alpha$ and [O III]-detected objects. The rich sample of
ELGs in each SFACT field will provide a statistically meaningful ensemble of
galaxies with which to probe the impact of local environment of both star-
formation and AGN activity.

###  6.5 Evolution of Galaxy Abundances to z = 0.9

The measurement of the metal abundances in galaxies is a key tool for
understanding how they evolve with time. As our picture of galaxy evolution
has taken shape, astronomers have come to realize that many physical processes
– beyond basic star evolution – can affect the chemical enrichment of a given
galaxy. For example, galaxy mergers with metal-poor but gas-rich dwarfs or the
infall of pristine gas can lower the overall metallicity of a galaxy. These
effects are likely to be dependent on the local environment of the system. The
outflow of metal-rich ejecta from supernovae can likewise lower the measured
abundance of a galaxy below the level expected based on its time-integrated
star formation history. This process will be dependent on the overall mass of
the galaxy. Disentangling all of the relevant processes to arrive at a more
complete picture of galaxy chemical evolution from the observational side
requires large, comprehensive samples of galaxies with metal abundances.

The SFACT survey holds much promise for providing the type of galaxy sample
necessary to make substantial progress in probing the metal abundances of
large samples of star-forming galaxies to cosmologically relevant distances.
With many hundreds of galaxies with measured abundances within each redshift
window, SFACT will be able to deliver very focused views of the metallicities
of galaxies. In particular, we envision constructing luminosity-metallicity
and mass-metallicity relations (LZRs and MZRs, respectively) for each redshift
window. We expect to be able to robustly map out the redshift evolution of the
LZR and MZR to the redshift limit of our data.

Our current round of follow-up spectroscopy should be adequate for providing
abundance estimates for the majority of star-forming galaxies in the H$\alpha$
and [O III]-selected subsamples, where the necessary nebular diagnostic lines
are present in our data. However, spectra that reach to redder wavelengths
will be necessary to yield metallicity estimates for any of the [O
II]-selected ELGs (see §7). With these new red-spectral-range data we expect
to be able to derive metallicities for galaxies detected via the [O II]
doublet discovered in NB1 and NB2. This will push our analysis of the LZRs and
MZRs out to z $\sim$ 0.88. Complete metallicity studies involving the very
faintest of the SFACT objects will likely require additional spectroscopy
using larger telescopes.

###  6.6 Detection of Rare Objects

Figure 11: Spectrum of the cataclysmic variable (CV) star SFF17-NB2-D24012.
This object was previously identified in a catalog of suspected CVs by Drake
et al. (2014). It was detected in our survey due to the strong H$\alpha$
emission located in NB2. Other lines visible in the spectrum include H$\beta$
as well as five Helium lines: He I 4921, 5015, 5876, 6678, 7065. The lines are
double peaked and broad, indicative of emission from a rapidly rotating disk.

One of the more enjoyable and unpredictable aspects of carrying out
astronomical surveys of this type is the fact that one commonly encounters
unusual objects. In extreme cases, one might even discover an entirely new
class of objects. While the SFACT survey is still in its early days, we have
already come across a number of interesting objects. For example, we have
“discovered” a cataclysmic variable (CV) star in our early survey data that
had previously been identified as a suspected CV candidate by Drake et al.
(2014). Its spectrum is shown in Figure 11. More to the point of our ELG
survey, we expect to detect a number of rare and potentially very interesting
objects such as Green Pea galaxies and extremely metal-poor (XMP) dwarf
galaxies.

On face value, our survey method may not appear to be the best approach for
discovering rare objects. As already discussed, a primary drawback of the NB
survey technique is the relatively small volumes covered with each pointing.
The SFACT survey methodology mitigates this problem to some extent by using
somewhat broader filters than have traditionally been used in the past
($\sim$90 Å rather than $\sim$50-60 Å). These broader filters, coupled with
the large FOV of the ODI camera, result in reasonably large volumes for each
redshift window. For example, the effective survey volume for a single field
with the NB1 filter detecting ELGs via the [O III]$\lambda$5007 line (redshift
range of 0.379 to 0.397) is of order 80,000 Mpc3. If one then multiplies this
by the three principle emission lines detected in each pointing, by the three
NB filters currently used by the survey, and by the 50-60 planned survey
fields, the total effective volume of the survey rises to the level of tens of
millions of cubic megaparsecs. Hence, our expectation is that many dozens of
interesting objects will be found during the course of the survey.

