# Gems of the Galaxy Zoos - a Wide-Ranging _Hubble Space Telescope_ Gap-Filler
Program111This research is based on observations made with the NASA/ESA Hubble
Space Telescope obtained from the Space Telescope Science Institute, which is
operated by the Association of Universities for Research in Astronomy, Inc.,
under NASA contract NAS 5-26555. These observations are associated with
program 15445.

William C. Keel Department of Physics and Astronomy, University of Alabama,
Box 870324, Tuscaloosa, AL 35487  Jean Tate  Private address222Deceased 6
November 2020. Jean Tate was among the most active volunteers in Galaxy Zoo
and Radio Galaxy Zoo, and oversaw virtually the entire selection process for
Radio Galaxy Zoo targets in this project. Jean set a high standard for the
professional scientists on the project, maintaining online material in such
good order that we were able to continue to retrieve and understand even work
left in progress. O. Ivy Wong CSIRO Space & Astronomy, PO Box 1130, Bentley,
WA 6102, Australia  ICRAR-M468, University of Western Australia, Crawley, WA
6009, Australia  Julie K. Banfield Research School of Astronomy and
Astrophysics, Australian National University, Canberra, ACT 2611, Australia
Chris J. Lintott Department of Physics, University of Oxford, Denys Wilkinson
Building, Keble Road, Oxford OX1 3RH, UK  Karen L. Masters Departments of
Physics and Astronomy, Haverford College, 370 Lancaster Ave, Haverford, PA
19041  Brooke D. Simmons Physics, Lancaster University, Lancaster LA1 4YB, UK
Claudia Scarlata School of Physics & Astronomy, University of Minnesota, 116
Church Street S.E., Minneapolis, MN 55455  Carolin Cardamone Center for the
Enhancement of Learning and Teaching, Tufts University, 108 Bromfield Road,
Somerville, MA 02144  Rebecca Smethurst Department of Physics, University of
Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK  Lucy Fortson
School of Physics & Astronomy, University of Minnesota, 116 Church Street
S.E., Minneapolis, MN 55455  Jesse Shanahan  Center for Astrophysics and Space
Sciences (CASS), Department of Physics, University of California, San Diego,
CA 92093, USA  Sandor Kruk European Space Agency, ESTEC, Keplerlaan 1, NL-2201
AZ Noordwijk, the Netherlands  Max-Planck-Institut für extraterrestrische
Physik (MPE), Giessenbachstraße 1, D-85748 Garching bei München, Germany”
Izzy L. Garland  Physics, Lancaster University, Lancaster LA1 4YB, UK  Colin
Hancock  Department of Physics and Astronomy, University of Alabama, Box
870324, Tuscaloosa, AL 35487  David O’Ryan  Physics, Lancaster University,
Lancaster LA1 4YB, UK

(Received June 1, 2019; Revised January 10, 2019; Accepted )

###### Abstract

We describe the Gems of the Galaxy Zoos (Zoo Gems) project, a gap-filler
project using short windows in the Hubble Space Telescope’s schedule. As with
previous snapshot programs, targets are taken from a pool based on position;
we combine objects selected by volunteers in both the Galaxy Zoo and Radio
Galaxy Zoo citizen-science projects. Zoo Gems uses exposures with the Advanced
Camera for Surveys (ACS) to address a broad range of topics in galaxy
morphology, interstellar-medium content, host galaxies of active galactic
nuclei, and galaxy evolution. Science cases include studying galaxy
interactions, backlit dust in galaxies, post-starburst systems, rings and
peculiar spiral patterns, outliers from the usual color-morphology relation,
Green Pea compact starburst systems, double radio sources with spiral host
galaxies, and extended emission-line regions around active galactic nuclei.
For many of these science categories, final selection of targets from a larger
list used public input via a voting process. Highlights to date include the
prevalence of tightly-wound spiral structure in blue, apparently early-type
galaxies, a nearly complete Einstein ring from a group lens, redder components
at lower surface brightness surrounding compact Green Pea starbursts, and
high-probability examples of spiral galaxies hosting large double radio
sources.

AGN host galaxies (2017) — Galaxy collisions (585) — Starburst galaxies (1570)
— Radio galaxies (1343) — Ring galaxies (1400)

††journal: AJ††facilities: HST(ACS), Lick(Shane)

##  1 Introduction

Astronomy enjoys a rich history of knowledge gained from objects at the
extremes of sample properties, and outliers to common correlations. Our
experience with spinoff studies from the Galaxy Zoo projects has certainly
borne this out, leading to further observation of rare and unusual galaxies
which in turn yielded insight in a range of questions in galaxy evolution.
This paper describes one such project, delivering _Hubble Space Telescope_
(HST) images of galaxies randomly selected from a list chosen for science
value in a number of contexts.

Galaxy Zoo has encompassed several iterations of classification based on
volunteer examination of galaxies in digital sky surveys. Initially, “classic”
Galaxy Zoo (Lintott et al., 2008) provided broad morphological information
(spiral/elliptical/merging, and direction of spiral arms) for over 900,000
galaxies from data release 7 (DR7, Abazajian et al. 2009) of the Sloan Digital
Sky Survey (SDSS; York et al. 2000). Galaxy Zoo 2 (Willett et al., 2013) built
on the demonstrated ability of volunteers to consistently provide finer-
grained morphological information, now working with about 250,000 of the
brightest SDSS galaxies. The approach was extended, broadening the decision
tree to encompass clumpy galaxies, to deep optical HST fields in Galaxy Zoo
Hubble (Willett et al., 2017) and the near-IR CANDELS data (Simmons et al.,
2017), and most recently images from the Legacy Survey (Dey et al., 2019). The
results of these studies led to recognition of the importance of blue early-
type galaxies (blue ellipticals for short; Schawinski et al. 2009a) and red
spiral galaxies (Masters et al. 2010, Bamford et al. 2009); Masters et al.
(2020) summarizes the first twelve years of Galaxy Zoo results. It quickly
became clear that the project discussion
forum333https://www.galaxyzooforum.org/, with content frozen 9 July 2014,
where volunteers could ask questions and exchange comments about galaxy
images, was drawing attention to very rare phenomena, leading to the
identification of Green Pea compact starburst systems (Cardamone et al.,
2009), nearly 2000 pairs of galaxies with overlapping images for dust analysis
(Keel et al., 2013), and giant extended emission-line regions (EELRs) around
active galactic nuclei (AGN), many of which are so luminous as to suggest that
the central AGN must have faded within the relevant light-travel time (Lintott
et al. 2009, Keel et al. 2012a). Beyond these, numerous other galaxy images of
special interest have been brought up for discussion on the Forum and its
successor in the project’s Talk interface444https://talk.galaxyzoo.org/ until
April 2019, discussion moved to
https://www.zooniverse.org/projects/zookeeper/galaxy-zoo/talk thereafter.,
providing ready sets of objects for followup observation. As this became
clear, team members could call for specific kinds of objects, and were often
answered with great energy by volunteers. Beyond the primary statistical goals
of the various iterations of the Galaxy Zoo public-participation project, some
of its hundreds of thousands of volunteer participants have identified rare
and unusual galaxies for which further data would be particularly interesting.

The more recent launch of Radio Galaxy Zoo (Banfield et al., 2015), in which
participants examine optical and near-infrared images in concert with radio
data, likewise makes use of the Talk interface for exchange of more detailed
information, particularly on the rare radio galaxies with possibly spiral host
morphology and on active galactic nuclei (AGN) with extensive emission-line
clouds. For many of the objects, their nature would become much clearer with
higher-resolution optical images than the SDSS data used for the initial
rounds of Galaxy Zoo classifications, and numerous specific science goals
could be addressed with even a modest set of such followup images.

Galaxy Zoo team members had long joked about the appropriate followup
observing proposal being “We have a bunch of weird galaxies, and need a closer
look to understand them better”. This was essentially what the 2017 STScI gap-
filler opportunity offered. We describe in this paper the resulting program,
“Gems of the Galaxy Zoos” (Zoo Gems for short, program 15445), which has
provided HST images relevant to a wide range of science cases drawn from
Galaxy Zoo and Radio Galaxy Zoo. In this paper, we describe these aims, detail
how we incorporated public input in selecting the target lists for many of the
science cases we could address, document the setup of the observations, and
present some initial results. While many of the results of Zoo Gems will
appear in further papers, we think it useful to provide here the common
background and rationale of the observations.

##  2 Science cases

This section sets out the science rationales for various object categories,
organized into broad morphological themes. Some categories have had only a
single example observed at this point.

###  2.1 Galaxy disks

####  2.1.1 Galaxy Zoo: unusual spirals

This category furnishes a catchall for spiral galaxies - 3-armed grand-design
systems, galaxies with very asymmetric patterns but no obvious interacting
companion, or dominant resonance ring structures. The 3-armed spirals took on
particular interest with the finding from Galaxy Zoo 2 classifications that
they are not more common in low-density environments, as had been expected
from relative growth properties of various Fourier modes (Elmegreen et al.
1992, Durbala et al. 2009), and that bars are just as common in these as in
the 2-armed examples where the 180∘ symmetry of bar and arms matches (Hancock,
2019).