We highlight a few categories of rare objects below.

####  6.6.1 Green Peas and Blueberries

Figure 12: Spectrum of a candidate Green Pea galaxy SFF15-NB2-D22777. The
combination of high equivalent width [O III] emission lines and high
luminonsity (Mg = $-$19.3) are typical of the Green Peas.

The Green Pea (Cardamone et al., 2009) and Blueberry (Yang et al., 2017)
galaxies are among the most extreme star-forming galaxies known. Their common
feature is the presence of very strong [O III]$\lambda\lambda$5007,4959
emission. The original samples of both types of galaxies were created using BB
colors that were sensitive to high-equivalent-width [O III] emission. Due to
this selection method, the two sets of galaxies are only detected in limited
redshift ranges that are not overlapping. Currently very little is known about
the redshift evolution of either the Green Pea or Blueberry populations, or
how their properties compare with less extreme star-forming galaxies. Do the
Green Peas and Blueberries form a continuum of extreme objects, or are they
distinctly different types of systems?

Emission-line-selected samples (e.g., Brunker et al., 2020) have shown that
these types of systems can be identified over a much broader range of
redshifts. We expect that the [O III]-selected subsample of the SFACT survey
should be particularly effective at detecting both types of galaxies over an
extended redshift range. SFACT should also be sensitive to lower-redshift
versions in the H$\alpha$-detected portion of the survey. The spectrum of an
example SFACT Green Pea candidate is shown in Figure 12. This galaxy,
SFF15-NB2-D22777, has a redshift of z = 0.3101 and a g-band absolute magnitude
of Mg = $-$19.3. The high luminosity coupled with the large equivalent widths
of the [O III] line (EW5007 $\sim$ 600 Å) are what identify this galaxy as a
Green Pea candidate. As the survey progresses, we expect to build up a large
population of both Green Peas and Blueberries. This will allow us to carry out
a complete study of their demographics and better place them into context with
the broader population of star-forming galaxies.

####  6.6.2 XMP galaxies

Figure 13: Spectrum of the metal-poor dwarf galaxy SFF06-NB2-C4238. It has an
estimated oxygen abundance log(O/H)+12 of 7.15 $\pm$ 0.10.

The discovery and subsequent study of XMP galaxies has a long and interesting
history (e.g., see McQuinn et al., 2020, for a recent review). Due to the
nature of the LZR, the most extremely-low-abundance systems should also be
extremely-low-luminosity, low-mass systems. This has made their detection a
challenge. Despite a focused effort to discover more of these galaxies (e.g.,
see the list of previous surveys mentioned in §1), the number of truly XMP
galaxies (e.g., log(O/H)+12 $<$ 7.3) remains modest.

The discovery of XMP galaxies was not among the list of expected SFACT survey
results when the survey began. The primary reason for this has already been
mentioned in §3.2 and verified in §4.2.2: the effective survey volume for the
H$\alpha$-detected NB2 redshift window is quite small. We expect that this is
the only one of the current survey redshift windows where we would be
sensitive to the very low-luminosity XMP objects. For example, the next-lowest
redshift window is the H$\alpha$-selected NB1 region, which covers the
redshifts z = 0.052 - 0.066. The XMP galaxies located at these redshifts are
likely to be below our detection threshold.

Despite this gloomy outlook, we can report the discovery of at least a few XMP
candidates in the early survey data. We illustrate one example in Figure 13.
The object is SFF06-NB2-C4338 (which is not from one of the three pilot-study
fields). It has a g-band magnitude of 23.05 and a redshift of z = 0.01144
(distance = 53.2 Mpc). The inferred absolute magnitude is Mg = $-$10.6. The
spectrum exhibits an [O III]$\lambda$5007 line that is weaker than H$\beta$,
plus strong H$\alpha$ but no detection of [N II]$\lambda$6583, both
characteristics of XMP systems. A spectrum covering bluer wavelengths (not
shown) reveals a weak detection of the [O II] doublet. This has allowed us to
generate a preliminary estimate of the metal abundance using the R23-O23
method (e.g., McGaugh, 1991) which yields log(O/H)+12 = 7.15 $\pm$ 0.10. More
details, as well as results from other low metallicity objects, will be
presented in forthcoming papers.

##  7 Status and Future Plans

The SFACT survey has been acquiring imaging data with the current set of three
NB filters since the Fall 2018 semester. Despite a significant loss of
observing time due to technical problems, weather, and health safety issues
(i.e., WIYN was closed for most of 2020 due to the Covid-19 pandemic), our
team has been making steady progress toward meeting the overall survey goals
outlined in §3.3. In the current section we summarize the status of our survey
observations. We also highlight future plans and possible new directions for
SFACT.

As of the end of the Spring 2022 semester, we have reached the $\sim$75% level
toward our goal of observing 50-60 fields. The SFACT team has acquired
complete NB and BB imaging for 43 fields. Of these, 24 are Fall fields and 19
are located in the Spring sky. Processing has been completed on 41 of these
fields, resulting in the detection of $\sim$5500 ELG candidates. Our current
analysis efforts are focusing on completing the processing for the remaining
fields for which data exist, in order to initiate spectroscopic follow-up
observations. Our current plans are for the acquisition of imaging survey data
with the three existing NB filters to continue through the 2023 observing
seasons. We expect to reach our target goal for the number of fields observed
at that point.

Naturally, the acquisition of follow-up spectroscopy lags the imaging portion
of the survey. However, for the past several semesters we have utilized
roughly 50% of our observing time to obtain these extremely important data.
Most of our fully-processed fields have at least some follow-up spectroscopy,
ranging between $\sim$25% and $\sim$90% coverage. As time goes on, an
increasing fraction of our observing efforts will shift to spectroscopic
confirmation. However, we already have a solid start on this phase of the
project. For example, there are 17 Fall-semester fields that have been
processed through the stage of ELG candidate selection that have significant
amounts of follow-up spectra. These fields include a total of $\sim$2300 ELG
candidates, of which 55% already possess follow-up spectra.

Two major modifications to our observing procedures are planned for the
future. First, as outlined in §3.2, we plan to add additional NB filters to
the survey. Both of these new filters are located in gaps in the telluric OH
spectrum, providing relatively dark spectral windows suitable for our survey
work. These filters serve three important roles for the survey. First, they
will create additional redshift windows, filling in gaps in our current
redshift coverage. For example, NB812 provides key redshift windows at z
$\sim$ 0.24 (H$\alpha$ detections) and z $\sim$ 0.62 ([O III] detections). See
Figures 1 and 6 for more details. Second, they significantly extend the
redshift coverage over which we will be able to detect star-forming ELGs (up
to as high as z $\sim$ 1.46). Finally, the addition of the NB912 filter allows
for the detection of objects within the same redshift window via both the
H$\alpha$ and [O III]$\lambda$5007 lines. The latter are detected using the
current NB1 filter. This overlapping selection capability will provide the
opportunity to directly compare the ELG populations found in the same volumes
of space via these two distinctly different lines, and to robustly calibrate
the SFRs found for the [O III]-selected galaxies, firmly placing them on the
same scale as the H$\alpha$ SFR measurements.

A second change to our survey methodology that will be implemented in the
coming semesters will be the acquisition of spectra at longer wavelengths. As
seen in Figure 6, the majority of the SFACT ELGs are detected via their [O
III] and [O II] lines. However, our current set of follow-up spectra only
cover the wavelength range of 4700 – 7600 Å. With this spectral coverage we
are not able to observe the important spectral region near H$\alpha$ for the
[O III]-detected SFACT galaxies, missing the important [N
II]$\lambda\lambda$6583,6548 and [S II]$\lambda\lambda$6731,6716 diagnostic
emission lines. The situation is even worse with the [O II]-detected galaxies,
where our current spectral coverage tends to cover only the [O II] doublet
itself plus the [Ne III]$\lambda$3869 line. Once our first round of follow-up
spectroscopy is complete, we plan to pursue a second round for our [O III]-
and [O II]-detected ELGs. Naturally, once we start observing SFACT fields
using the proposed new NB812 and NB912 filters we will need to observe further
into the red simply to obtain verification spectra. Hence we expect to carry
out the second-pass red spectroscopic campaign simultaneously with the first-
pass follow-up spectra for objects detected in the new longer-wavelength
filters.

##  8 Summary & Conclusions

We present a description of the new NB imaging survey SFACT: Star Formation
Across Cosmic Time. SFACT is a long-term program being carried out on the WIYN
3.5 m telescope using both the ODI wide-field imaging camera and the Hydra
multi-fiber positioner for spectroscopic follow-up. In addition, we present
preliminary results from newly discovered SFACT objects detected in three
pilot-study fields.