####  2.1.2 Galaxy Zoo: nuclear disks and bars in spirals

High-resolution images have shown some spiral galaxies, especially those with
bars and outer rings, to have analogous circumnuclear structures - known as
nuclear disks. We included galaxies seen (or very likely) to have unusually
large central disks within bars, or central bars misaligned with the outer
structures.

####  2.1.3 Galaxy Zoo: backlit galaxies

Noninteracting galaxy pairs whose images overlap in projection offer a way to
study dust attenuation independent of dust temperature or structure of the
galaxy, and subject to completely different systematics than methods relying
on IR emission or modeling of the galaxy’s spectral-energy distribution (SED).
This approach has been applied to very limited sets of galaxies, using ground-
based data by White & Keel (1992), Berlind et al. (1997), and White et al.
(2000), and with the improved angular resolution of HST imaging by Elmegreen
et al. (2000) Keel & White (2001), Keel & White (2001) and Holwerda et al.
(2009). Applicability of this technique remained limited by the very modest
number of suitable backlit galaxies then known, a situation which was
dramatically improved by the sensitivity and (especially) dynamic range of
SDSS data. Galaxy Zoo participants provided an extensive finding list of
candidate pairs, for a catalog of nearly 2000 such pairs after validation from
their initial examination of DSSS DR7 images alone (Keel et al., 2013). This
list has been supplemented by the second pass through DR7 images during Galaxy
Zoo 2. Similar pairs were also noted during Hubble Zoo, but not considered
here since Zoo Gems images would not improve their data quality. The ideal
pair would have galaxies with redshifts so different as to rule out physical
association, containing a smooth early-type galaxy behind a relatively
symmetric spiral. In practice these criteria can be relaxed, for example to
include pairs with particular geometries or evidence for very distant
attenuation regions, as long as the image information is properly used to
estimate uncertainties due to departures from symmetry.

###  2.2 Starbursts and star-forming regions

####  2.2.1 Galaxy Zoo: Green Peas

Green Pea systems, as described by Cardamone et al. (2009), were initially
identified as a class by Galaxy Zoo volunteers. The name arises from their
combination of small size (SDSS Petrosian radius $petrorad\\_r<2.0$″) and
green appearance in SDSS $gri$ composite images, arising from a strong
emission line in the $r$ band (refined to $ugriz$ color criteria by Cardamone
et al. 2009). The dominant emission line for these “green” objects is
redshifted [O III] $\lambda 5007$. Inspection of the SDSS spectra showed most
of these to be star-forming systems; their small angular sizes and redshifts
meant these are therefore among the most compact star-forming galaxies, and
the large emission-line equivalent widths responsible for their color
selection mark them firmly as starburst systems. Only a handful of Green Peas
appeared serendipitously in previous HST imagery, so we incorporated Peas into
the Zoo Gems list to enable a more systematic study of their structures. In
particular, we selected filters that emphasized the stellar continuum, for a
better view of the structure of the galaxies themselves, and included systems
in three redshift slices. Improved measurements or limits on the sizes of the
stellar structures would lead to better understanding of how intense the
starbursts are, and to what extent these extreme star-forming regions reside
in systems with previous histories of star formation, or show evidence of
tidal disturbance which could trigger these extreme starbursts. These red ACS
WFC passbands also provide a contrast with the UV bands previously observed
for some Green Pea samples using HST, which represent the young starburst
populations well but not any older stellar components.

The Galaxy Zoo sample of Green Peas has generated extensive followup work as
sources of Lyman-continuum leakage (Yang et al. 2017, Malkan & Malkan 2021),
Lyman $\alpha$ emission sources (Orlitová et al., 2018), extreme starbursts
driving global winds (Jaskot et al. 2017, Bosch et al. 2019, Hogarth et al.
2020), and testbeds for chemical evolution scenarios at high star-formation
rates (Hawley 2012, Amorín et al. 2012). These compact, intense starbursts are
likely related to the less massive “Little Blue Spheroids” (Moffett et al.,
2019).

####  2.2.2 Galaxy Zoo: Poststarbursts

Spectroscopically-selected post-starburst galaxies555As it happened, all of
the galaxies in this input list were affected by an initial bug in SDSS DR7
redshifts, which applied to galaxies having strong Balmer absorption lines and
gave erroneously high redshifts when the gaps between Balmer absorption lines
were matched to broad emission features. This was quickly corrected in the
SDSS pipeline, before the release of DR8, and did not affect our sample
construction. (using the combination of H$\delta$ in absorption with
equivalent width $>3$ Å and H$\alpha$ emission undetected at the $4\sigma$
level) show central concentrations (SDSS _fracdev_ parameter) intermediate
between disk and early-type galaxies (Wong et al., 2012). This could reflect
genuine morphological transformation or preferential scales for the starburst,
both issues which higher-resolution images could shed light on. This selection
on specific absorption line properties is distinct from the color/luminosity
selection often defining the “Green Valley”, being influenced by star-
formation events which are more localized or involve smaller gas masses, but
the possibility of post-starburst systems being seen during morphological
transformation does parallel the inferences about galaxies in the Green Valley
largely undergoing a one-time morphological change (Mendez et al. 2011,
Schawinski et al. 2014, Smethurst et al. 2015, Kelvin et al. 2018). In
addition, the fading starbursts are often concentrated in knots, which are
often well-detected in snapshot observations, allowing broad comparison of the
starburst properties. These properties in combination could suggest to what
extent processes in local post-starburst galaxies are, or are not, useful
analogs for the quenching of star formation in the galaxy population more
generally.

####  2.2.3 Galaxy Zoo: blue ellipticals

One of the earliest Galaxy Zoo science results was the existence of galaxies
robustly classified as early-type, with colors much bluer than the usual red
sequence (Schawinski et al., 2009a), indicating an unusual level of recent
star formation or contamination by the light of an AGN. High-resolution images
can trace the distribution of star-forming regions, indicating whether they
form disks, rings, or the kinds of asymmetric patches that could be infalling
star-forming regions. We tracked three separate subcategories among blue
ellipticals (selected using a color cut based on the red sequence track, as in
Schawinski et al. 2009a), so the final list included the highest-ranked
examples of blue early-type galaxies with emission-line ratios from SDSS
spectra indicating that AGN or star formation is dominant (5 and 4 targets,
respectively), and star-forming examples with detected CO or H I emission (3
targets; Schawinski et al. 2009b, Wong et al. 2015).

####  2.2.4 Galaxy Zoo: Red spirals

Analogous to blue ellipticals, Galaxy Zoo classifications led to
identification of a population of red spiral galaxies (Masters et al., 2010),
defined by color offset from typical spirals at a given luminosity. While
present in all environments, they are most abundant in the high-density
regions just outside cluster cores (Bamford et al., 2009). Galaxies for voting
were selected to be nearly face-on, so the red color is not due solely to
attenuation in the disk. The HST images could show whether the star-forming
regions in these systems are of unusually low luminosity or unusually sparse
compared to spirals of typical colors, and address the incidence of bars at
small scales for comparison with the global bars which are common in these
systems (Masters et al. 2011, Kruk et al. 2018). The properties of red spirals
may give clues to the processes quenching star formation when seen
independently of morphological transformations.

####  2.2.5 Galaxy Zoo: luminous star-forming clumps in galaxies

While rare in the local Universe, these may be helpful analogs to the luminous
star-forming clumps which are ubiquitous in the high-redshift galaxy
population (Cowie, Hu, & Songaila 1995, Elmegreen, Elmegreen, & Sheets 2004).
The relative handful of nearby analogous objects will be much better resolved,
providing information on scales of star formation (and sometimes the
properties of the most luminous clusters in these regions).

###  2.3 Interacting and merging galaxies

####  2.3.1 Galaxy Zoo: mergers which are very distorted or have very
long/luminous tails

Among the many interacting and merging systems flagged by Galaxy Zoo (Darg et
al., 2010), some stand out even in such exceptional company as having tidal
tails of unusual length or brightness, or main galaxy bodies which are
unusually distorted. We include some of these in the target list, to sample
the properties of galaxy interactions which produce such extreme stages.
Outcomes might include populations of luminous star clusters, morphological
information on scales beyond the SDSS resolution limit, and the role of dust
attenuation (which can change our interpretation of a system’s components and
their spatial relationships).

####  2.3.2 Galaxy Zoo: collisional and polar rings (including possible
lenses)

Numerous candidates for these rare interaction signatures were noted by Galaxy
Zoo participants. They offer particular insight into not only the prevalence
of these kinds of galaxy encounters, but such disparate issues as the shapes
of dark-matter halos, properties of star formation with time mapped to
location, and the degree of self-gravitation in polar rings ( Reshetnikov &
Combes 1994, Bizyaev et al. 2007, Egorov & Moiseev 2019). The top-voted polar-
ring candidate was earlier catalogued as PRC A-1 in the catalog by Whitmore et
al. (1990), supporting the identification of this system as hosting a classic
polar-ring structure. This category also includes objects which may turn out
to be gravitationally lensed arcs when examined at high angular resolution, as
indeed happened in one spectacular and near-complete Einstein ring (section
5.5).

####  2.3.3 Galaxy Zoo: Red/blue pairs

Particularly in the earliest examination of Legacy Survey images (Dey et al.,
2019), participants have found a small category of close pairs of marginally
resolved images with very different colors. At first we suspected these might
be star/galaxy superpositions, but one Zoo Gems image shows at least some to
be interesting galaxy interactions, of the general kind long discussed as
mixed-morphology pairs (Rampazzo & Sulentic 1992, Hernández Toledo et al.
1999) with contrasting morphologies, colors, and star-forming properties.

####  2.3.4 Galaxy Zoo: regrowing disks

The “merger hypothesis” for making elliptical galaxies by merging disk systems
(Toomre, 1977) was largely suggested by the tendency for disk mergers in
simulations to yield elliptical-like, diskless remnants for mass ratios near
unity (in practice, a cutoff near 3:1 has often been taken to give the right
fraction of mergers, if not always the right outcomes in individual cases when
such parameters as approach geometry and gas fraction are added to the mass
ratio). However, as merger simulations achieve higher fidelity and are run for
more combinations of the numerous initial conditions, some mergers of near-
equal mass galaxies are shown to retain disks afterward (Barnes & Hernquist
1996, Governato et al. 2009, Hopkins et al. 2009). In parallel, we have
noticed a category of interacting-galaxy pairs which show single disks and
spiral patterns surrounding two distinct bulges, which could represent this
process in action (and existence of a remnant disk before the nuclei merge).
We included all these systems in the Zoo Gems object list, to give the
possibility of empirical information on the survival or re-formation of disks
in major mergers. Such events may be important in producing the population of
high-luminosity “super spiral galaxies” identified by Ogle et al. (2016) and
Ogle et al. (2019). They note that a significant fraction of these gigantic
disk systems show signs of at least minor meters; retaining or regrowing disks
after some major mergers would make it easier to understand the existence of
disk-dominated systems at the highest galaxy luminosities.

###  2.4 Active galactic nuclei and their host galaxies

####  2.4.1 Galaxy Zoo: EELRs

Galaxy Zoo participants have proven to be adept at identifying candidates for
extended emission-line regions (EELRs) associated with AGN, based on the
distinctive colors in the SDSS _gri_ composite images produced by strong [O
III] emission appearing in $g$ or $r$ at different redshifts (Lintott et al.
2009, Keel et al. 2012a). The distance of these clouds from the AGN, and their
luminosity compared to what we see from the AGN directly, constrain both the
duty cycle of rapid accretion and characteristic duration of accretion
episodes. Fine structure in these clouds strengthens constraints on the
required ionizing luminosity of the associated AGN (Keel et al. 2012b, Keel et
al. 2017), motivating us to include the strongest EELR candidates in filters
matching the SDSS detection bands, even in the absence of separate HST
continuum data.

####  2.4.2 Radio Galaxy Zoo: EELRs

As in Galaxy Zoo, Radio Galaxy Zoo participants identified many galaxies, or
galaxy components, with such extreme colors that there are almost certainly
strong, resolved emission-line clouds (some tagged as “RGZ Green” objects,
since many are at redshifts $z\approx 0.25$ where [O III] emission appears in
the $r$ band, mapped to green in the SDSS and analogous color-composite
displays). For radio-loud AGN, in addition to photoionization, emerging jets
add the possibility of shock ionization on large scales. Here again, we
specified filters closely matching the detection band from Radio Galaxy Zoo.
The input list for voting incorporated both SDSS colors of the host and
spatially resolved structure in the $r$ filter.

####  2.4.3 Radio Galaxy Zoo: SDRAGNs

For convenience, we follow Leahy (1993) in using DRAGNS (Double-lobed Radio
sources Associated with Galactic Nuclei) to describe typical double-lobed
radio galaxies. A long-known property of the population of galaxies hosting
DRAGNs is that they are overwhelmingly elliptical galaxies or merger remnants.
However, detailed study has revealed a handful of galaxies, associated at high
probability with DRAGNs, with clear spiral structure (Ledlow et al. 2001, Keel
et al. 2006, Hota et al. 2011, Mao et al. 2015, Mulcahy et al. 2016) We will
call these rare DRAGNs with spiral host galaxies SDRAGNs. We concentrate on
these because they go against the dominant correlation of luminous double
sources with elliptical or post-merger host galaxies, offering the possibility
of a way to understand which host-galaxy properties drive (or allow)
production of large-scale double sources. The initial list was selected based
on location of the putative host galaxy with respect to the radio lobes, and
evidence of a disk (a large exponential-disk fraction in the SDSS image
analysis or from a full 2-dimensional fit to the SDSS images using GALFIT
(Peng et al., 2002)).

The angular resolution of HST imaging can make the morphology of these
galaxies, many as distant as $z\approx 0.2$, much clearer, resolving spiral
arms, dust lanes, and star-forming regions, allowing a much more confident
morphological assessment than the photometric bulge-disk decompositions that
were our starting points. By the same token, the bulge properties will also be
much better determined, giving an improved understanding of likely black-hole
masses and evolutionary paths of these galaxies. For those radio sources which
actually arise in a more distant galaxy than the spiral observed, improved
astrometry with the HST data can identify such false associations.

###  2.5 Galaxy Zoo: unusual bulges

This category includes bulges which are unusually shaped, unusually prominent
for the galaxy’s morphological type, or appear prolate with respect to the
disk orientation. The latter may indicate that the disk is more like a polar
ring, resulting from material starting in a second galaxy. An important
subtype is formed by so-called X-bulges (extreme versions of “peanut bulges”)
which have attracted attention as being edge-on views of bars, allowing us to
visualize the distribution of stars along a direction which remains unseen in
images of more face-on systems (Bureau & Freeman 1999, Kruk et al. 2019).

##  3 Final target list

Our original proposal estimated 1100 targets available. The program was
allocated 300 targets for entry into the whole gap-filler coordinate
pool666One later became impossible to schedule after tighter restrictions
became necessary on guide-star flux., so much of the time available for
detailed preparation went into winnowing the target list. Sparse categories
(in practice, those with fewer than 10 examples) were carried along “as is”.

Because of the public-participation nature of the Galaxy Zoo projects, and
further encouraged by comments from STScI, we solicited public input to select
the objects to be listed from categories with a large enough number of
galaxies. After members of the science team inspected them for suitability, we
set up a web-based voting system, and advertised this opportunity widely
through each project’s Web and social-media presence, and coordinated with the
STScI social-media team for announcement through their accounts as well. We
used the Zooniverse Project Builder interface777https://www.zooniverse.org/lab
(Trouille et al., 2019), producing parallel selection interfaces for the
Galaxy Zoo and Radio Galaxy Zoo subsets. The voting was open from 2-18
February 2018, to meet the 28 February deadline for submission of the Phase 2
proposal with target names, coordinates, and observation details. The Galaxy
Zoo categories drew 6199 votes among 292 galaxies (mean 21.2 per object),
while the Radio Galaxy Zoo objects attracted 9730 votes among 286 galaxies
(mean 34.0 each). Numbers of votes per object varied as users did different
numbers per session. The Project Builder interface was set up so that
volunteers would cycle through all members of each science category in
sequence, with the option to see a montage of available objects in that
category to allow a more informed comparative ranking. As illustrated in Fig.
1, each was presented with a question of the form “What priority would you
give this galaxy with an unusual spiral pattern for possible Hubble Space
Telescope observations?” and possible answers lowest priority, low, medium
priority, high, or highest priority.

Figure 1: Screen capture of the voting interface for Galaxy Zoo targets.
Clicking on “Need some help with this task?” displayed a longer description
and a montage of all galaxies in the same voting category. The red banner at
upper left indicates that a user has already seen this object.

In collating votes, we could distinguish between users who signed in with a
Zooniverse account and those participating anonymously. For the former,
registered users, we could recognize multiple votes for a single object, and
counted only the last one. We examined the anonymous votes for any evidence of
such misbehavior as robotic software packing votes, which could affect the
outcome. There were very few anonymous votes (180 for Galaxy Zoo, 247 for
Radio Galaxy Zoo), widely spread across galaxies, so we saw no evidence of
problematic behavior and included these votes in our rankings. Within each
science category being voted on, we ranked objects using a straightforward
mean of votes weighted by priority. Users were given a 5-point scale to select
on seeing each galaxy image; we ranked on the mean with highest priority=1,
lowest priority=5. The highest-ranked objects in each science category were
carried into the final target list . (The number of slots allocated for each
science category was arbitrarily set by the PI, attempting to reflect the
number of input objects and scientific interest).

The preliminary target list was shared for discussion with the Galaxy Zoo
community on the Talk interface. Volunteers identified some duplicates,
because objects could enter in different science categories by different
names. We also coordinated our target list with that of gap-filler program
15446 (Arp and Arp-Madore interacting galaxies, P.I. Julianne Dalcanton), to
eliminate duplications while ensuring that some especially interesting systems
were on one list or the other.