The imaging portion of the survey utilizes the ODI camera, which has a field-
of-view of 48 $\times$ 40 arcmin ($\sim$0.53 deg2). Currently, three specially
designed NB filters are used to detect ELGs at a range of redshifts up to z
$\sim$ 1.0 and QSOs to z $\sim$ 5.2. Future expansion of the survey by adding
additional NB filters is planned. In addition to the NB images, survey fields
are imaged through gri BB filters. The latter images provide deep calibrated
photometry as well as providing the necessary continuum subtraction for our NB
data. Our overall survey plan calls for observing between 50 and 60 survey
fields, which will result in the detection of on the order of 1000 ELGs in
each redshift window covered by the survey.

The three pilot-study fields yielded a total of 533 emission-line sources from
the imaging survey. This represents a surface density of 355 objects deg-2,
which is entirely consistent with the expectations based on our projections
for the survey depth. The median r-band magnitude of the SFACT sources is
22.51, and the faintest objects have r $\sim$ 25\. The median NB line flux
measured for our ELG candidates is 2.97 $\times$ 10-16 erg s-1 cm-2, and the
limiting flux is 1.01 $\times$ 10-16 erg s-1 cm-2. We find a good distribution
of detections between the three NB survey filters. Overall we interpret the
results from the pilot study as a strong verification of the validity of our
survey method.

The second component of the SFACT survey entails the acquisition of spectra of
the ELG candidates. These spectra are necessary for confirming the candidate
ELGs detected in the imaging data, for determining which line was present in
the NB filter, and for determining redshifts which in turn allows us to derive
important physical properties of our sample galaxies. While the spectroscopic
follow-up naturally must lag the imaging portion of the survey, we have
nonetheless already acquired substantial amounts of spectra in the early days
of the survey. For the pilot-study fields, 453 out of the 533 ELG candidates
(85.0%) have follow-up spectra. Of these, 415 are confirmed to be bona fide
emission-line objects (91.6%). We use the results of the spectroscopy to
determine the redshift distribution of the SFACT galaxies, and present
preliminary looks at the luminosity and SFR distributions of our sample.

Two companion papers present our initial survey catalogs of extragalactic
emission-line objects as well as the available follow-up spectroscopy for
these sources. Sieben et al. (2023, SFACT2) describes the ODI imaging
observations of our three pilot-study fields, details the data processing and
analysis steps applied to the data to yield our survey lists, and presents the
first three lists of SFACT ELGs. Numerous examples of newly discovered ELG
candidates are shown to illustrate the range of objects discovered in SFACT.
Carr et al. (2023a, SFACT3) presents the results of our spectroscopic follow-
up of the objects from the pilot-study fields. After detailing our observing
methods, data-processing steps, and emission-line-measurement procedures, the
paper tabulates all of the relevant spectral information. Many example
spectral plots are used to illustrate the types of ELGs detected in SFACT.

As described above, we have already acquired imaging and spectra data for a
substantial number of additional survey fields. We expect that we will be
publishing survey lists and follow-up spectroscopy at regular intervals. We
are also open to sharing our survey lists prior to publication. Colleagues who
would like to work with our deep survey data are encouraged to contact us.

We gratefully acknowledge the longterm financial support provided by the
College of Arts and Sciences at Indiana University for the operation of the
WIYN Observatory. Additional funds have been provided by the Department of
Astronomy and the Office of the Vice Provost for Research at Indiana
University to help support this project. The authors express their
appreciation to the anonymous referee who made a number of insightful
suggestions that improved the quality of this paper. The entire SFACT team
wishes to thank the entire staff of the WIYN Observatory, whose dedication and
hard work have made this survey possible. In particular, we acknowledge the
contributions of Daniel Harbeck, Wilson Liu, Susan Ridgeway, and Jayadev
Rajagopal. We also thank Ralf Kotulla (U. Wisconsin) for his development and
continued support of the ODI image processing software (QuickReduce), and
Arvid Gopu and Michael Young (Indiana U) for their support of the ODI
Pipeline, Portal & Archive. Finally, we acknowledge the contributions made at
various stages of this project by students in the Department of Astronomy at
Indiana University who assisted with the data processing: Bryce Cousins,
Anjali Dziarski, Sean Strunk, and John Theising.

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