For Green Peas, whose unresolved SDSS images made visual selection
superfluous, the input list consisted of SDSS DR12 objects with secure matches
to the Portsmouth spectral fitting catalog (Thomas et al., 2013), and BPT type
“Star Forming” from that catalog. Further selection was for the brightest
objects in each of three redshift ranges where ACS filters (mostly) isolate
continuum.

Figure 2: Emission-line contribution in the F555W, F775W, and F850LP passbands
for a typical Green Pea spectrum, SDSS J131036.73+214817.0 at $z=0.2832$,
evaluated at various redshifts, showing the range near $z=0.24$ where both the
very broad F555W and F850LP filters have $<10$% contribution from emission
lines, and the very deep minimum in the F775W filter from $z=0.32-0.36$ where
the contamination from line emission is well below 1%.

Our category for each targeted object was entered in the phase 2 proposal
file888Available at https://www.stsci.edu/hst/phase2-public/15445.pdf under
the “Comments” field.

Some of the flavor of the voting outcomes can be seen in Fig. 3, showing the
top-ranked galaxies in four categories with numerous objects examined.

Figure 3: Montage of SDSS $gri$ images of galaxies highest-ranked in the
voting among four large categories. Object names are shown for those which
have been observed at the time of submission.

Table 1 lists the number of target objects in each category, and the number
observed as of submission of this paper. Only primary categories are given,
although some fit in multiple categories. For example, all five of the Galaxy
Zoo EELR systems occur in interacting galaxies, and one Radio Galaxy Zoo lens
candidate lies in the same ACS field as the Galaxy Zoo EELR candidate SDSS
J160646.74+565139.2. For categories where targets were selected by voting, the
table also lists the number of objects voted on in each category and the
number of votes received. “Number input” is the number of targets from that
category included in the final observing list, and ”Observed” is the number
actually observed to date.

Table 1: Target and Filter selection by Object Categories Category | Number voted | Votes | Number Input | Observed | Filter | Goal
---|---|---|---|---|---|---
Galaxy Zoo ring systems | 26 | 799 | 15 | 6 | F606W | Overall structure
Galaxy Zoo EELRs | — | — | 5 | 1 | F475W | Line emission
Galaxy Zoo regrowing disks | — | — | 7 | 6 | F606W | Overall structure
Galaxy Zoo bulges | — | — | 9 | 5 | F814W | Old populations
Galaxy Zoo interacting systems | 15 | 515 | 10 | 7 | 606W | Overall structure
Galaxy Zoo red/blue pairs | — | — | 5 | 2 | F606W | Overall structure
Galaxy Zoo star-forming clumps | 36 | 904 | 7 | 1 | F606W | Overall structure
Galaxy Zoo spiral patterns | 17 | 490 | 7 | 4 | F475W | Young populations
Galaxy Zoo poststarbursts | 56 | 1396 | 20 | 5 | F475W | Young clusters
Galaxy Zoo overlapping pairs | 63 | 1566 | 21 | 7 | F606W | Dust structure
Galaxy Zoo nuclear disks/bars | — | — | 8 | 4 | F814W | Old populations
Galaxy Zoo blue ellipticals | 61 | 1569 | 12 | 6 | F475W | Spiral structure
Galaxy Zoo red spirals | 19 | 517 | 5 | 1 | F606W | Overall structure
Galaxy Zoo Green Peas: all | — | — | 74 | 38 |  | Stellar continuum
Green Peas: single filter | — | — | 47 | 23 | F775W | Stellar continuum
Green Peas: 2 filters | — | — | 27 | 15 | F555W+F850LP | Stellar continuum
Radio Galaxy Zoo SDRAGNs | 214 | 7110 | 65 | 32 | F475W | Spiral structure
Radio Galaxy Zoo EELRs | 72 | 2620 | 36 | 18 | F625W | Line emission

##  4 Observation setup

The gap-filler proposal category originated with the realization within STScI
that there were schedule gaps too short for typical snapshot programs, so that
the observational output of HST could be further enhanced by programs with
large target lists, ideally spread around the sky or at least around all right
ascensions, which could make effective use of short observation windows. This
rationale, and the review process for gap-filler programs, are described by
MacKenty (2017). Use of the Wide-Field Camera (WFC) mode of the Advanced
Camera for Surveys (ACS; Ford et al. 1998) was mandated for gap-filler
observations, because of its larger field of view than the Wide Field Camera 3
(WFC3), and to minimize use of a moving mirror in WFC3 identified as a
potential failure mode.

An internal STScI pilot project (program 14840) observed bright NGC galaxies,
using pairs of 337-second exposures with short dither motions in between. All
three gap-filler programs finally scheduled use these same exposure times,
since the pilot program had demonstrated that there were a significant number
of schedule gaps which could accommodate this sequence after guide-star
acquisition.

Having settled on the exposure strategy for each object, the available choices
were filter, dither strategy, target location in the ACS field, and (in a few
cases among Green Peas) whether to do 2 dithered exposures or one exposure
each in two filters. The filter selection followed the science goal - if we
were interested mostly in spiral structure, the bluer F475W (roughly SDSS $g$)
filter was chosen to enhance its contrast. If the goal was bulge structure,
its signal-to-noise ratio would be best in the F814W filter. When the goal was
emission-line structure already identified in the SDSS $r$ filter, we used the
closely-matching F625W filter. The filter choices and rationale for each
object category are listed in Table 1.

For Green Pea systems, our aim was to study the continuum structure, using
filters dominated by the stellar population rather than ionized gas. We were
guided by the results of numerically redshifting a typical Pea spectrum and
folding through the system response for several filters (Fig. 2) . For 35
Green Peas in the redshift range $z=0.32-0.36$, we used the F775W filter, with
emission-line fraction $<0.3$%. For Green Peas near $z=0.15$ (12 objects), we
used F850LP where that is dominated by continuum, while for the 27 targets
near $z=0.25$, we took single exposures in F555W and F850LP which are each
mostly continuum, tolerating the numerous cosmic-ray events since the targets
are small. Except for this small number of two-filter Green Pea observations,
we used a common two-point dither pattern designed to fill the chip gap
(albeit with a central band of cosmic-ray features resulting from coverage
with only one exposure in this area).

Target locations on the ACS chips were set in view of the charge-transfer
trailing occurring in these CCDs. While the effects can be substantially
reduced with the pixel-based algorithm, essentially a deconvolution, by
Anderson & Bedin (2010), the effect is reduced in the first place if the
number of charge transfers can be minimized by placing the target close to the
readout amplifier. Since orientation had to be unconstrained for snapshot
observations, we identified a circular region of interest for each target,
including the SDSS extent of the galaxy and any nearby obvious companions, and
used the interactive ALADIN viewing feature of the Astronomers’ Proposal Tool
(APT) to define a POS TARG coordinate offset, so that circular region closely
abutted the edge of CCD WFC1. We considered putting targets closer to the
corner of the overall ACS WFC field where optical distortion gives the largest
pixels on the sky, in order to improve surface-brightness sensitivity, but
that region falls on CCD WFC2 whose slightly higher readout noise more than
compensates for the pixel-area difference in sensitivity.

During this stage, we also caught some pasting errors in the sign of target
declinations, possibly fostered by having target lists and formats from
multiple sources.

##  5 Sample results

The Zoo Gems observations to date are listed in Table 2, by science topic and
observation date. As of 31 January 2022, 146 objects had been observed, 49% of
the input list. The total exposure time in each case is 674 seconds, in two
dithered exposures when a single filter was used, or two individual 337-second
exposure for those Green Peas with two filters listed. For these 2-filter
objects, we list the final two characters of the second data set identifier
after the complete identifier for the first exposure. In a few cases, one
exposure was terminated onboard before the planned duration.

The ACS images showed some objects to be morphologically rather different than
we anticipated from SDSS images. In these cases, we show them in Table 2 and
in our further discussion according to the category where they fit most
closely rather than the category listed in the proposal. One object, SDSS
J222024.58+010931.3, turned out to be a superposition of a star and faint
background galaxies which mimicked a ring structure in the SDSS data, and is
not included in Table 2.

The following sections highlight some initial results from the Zoo Gems
observations, and demonstrate the value of even such shallow exposures in
addressing a variety of scientific questions.

Table 2: Zoo Gems Observations Dataset | Target Name | $\alpha_{2000}$ | $\delta_{2000}$ | UT Start Time | Filter
---|---|---|---|---|---
Unusual spiral patterns: |  |  |  |
JDS452010 | SDSS-170414.33+620234.0 | 17 04 16.225 | +62 02 57.91 | 2018-07-06 08:19:20 | F475W
JDS450010 | UGC-4250 | 08 10 05.486 | +46 11 34.90 | 2021-01-11 05:23:05 | F475W
JDS451010 | NGC-2595 | 08 27 42.024 | +21 28 44.76 | 2021-03-16 11:57:55 | F475W
JDS453010 | MCG+10-21-019 | 14 36 35.851 | +57 47 49.08 | 2021-08-03 20:07:36 | F475W
JDS454010 | SDSS-233906.23-002615.0 | 23 39 06.236 | -00 26 15.10 | 2021-11-13 12:36:39 | F475W
X-Bulges: |  |  |  |  |
JDS439010 | NGC-1175 | 03 04 30.752 | +42 20 07.55 | 2019-07-18 02:34:29 | F814W
JDS436010 | SDSS-1237661418212229211 | 14 09 04.333 | +54 52 19.88 | 2019-09-07 11:43:00 | F814W
Nuclear disks/bars: |  |  |  |  |
JDS498010 | CGCG-245-033 | 13 12 56.707 | +47 27 23.84 | 2021-03-12 22:49:26 | F814W
JDS40A010 | NGC-2771 | 09 10 41.192 | +50 22 36.54 | 2021-03-14 01:09:33 | F814W
JDS40B010 | NGC-5945 | 15 29 45.007 | +42 55 07.13 | 2021-03-14 04:40:37 | F814W
JDS40F010 | MCG+09-19-030 | 11 17 43.510 | +53 47 36.25 | 2021-09-27 04:08:05 | F814W
Large or prolate bulges: |  |  |  |
JDS420010 | NGC-0810 | 02 05 28.560 | +13 15 05.76 | 2019-06-24 18:54:23 | F606W
JDS435010 | CGCG-308-012 | 06 13 05.304 | +64 33 41.04 | 2019-12-19 07:05:33 | F814W
JDS47T010 | UGC-10374 | 16 23 39.166 | +50 58 09.96 | 2020-03-03 13:11:57 | F814W
Red/Blue Pairs: |  |  |  |  |
JDS432010 | SDSS-1237678618479755694 | 22 12 06.397 | +01 46 06.37 | 2019-11-28 19:29:10 | F606W
JDS431010 | GAMA-636444 | 09 23 21.888 | -01 43 33.96 | 2022-01-29 18:48:26 | F606W
Star-forming knots: |  |  |  |  |
JDS430010 | GAMA-302130 | 09 05 46.392 | +01 15 32.76 | 2021-10-06 04:06:12 | F606W
Overlapping Galaxies: |  |  |  |  |
JDS437010 | SDSS-1237661417676800323 | 14 28 14.096 | +53 14 29.00 | 2019-05-08 08:33:55 | F814W
JDS490010 | SDSS-115331.86+360024.2 | 11 53 31.865 | +36 00 24.27 | 2019-05-28 18:56:44 | F606W
JDS478010 | UGC-7064A | 12 04 44.973 | +60 40 24.56 | 2019-07-12 11:33:01 | F606W
JDS488010 | NGC-5021 | 13 12 06.265 | +46 11 45.75 | 2020-07-16 12:05:54 | F814W
JDS489010 | UGC-12281 | 22 59 14.839 | +13 36 16.74 | 2020-09-01 04:48:39 | F606W
JDS485010 | IC-720 | 11 42 22.336 | +08 46 11.51 | 2021-03-14 06:10:29 | F606W
JDS481010 | MCG+07-34-030 | 16 25 58.133 | +43 57 46.47 | 2021-10-10 11:32:38 | F606W
Interacting/Merging Systems: |  |  |  |
JDS449010 | SDSS-081913.94+591926.4 | 08 19 13.920 | +59 19 26.76 | 2019-02-22 21:23:13 | F606W
JDS47W010 | UGC-00240 | 00 25 10.106 | +06 29 27.17 | 2019-10-05 06:19:39 | F606W
JDS44M010 | VII-ZW-090 | 10 36 35.625 | +02 21 31.41 | 2020-02-27 03:33:10 | F475W
JDS428010 | SDSS-095346.77-012746.1 | 09 53 46.680 | -01 27 45.00 | 2020-03-03 05:50:45 | F606W
JDS406010 | SDSS-1237668504364187727 | 16 12 24.606 | +59 46 10.47 | 2021-03-14 03:53:56 | F475W
JDS444010 | CGCG-396-002 | 05 37 35.976 | +01 20 04.20 | 2021-03-18 22:36:26 | F606W
JDS442010 | VV-689 | 10 01 39.502 | +19 47 32.58 | 2021-04-30 17:06:39 | F606W
JDS441010 | IC-2431 | 09 04 34.776 | +14 35 45.96 | 2021-10-03 02:54:35 | F606W
Ring(ed) Galaxies, Lenses: |  |  |  |
JDS425010 | SDSS-1237679438812676365 | 02 03 28.727 | -06 59 49.72 | 2019-02-17 01:28:33 | F606W
JDS426010 | CGCG-087-009 | 07 33 17.712 | +18 17 24.36 | 2019-03-20 14:17:24 | F606W
JDS418010 | GAMA70579 | 12 01 43.464 | +00 10 59.16 | 2019-05-17 06:34:36 | F606W
JDS424010 | SDSS-133145.32+513431.2 | 13 31 45.326 | +51 34 31.22 | 2019-09-08 17:53:10 | F814W
JDS495010 | IC-3828 | 12 50 20.695 | +37 56 56.19 | 2020-06-13 17:27:34 | F606W
JDS405010 | SDSS-1237678595932094536 | 22 20 24.589 | +01 09 31.30 0 | 2020-12-16 21:52:36 | F475W
JDS427010 | SDSS-081740.08+042952.3 | 08 17 40.080 | +04 29 52.44 0 | 2021-01-12 21:07:04 | F606W
Galaxy Zoo EELRs: |  |  |  |
JDS403010 | SDSSJ160646.74+565139.2 | 16 06 46.740 | +56 51 39.20 | 2021-12-24 16:13:14 | F606W
Radio Galaxy Zoo EELRs: |  |  |  |
JDS46N010 | SDSS-130854.52+562155.6 | 13 08 52.460 | +56 22 42.40 | 2019-02-15 02:12:01 | F625W
JDS46Z010 | SDSS-075529.95+520450.6 | 07 55 26.309 | +52 03 51.25 | 2019-04-02 07:46:09 | F625W
JDS46U010 | SDSS-102733.29+544227.9 | 10 27 30.688 | +54 41 30.79 | 2019-06-04 03:47:14 | F625W
JDS46R010 | SDSS-121849.88+502617.6 | 12 18 45.908 | +50 25 30.87 | 2019-06-07 00:40:21 | F625W
JDS46E010 | SDSS-160344.95+524220.6 | 16 03 42.715 | +52 41 25.34 | 2019-08-20 14:58:52 | F625W
JDS47M010 | SDSS-010206.98+093427.6 | 01 02 03.668 | +09 35 04.12 | 2019-10-08 21:44:09 | F625W
JDS47G010 | PKS-0236+02 | 02 38 34.584 | +02 34 46.84 | 2019-10-16 03:00:46 | F625W
JDS47V010 | SDSS-101147.31+071915.2 | 10 11 50.739 | +07 19 42.52 | 2020-02-15 07:07:13 | F625W
JDS46H010 | SDSS-025210.17+025430.1 | 02 52 10.017 | +02 53 32.90 | 2020-03-06 11:15:15 | F625W
JDS47C010 | SDSS-141119.04+094225.3 | 14 11 19.04 | +09 42 25.3 | 2020-08-02 17:12:10 | F625W
JDS46I010 | SDSS-105426.23+573649.1 | 10 54 32.316 | +57 37 16.16 | 2020-10-17 09:26:49 | F625W
JDS47F010 | SDSS-082400.50+031749.4 | 08 24 04.353 | +03 18 07.73 | 2021-01-14 06:29:16 | F625W
JDS46C010 | B2-0832+34 | 08 35 15.391 | +34 34 01.90 | 2021-03-12 06:17:49 | F625W
JDS46W010 | SDSS-083512.43+175441.0 | 08 35 10.668 | +17 53 50.09 | 2021-03-19 01:55:27 | F625W
JDS47D010 | SDSS-123300.30+060326.1 | 12 32 58.539 | +06 04 16.38 | 2021-03-21 08:13:26 | F625W
JDS46P010 | 3C-458 | 23 12 48.445 | +05 17 05.36 | 2021-09-10 03:03:35 | F625W
JDS46Q010 | 4C+08.70 | 23 36 40.401 | +08 49 55.09 | 2021-09-12 04:19:16 | F625W
JDS46O010 | SDSS-141408.44+484156.0 | 14 14 08.445 | +48 41 56.00 | 2021-09-22 01:59:54 | F625W
Blue Elliptical Galaxies: |  |  |  |
JDS40O010 | CGCG-315-014 | 12 06 17.055 | +63 38 19.08 | 2019-04-15 14:34:09 | F475W
JDS40M010 | MKN-0888 | 16 44 30.755 | +19 56 26.73 | 2019-08-07 09:15:53 | F475W
JDS40P010 | SDSS-111850.04+422541.8 | 11 18 50.047 | +42 25 41.84 | 2019-12-28 23:11:30 | F475W
JDS40H010 | SDSS-031749.30+011337.1 | 03 17 49.304 | +01 13 37.25 | 2020-11-28 06:10:32 | F475W
JDS40J010 | SDSS-000907.90+142755.8 | 00 09 07.908 | +14 27 55.83 | 2021-08-06 09:06:11 | F475W
JDS40G010 | CGCG-432-030 | 23 47 03.791 | +14 50 30.36 | 2021-09-17 06:36:51 | F475W
Red spiral galaxies: |  |  |  |  |
JDS40T010 | UGC 3935 | 07 37 49.410 | +46 23 51.53 | 2020-10-18 00:25:12 | F606W
Regrowing-disk Mergers: |  |  |  |
JDS414010 | NGC-2292 | 06 47 40.830 | -26 45 05.00 | 2020-01-23 04:22:48 | F606W
JDS412010 | CFHTLS1220-215555 | 08 50 58.169 | -04 02 12.85 | 2020-03-12 02:41:03 | F606W
JDS410010 | UGC-4052 | 07 51 16.564 | +50 14 03.27 | 2020-05-01 11:33:05 | F606W
JDS408010 | SDSS-1237659936978568047 | 00 43 41.784 | +43 02 35.16 | 2020-12-19 19:47:02 | F606W
JDS413010 | SDSS-1237680246274064522 | 23 26 23.853 | +19 27 09.12 | 2021-09-20 01:17:10 | F606W
JDS409010 | SDSS-1237678439701807265 | 02 49 03.312 | +03 12 12.60 | 2021-09-21 09:16:29 | F606W
Post-Starburst Galaxies: |  |  |  |
JDS464010 | CGCG-292-024 | 11 44 52.092 | +57 52 24.67 | 2018-08-09 03:21:12 | F475W
JDS474010 | SDSS-124354.11+163250.5 | 12 43 54.178 | +16 32 50.85 | 2019-03-19 13:30:53 | F475W
JDS460010 | NGC-3156 | 10 12 41.183 | +03 08 04.71 | 2020-02-24 16:44:52 | F475W
JDS457010 | UGCA-188 | 09 55 29.700 | +08 23 26.28 | 2020-06-12 20:44:08 | F475W
JDS476010 | VCC-1711 | 12 37 22.147 | +12 17 13.32 | 2021-03-21 19:20:48 | F475W
SDRAGNs: |  |  |  |  |
JDS45H010 | SDSS-091949.07+135910.7 | 09 19 47.195 | +13 58 22.68 | 2018-05-15 21:49:55 | F475W
JDS43Y010 | UGC-1797 | 02 19 58.728 | +01 55 48.72 | 2018-07-03 02:46:37 | F475W
JDS44C010 | SDSS-16562058+6407529 | 16 56 16.945 | +64 07 14.62 | 2018-08-24 13:52:36 | F475W
JDS45T010 | SDSS-112811.63+241746.9 | 11 28 09.853 | +24 18 39.94 | 2019-02-23 05:51:12 | F475W
JDS45Z010 | B3-1352+471 | 13 54 30.924 | +46 56 44.51 | 2019-04-28 00:35:49 | F475W
JDS44X010 | SDSS-132809.31+571023.3 | 13 28 03.443 | +57 10 13.25 | 2019-05-14 07:06:26 | F475W
JDS45J010 | SDSS-163300.85+084736.4 | 16 32 58.024 | +08 47 03.84 | 2019-07-11 23:11:15 | F475W
JDS44Z010 | B2-1644+38 | 16 46 25.987 | +38 31 03.31 | 2019-07-19 17:39:53 | F475W
JDS43V010 | SDSS-172107.89+262432.1 | 17 21 05.558 | +26 23 54.34 | 2019-08-22 16:18:28 | F475W
JDS47J010 | SDSS-134900.13+454256.5 | 13 49 06.051 | +45 43 03.52 | 2019-11-13 21:02:54 | F475W
JDS45V010 | SDSS-214110.61+082132.6 | 21 41 11.564 | +08 20 35.91 | 2019-12-11 14:04:35 | F475W
JDS45G010 | SDSS-081303.10+552050.7 | 08 13 00.417 | +55 21 37.26 | 2019-12-25 01:19:10 | F475W
JDS44D010 | SDSS-150903.21+515247.9 | 15 09 08.415 | +51 53 28.37 | 2020-01-14 13:19:06 | F475W
JDS44R010 | B2-0938+31A | 09 40 59.773 | +31 26 29.12 | 2020-02-13 02:39:49 | F475W
JDS47H010 | SDSS-095605.87+162829.9 | 09 56 01.712 | +16 28 52.37 | 2020-02-14 21:34:14 | F475W
JDS45A010 | IC-4234 | 13 22 58.535 | +27 07 09.45 | 2020-04-09 03:03:16 | F475W
JDS44J010 | SDSS-080658.46+062453.4 | 08 06 58.46 | +06 24 53.4 | 2020-05-28 23:07:53 | F475W
JDS44P010 | SDSS-095833.44+561937.8 | 09 58 39.333 | +56 20 16.06 | 2020-10-17 13:19:01 | F475W
JDS44I010 | SDSS-080259.73+115709.7 | 08 03 04.152 | +11 57 33.22 | 2021-01-10 03:57:52 | F475W
JDS47K010 | SDSS-113648.57+125239.7 | 11 36 48.57 | +12 52 39.7 | 2021-03-14 02:58:57 | F475W
JDS41L010 | B3-0852+422 | 08 55 44.151 | +42 03 44.70 | 2021-03-14 23:24:25 | F475W
JDS45L010 | SDSS-090305.84+432820.4 | 09 03 02.290 | +43 27 51.56 | 2021-03-15 00:59:46 | F475W
JDS44G010 | SDSS-082312.91+033301.3 | 08 23 11.663 | +03 32 03.79 | 2021-03-15 05:49:08 | F475W
JDS45B010 | SDSS-083351.28+045745.4 | 08 33 50.118 | +04 56 54.67 | 2021-03-18 00:32:22 | F475W
JDS45F010 | SDSS-163624.97+243230.8 | 16 36 21.753 | +24 32 46.26 | 2021-06-05 19:19:04 | F475W
JDS44K010 | SDSS-083224.82+184855.4 | 08 32 27.328 | +18 49 54.04 | 2021-09-28 05:30:40 | F475W
JDS45E010 | SDSS-130300.80+511954.7 | 13 03 00.803 | +51 19 54.70 | 2021-10-01 05:04:08 | F475W
JDS45I010 | SDSS-084759.90+124159.3 | 08 47 59.90 | +12 41 59.3 | 2021-10-02 04:44:49 | F475W
JDS44T010 | SDSS-020904.75+075004.5 | 02 09 04.750 | +07 50 04.50 | 2021-10-22 03:54:22 | F475W
JDS45W010 | SDSS-090147.17+164851.3 | 09 01 47.17 | +16 48 51.3 | 2021-11-13 10:05:19 | F475W
JDS43Z010 | B3-0911+418 | 09 14 45.528 | +41 37 14.52 | 2021-12-29 08:17:30 | F475W
JDS47L010 | SDSS-092605.17+465233.9 | 09 26 05.17 | +46 52 33.9 | 2021-12-29 19:23:05 | F475W
Green Peas: |  |  |  |  |
JDS42MO4Q/5Q | SDSS-1237651537646977181 | 15 04 57.987 | +59 54 07.27 | 2018-07-10 14:11:15 | F555W, F850LP
JDS42KCEQ/FQ | SDSS-1237659330316140926 | 16 33 37.941 | +37 53 14.30 | 2018-07-14 03:38:45 | F555W, F850LP
JDS41D010 | SDSS-1237648720155902209 | 11 49 46.471 | -01 02 17.65 | 2018-11-29 21:59:41 | F775W
JDS41F010 | SDSS-1237661386530882021 | 14 10 05.248 | +53 50 37.89 | 2018-12-27 01:56:41 | F775W
JDS40X010 | SDSS-1237651271358349509 | 10 26 15.207 | +63 33 08.49 | 2019-01-02 02:51:41 | F775W
JDS40Y010 | SDSS-1237657878078751164 | 08 08 16.907 | +28 14 31.14 | 2019-02-24 00:38:23 | F775W
JDS42IUGQ/HQ | SDSS-1237654398623023110 | 13 36 07.914 | +62 55 30.77 | 2019-04-12 02:35:01 | F555W, F850LP
JDS40Z010 | SDSS-1237651271899939173 | 12 05 17.538 | +66 40 29.64 | 2019-04-19 05:30:54 | F775W
JDS42ZV5Q/6Q | SDSS-1237667551414452597 | 10 15 41.152 | +22 27 27.52 | 2019-05-16 16:09:08 | F555W, F850LP
JDS42QJPQ/QQ | SDSS-1237661851469021323 | 12 14 23.180 | +45 20 40.91 | 2019-07-17 04:52:31 | F555W, F850LP
JDS42B010 | SDSS-1237669698364768508 | 21 08 03.059 | +05 27 07.14 | 2019-08-10 04:15:11 | F775W
JDS41A010 | SDSS-1237651273498755389 | 08 38 40.165 | +54 44 03.49 | 2019-10-01 17:20:01 | F775W
JDS43DUMQ/NQ | SDSS-1237666300559098162 | 03 53 32.464 | -00 10 28.88 | 2019-11-14 06:16:30 | F555W, F850LP
JDS42F010 | SDSS-1237680507722793618 | 23 19 27.467 | +33 23 24.76 | 2019-11-25 15:15:51 | F775W
JDS43EDKQ/LQ | SDSS-1237665369575981438 | 10 20 57.462 | +29 37 26.47 | 2020-01-26 23:25:54 | F555W, F850LP
JDS42WWCQ/DQ | SDSS-1237667536933945523 | 10 04 00.641 | +20 17 19.25 | 2020-02-21 03:03:38 | F555W, F850LP
JDS43T010 | SDSS-1237664668421849521 | 08 15 52.002 | +21 56 23.65 | 2020-04-27 20:20:00 | F850LP
JDS42LSNQ/OQ | SDSS-1237658491735507237 | 10 55 30.41 | +08 41 32.8 | 2020-06-03 14:22:26 | F555W, F850LP
JDS43M010 | SDSS-1237657632187613477 | 09 24 38.718 | +47 07 58.93 | 2020-06-10 20:58:36 | F850LP
JDS41Q010 | SDSS-1237662300818702524 | 13 01 28.316 | +51 04 51.18 | 2020-08-01 18:46:22 | F775W
JDS42NVWQ/XQ | SDSS-1237653653450064110 | 00 42 36.92 | +16 02 02.7 | 2020-11-12 16:48:32 | F555W, F850LP
JDS42HVJQ/KQ | SDSS-1237660343930782057 | 08 45 11.669 | +32 51 53.92 | 2020-12-09 06:05:51 | F555W, F850LP
JDS41G010 | SDSS-1237654383585395027 | 08 34 40.056 | +48 05 40.91 | 2021-01-11 21:16:54 | F775W
JDS42E010 | SDSS-1237667254538142046 | 09 51 03.165 | +24 54 35.70 | 2021-02-07 10:29:06 | F775W
JDS42XQMQ/NQ | SDSS-1237667211050680599 | 09 41 49.637 | +23 37 30.07 | 2021-02-14 12:28:58 | F555W, F850LP
JDS41U010 | SDSS-1237662640662905610 | 16 46 12.15 | +20 54 11.5 | 2021-03-13 05:34:25 | F775W
JDS43J010 | SDSS-1237658423543529721 | 09 05 35.161 | +04 53 34.51 | 2021-03-18 02:09:30 | F850LP
JDS42SAJQ/KQ | SDSS-1237648704041976118 | 12 29 33.142 | -00 18 01.68 | 2021-03-26 05:48:14 | F555W, F850LP
JDS42UHCQ/DQ | SDSS-1237667781210145026 | 10 04 34.733 | +17 47 35.35 | 2021-04-27 00:09:42 | F555W, F850LP
JDS43CVFQ/GQ | SDSS-1237679476939882858 | 01 03 21.059 | +21 32 15.91 | 2021-09-09 17:36:16 | F555W, F850LP
JDS43N010 | SDSS-1237655373039927410 | 16 53 04.490 | +33 39 37.74 | 2021-09-09 18:57:15 | F850LP
JDS43O010 | SDSS-1237663782590021909 | 00 29 38.169 | -01 12 16.05 | 2021-09-17 05:07:19 | F850LP
JDS41R010 | SDSS-1237657589239382254 | 09 54 22.599 | +47 51 44.02 | 2021-09-22 03:22:49 | F775W
JDS41S010 | SDSS-1237661382770098472 | 09 20 36.046 | +32 42 52.63 | 2021-09-29 03:39:51 | F775W
JDS41J010 | SDSS-1237663204918952341 | 00 44 00.266 | +00 47 24.68 | 2021-09-30 23:56:42 | F775W
JDS41I010 | SDSS-1237657628456386802 | 12 33 38.626 | +51 41 59.34 | 2021-10-01 01:52:15 | F775W
JDS42JI3Q/4Q | SDSS-1237660635458568341 | 10 27 16.725 | +43 42 02.18 | 2021-10-03 06:30:17 | F555W,F850LP
JDS41P010 | SDSS-1237661361304568164 | 14 46 42.608 | +40 48 44.21 | 2021-12-11 16:35:48 | F775W

###  5.1 Green Peas

The great majority of Green Peas (34/38, 89%) are resolved into multiple
distinct components or show surrounding non-axisymmetric structure. The
structures include double components, tails, and apparent disks. Four systems
(SDSS J004236.92+160202.7, SDSS J092438.71+470758.9, SDSS J100400.64+201719.2,
and SDSS J165304.48+333937.7) have central peaks only marginally resolved in
the ACS data; a simple Gaussian comparison with star images suggests intrinsic
FWHM $<1.8$ pixels (0.09″) or 0.36 kpc at the typical $z=0.25$. Some of the
Green Pea systems show tidal tails or patchy spiral patterns; most have either
multiple knots or show well-resolved surrounding galaxies (shown in Fig. 4 for
those observed in a single filter, allowing easy rejection of cosmic-ray
artifacts). Analysis of the 2-filter Zoo Gems data by Clarke et al. (2020)
indicates that the intense starburst regions are surrounded by redder
components, most likely older stellar populations. The smallest sizes we
measure in these deep-red bands are comparable to the UV sizes measured
(predominantly for the brightest components) by Yang et al. (2017) and Kim et
al. (2021).

Figure 4:  The first 15 Green Pea systems observed in single filters, as
listed in Table 2. Abbreviated names showing the first 4 digits of the right
ascension are used for convenience. Each panel is a $6\times 6$″region,
typically $24\times 24$ kpc, and all are shown to the same intensity scale,
logarithmic above a slight negative offset. North is up, and east to the left
in each case. The 2″scale bar matches the limiting Petrosian radius from SDSS
data used in the sample selection. All were observed in the F775M band except
0815, 0905, and 0924, observed in F850LP.

###  5.2 Blue ellipticals

Each of the six blue early-type galaxies observed in Zoo Gems shows a distinct
spiral structure, which in each case is too tightly wound to have been
resolved in SDSS data (Fig. 5). These are not simply otherwise-normal
elliptical galaxies with scattered star-forming regions, although in some
cases the outer light distribution is less disc-like than the inner regions.
In fact, two of these galaxies, Mkn 888 and SDSS J031749.30+011337, have a
nearly pure $r^{1/4}$ profile as assessed in the SDSS fracDeV parameter, with
values 0.97–1 among all SDSS filters. The other four have values 0.49–0.96 in
the well-measured $griz$ bands. Among the galaxies observed, SDSS
031749.30+011337.1 and CGCG 432-030 have AGN as classified by Schawinski et
al. (2009a) using emission-line ratios from the SDSS spectra. Schawinski et
al. (2009b) reported a CO detection of SDSS 111850.04+422541.8, with a double-
peaked disk-like profile and implied total molecular-gas mass near $6\times
10^{8}$ M⊙. These small-scale spiral patterns are similar to those sometimes
seen in recent major mergers, such as NGC 7252 (Whitmore et al., 1993), NGC
3256 (as shown in the figures by Mulia et al. 2016), and even 2MASX
J01392400+2924067 seen before coalescence of the nuclei (Koss et al., 2018),
which could support the conjecture of Schawinski et al. (2009b) that such
mergers are one route to producing blue galaxies with elliptical-like
properties. The galaxies in our sample share the radial scales of central
spiral patterns with the nearby post-merger systems, as traced both by star
clusters and by dust lanes. The dust spirals have radial extents 1.2–3.3 kpc,
and the patterns traced by bright star-forming regions span 1.2–2.6 kpc. These
are comparable to the values 1.8–6.4 kpc (dust) and 1.2–3.0 kpc (star
clusters) seen in nearby merging and post-merger systems. The smaller values
apply to NGC 7252, which is the oldest local merger based on comparison with
simulations and ages of star clusters, and thus more comparable to our systems
where merger signatures in the starlight distribution must be even more
subtle.

There is clearly more to be done in defining how these blue early-type
galaxies relate to normal ellipticals, mergers, and even rejuvenated spirals.
We plan to consolidate these HST images along with the new deep ground-based
surveys to address this in future work.

Figure 5: Blue early-type galaxies, in the F475W filter to emphasize young
stellar populations. The bottom two are classified as having spectroscopic AGN
by the SDSS automated system. Each galaxy shows a tightly wound spiral pattern
near the nucleus; the superimposed circles have radius 2.5″, showing how
strongly these patterns are blended together in typical survey images. Insets
show SDSS composite images, 60″square, as used by Galaxy Zoo participants in
the initial classifications. Some show various levels of structure in these
images, but only CGCG 432-030 might have been classified as a clear spiral
from SDSS data. North is up, and east to the left in each case.

###  5.3 Red spirals

One of these, UGC 3935, has been observed. The arms include star-forming
knots, while the dust arms include a spiral pattern cutting across stellar
structure near the core. This object was included in the MaNGA survey’s
integral-field spectroscopy (Bundy et al., 2015), and in the associated and
ongoing H I survey (Masters et al. 2019, Stark et al. 2021), which shows
$3.7\times 10^{10}$ M⊙ of neutral hydrogen. This is close to the derived
stellar mass $4.6\times 10^{10}$ M⊙ from SDSS data using the Portsmouth models
(Maraston et al., 2009), so the galaxy’s red color is not due purely to gas
exhaustion. The HST image is compared to an SDSS color-composite in Fig. 6.
The arms contain blue star-forming knots, and this could be classified as a
3-armed system as well.

Figure 6: Red spiral galaxy UGC 3935, comparing the usual $gri$ composite from
SDSS data with the HST ACS F606W image. The region shown is $77\times 78$″with
north at the top.

###  5.4 Disk structures

The range of disk structures included in Zoo Gems data is sampled in Fig. 7.

####  5.4.1 Circumnuclear disks and bars

The two systems shown in Fig. 7 both have nuclear bars and surrounding rings
or barlenses (as defined by Laurikainen et al. 2011), which extend beyond the
bar width in each case. The inner region of NGC 2771 forms a striking echo,
rotated nearly 90∘, of the outer bar and ring of the galaxy disk. NGC 2595
from the ”unusual spiral patterns” category shows similar features. UGC 10374
has an outer pseudoring beyond the area shown. The backlit spiral in IC 720
shows a nuclear spiral and possibly nuclear bar which were not well resolved
even in subarcsecond ground-based images.

Figure 7: Some of the kinds of disk structures included in Zoo Gems data. Top
row, nuclear bars and barlenses. Middle row, 3-armed spirals. Bottom row,
backlit spiral arms and disk with dust attenuation. All images have north at
the top and east to the left; vertical white scale bars indicate 10″ in each
case. The gray scale is logarithmic, with zero levels and contrast tailored to
show the structures in each object. In the object names, SDSS destinations are
truncated for legibility.

####  5.4.2 Three-armed spirals

Details in the arm structures of these systems may help understand why these
do not show a preference for low-density environments, as might be expected
from a straightforward analysis of spiral modes after perturbation (Elmegreen
et al. 1992, Hancock 2019). In SDSS J170414.33+620234.0, among the first
targets observed in this program, the enhanced angular resolution of HST
images reveals quite different distributions of star-forming knots along each
arm, and the arms starting from an off-center ring around the core.

####  5.4.3 Backlit dust

Fig. 7 shows two of the most striking backlit-galaxy systems observed in Zoo
Gems. While we will present a full modeling of these systems elsewhere, it is
already noteworthy that each case shows at least thin arms of attenuation
farther out than the detected starlight, and nearly transparent regions
between the dust arms (and within the resonance ring in the case of IC 720).
The outer dust lanes illustrate in a vivid way one advantage of dust detection
with this method - arbitrarily cold dust is detectable, unlike direct far-IR
measurements which rely on reradiation of absorbed starlight (Domingue et al.,
1999). These images also illustrate the gain in understanding structure on
going from SDSS to HST angular resolution. As has been shown with WFPC2 images
of two backlit spirals (Keel & White, 2001), better linear resolution reduces
the confusion between unresolved dust structure and sleep of the reddening
law, as there is less blending of areas with quite different attenuation. The
Zoo Gems overlap systems provide a significant addition to the range of disk
structures and backlit regions observed with Hubble’s resolution.

###  5.5 Ring features

Among these, SDSS 133145.32+513431.2 is especially noteworthy. This object
shows a subtle annular color pattern in SDSS and, more clearly, DES data
(Abbott et al., 2018). The ACS image reveals a partial Einstein ring (Fig. 8),
with typical radius 4.0″, and two main segments together spanning nearly 300∘
around the brightest galaxy in a group (SDSS data give $z=0.2894$ for this
galaxy). An inflection at its northern end is associated with an individual
luminous galaxy. While the source redshift remain unknown, pending further
information on the lensed source and additional foreground group members,
knowing the lens redshift we can bound the group mass enclosed within the
Einstein ring (radius 4.0″or 17.5 kpc). If the source-lens distance is equal
to the lens-observer distance (0.90 Gpc), the mass within 17.5 kpc would be
$9\times 10^{12}$ solar masses, dropping to $3\times 10^{12}$ for the
unrealistic case of arbitrarily high redshift.

Figure 8: Deep-red F814W image of the lensed arc around the central galaxy
SDSS 133145.32+513431.2 ($z=0.2894$). The upward inflection to the north may
indicate perturbation by the separate potential of the bright galaxy just
outside the arc. The partial ring has characteristic radius 4.0″, projecting
to 17.5 kpc at the distance of the lensing system.  Figure 9: Montage of
merging galaxies, including two typical mergers (upper row) and two potential
regrowing disks (lower row). The layout is as in Fig. 7, with 10″scale bars.
The regrowing disks illustrated here span large enough areas on the sky to
include the stripe of cosmic-ray events across the middle, where only a single
exposure was obtained as the telescope made a dither motion to give double
coverage elsewhere. For NGC 2292, the edges of the ACS combined field appear
in the corners.

###  5.6 Regrowing disks

The brightest of these are shown in Fig. 9, in comparison to two more typical
advanced merging systems. The images illustrate the characteristics of two
bulges, jointly surrounded by a disk with incomplete spiral patterns
containing dust lanes and patchy star-forming regions.

After program submission, one of these objects (CFHTLS1220-215555) was seen to
have a star superimposed on a single bulge as seen in Legacy Survey images
(Dey et al., 2019), which were not clearly separated in SDSS data and appeared
as a double bulge. This object does have the unusual combination of a
dominant, off-center ring without a bar, and short spiral features.

The overlapping-galaxy system UGC 7064A may more exactly belong with these
systems, on examination of the dust geometry. The eastern bulge component is
surrounded by a ring including stars and dust, which may also encircle the
inclined spiral disk to its west.

###  5.7 Reassignment of galaxy categories

Some objects turn out to be something quite different than we inferred from
the SDSS or Legacy Survey images. For example, the components of the NGC 5021
system show obvious signs of tidal interaction (off-center nucleus, helical
dust lanes, spokes), rather than being a superposition of relatively
undisturbed galaxies. SDSS-1237668504364187727 (SDSS-J 161224.60+594610.4),
observed as a potential reddened AGN with bluer outflow regions, looks like a
multicomponent merger; a Lick Observatory spectrum, using the Kast
spectrograph at the 3m Shane telescope, shows the core to host highly reddened
star formation, with relatively unreddened star formation on either side. The
central region has emission-line redshift $z=0.1019$.

##  6 Conclusions

We have described the diverse scientific cases addressed in the “Gems of the
Galaxy Zoos” HST gap-filler program, and detailed the target selection and
public input involved in its object list. The results so far illustrate the
value of even short-exposure images at Hubble’s high angular resolution,
deriving additional results from the effort invested by volunteers in the
Galaxy Zoo and Radio Galaxy Zoo projects. These data have revealed extended
redder components around Green Pea starburst systems, small-scale spiral
patterns in blue early-type galaxies, and suggested that some merging galaxies
quickly reform star-forming disks. Additional uses will certainly be
forthcoming. The variety of results already found from this unusually wide-
ranging program may encourage the community to consider ways to achieve a
similar richness of use for projects on other facilities. This includes
availability of high angular resolution, which has proven crucial in such
applications as substructure of Green Pea starbursts, using dust attenuation
to unravel the geometry of disks and merging systems, and distinguishing key
morphological features such as spiral arms in radio-AGN hosts. Many of our
results illustrate the complementary roles of deep survey images and even
shallow high-resolution images.

This work was enabled by the many volunteer participants in Galaxy Zoo, and
especially by the beta testers and commentators on the voting interface, those
who pointed to interesting objects discussed in the project Forum and Talk
sites, and all who voted on target selection. We particularly note
contributions by Ivan Terentiev, Chris Molloy, Victor Linares, Alexander
Jonkeren, Christine MacMillan, Richard Nowell, Graham Mitchell, Claude Cornen,
and Michael Peck. At STScI, program coordinator Blair Porterfield gave tips
which improved the quality of our data, while John Mackenty was helpful in
scheduling questions and in tracking down changes in scheduling priority
during the program. We thank Julianne Dalcanton for conversations on
observation setup and on coordinating object lists between two gap-filler
projects. A timely Excel suggestion from Nathan Keel greatly speeded the
production of Table 1. Funding for the SDSS and SDSS-II has been provided by
the Alfred P. Sloan Foundation, the Participating Institutions, the National
Science Foundation, the U.S. Department of Energy, the National Aeronautics
and Space Administration, the Japanese Monbukagakusho, the Max Planck Society,
and the Higher Education Funding Council for England. The SDSS Web Site is
http://www.sdss.org/. The SDSS is managed by the Astrophysical Research
Consortium for the Participating Institutions. The Participating Institutions
are the American Museum of Natural History, Astrophysical Institute Potsdam,
University of Basel, University of Cambridge, Case Western Reserve University,
University of Chicago, Drexel University, Fermilab, the Institute for Advanced
Study, the Japan Participation Group, Johns Hopkins University, the Joint
Institute for Nuclear Astrophysics, the Kavli Institute for Particle
Astrophysics and Cosmology, the Korean Scientist Group, the Chinese Academy of
Sciences (LAMOST), Los Alamos National Laboratory, the Max-Planck-Institute
for Astronomy (MPIA), the Max-Planck-Institute for Astrophysics (MPA), New
Mexico State University, Ohio State University, University of Pittsburgh,
University of Portsmouth, Princeton University, the United States Naval
Observatory, and the University of Washington.

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