# The Hosts of X-Ray Absorption Lines Toward AGNs

Maggie C. Huber Department of Astronomy, University of Michigan
Ann Arbor, MI 48109, USA<EMAIL_ADDRESS><EMAIL_ADDRESS>Joel N.
Bregman Department of Astronomy, University of Michigan
Ann Arbor, MI 48109, USA

###### Abstract

Most baryonic matter in the universe exists in gaseous form and can be found
in structures such as galactic halos and the low-density intergalactic medium.
Proposed X-ray spectroscopy missions such as Athena, Arcus, and Lynx will have
the capability to identify absorption lines in spectra toward bright active
galactic nuclei (AGNs), which can be used as a tool to probe this missing
matter. In this study, we examine the optical fields surrounding 15 primary
observing targets and identify the foreground galaxies and galaxy groups that
are potential hosts of absorption. We record the basic properties of the
potential host and their angular and physical separation from the AGN line of
sight. This process is done by marking the location of various galaxies and
groups in optical images of the field surrounding the target and plotting
their angular separation vs. redshift to gauge physical proximity to the
background source. We identify the surrounding objects according to those
which have measured redshifts and those that require them.

##  1 Introduction

In spectra of distant quasars, gaseous material from galaxies along the line
of sight causes absorption lines which provide a precise way to examine the
baryonic matter of galaxies. Quasar absorption line studies have revealed that
most baryons in the Universe are in gaseous form and in a variety of
structures, from galaxy disks and halos to the lower density intergalactic
medium (IGM) (Fukugita et al., 1998). These studies are carried out in the
ultraviolet, optical, and radio bands, which are sensitive to gas in the $10$–
$10^{5.5}$ K range. The baryons accounted for in these studies result in an
overall census that falls 30-50% short of what is predicted by Big Bang
Nucleosynthesis (Shull et al., 2012). It is important to note the temperature
range of these studies are below the virial temperature of L* galaxies
($\approx 10^{6.3}$ K), or the common galaxy groups ($10^{6.5}$-$10^{7.5}$ K).
In these higher temperature ranges, the prominent resonance absorption lines
lie in the X-ray band (0.3-0.8 keV) (Smith et al., 2011). Observations of the
Sunyaev-Zeldovich effect in locally brightest galaxies (Planck Collaboration
et al., 2013; Bregman et al., 2021) imply that we should observe most missing
baryons at the virial temperatures of galaxies. Thus, we expect to account for
the missing baryons by extending absorption line studies to the X-ray waveband
where we can probe these temperatures. This contradicts other explanations
based on UV absorption lines that place the majority of baryons in the
circumgalactic medium (CGM) at temperatures of 104 K (Werk et al., 2014). The
results of X-ray spectroscopy should be able to clarify to some extent whether
this is the case or if virial temperatures of galaxies dominate the
distribution of baryons.

X-ray absorption line studies of this hot halo material in galaxies and galaxy
groups are in their infancy due to the modest spectroscopic capabilities of
current X-ray observatories, such as XMM-Newton and the Chandra X-ray
Observatory (Yao et al., 2012). Milky Way halo gas is detected by these
observatories in several X-ray absorption lines, from which one obtains
information about the extent of the halo and thick disk, its composition, and
that the extended halo rotates (Miller & Bregman, 2013). Only one
extragalactic absorption line was detected (Nicastro et al., 2018), and a
likely galaxy host has been identified. X-ray and UV spectroscopy have also
been successful in revealing the physical origins of the warm-hot
intergalactic medium (Johnson et al., 2019) and intragroup gas (Bowen et al.,
2002) through absorption lines in the spectra toward AGNs.

Although current X-ray observatories do not possess the spectral resolution or
collecting area to probe the virial temperatures of L* galaxies, we should be
able to achieve this soon. Greatly improved X-ray absorption line sensitivity
will be possible with upcoming missions, such as Athena, Arcus, and Lynx
(Barcons et al., 2017; Gaskin et al., 2019; Smith, 2020). These observatories
will likely observe more than $100$ extragalactic absorption line systems in
their first few years of operation. The objects that will provide the best
continuum to measure absorption against are AGNs, especially blazars. The
highest priority background sources are already identified, based on mean
X-ray brightness and redshift; nearly all are blazars (Bregman et al., 2015).
In most of these blazar fields, the potential galaxy and galaxy group hosts
have yet to be identified. It is important to note that progress has been made
by dedicated efforts to conduct surveys of galaxies surrounding UV bright
quasars (Prochaska et al., 2011, 2019; Johnson et al., 2019).

Our goal is to identify likely hosts along the lines of sight toward the
blazars most likely to be observed with these new observatories.
Identification is helped considerably by the various optical and infrared
surveys that have been carried out, including the Two Micron All-Sky Survey
(Skrutskie et al., 2006), the Wide-field Infrared Survey Explorer (Wright et
al., 2010), the Sloan Digital Sky Survey (Blanton et al., 2017), Pan-STARRS
(Flewelling et al., 2020), the Dark Energy Survey (DES Collaboration et al.,
2021) and the VLT-ATLAS survey (Shanks et al., 2015).

Here, we examine each high priority field and identify possible hosts that
already have redshifts and others that need redshifts. Section 2 covers the
methods and selection criteria, section 3 goes through each field
individually, section 4 discusses applications of this study and future work,
and finally section 5 summarizes our findings.

##  2 Methods

In Table 1, we show the observing targets that we cover in this study chosen
from a previously curated list of 104 AGN sources (Bregman et al., 2015) that
provide the brightest X-ray continuum for detecting absorption lines.

Table 1: Summary of the AGN sources we selected for this study.

Object Name | Right Ascension | Declination | Redshift | Merit
---|---|---|---|---
1RXS J003334.6-192130 | 00:33:34.200 | -19:21:33.30 | 0.610 | 4.08E-12
1RXS J022716.6+020154 | 02:27:16.580 | +02:02:00.50 | 0.457 | 5.92E-12
S5 0836+71 | 08:41:24.3652 | +70:53:42.173 | 2.172 | 6.70E-12
1ES 1028+511 | 10:31:18.518 | +50:53:35.82 | 0.360 | 5.94E-12
1RXS J110337.7-232931 | 11:03:37.610 | -23:29:31.20 | 0.186 | 5.32E-12
Mrk 421 | 11:04:27.3139 | +38:12:31.799 | 0.030 | 7.44E-12
PG 1553+113 | 11:04:27.3139 | +11:11:24.365 | 0.450 | 8.32E-12
1RXS J111706.3+201410 | 11:17:06.260 | +20:14:07.40 | 0.139 | 4.05E-12
1RXS J122121.7+301041 | 12:21:21.941 | +30:10:37.11 | 0.184 | 4.19E-12
3C 273 | 12:29:06.6997 | +02:03:08.598 | 0.158 | 8.20E-12
Ton 116 | 12:43:12.7362 | +36:27:43.999 | 1.065 | 8.97E-12
3C 279 | 12:56:11.1665 | -05:47:21.523 | 0.536 | 4.14E-12
1RXS J142239.1+580159 | 14:22:38.895 | +58:01:55.50 | 0.635 | 6.48E-12
1RXS J150759.8+041511 | 15:07:59.7324 | +04:15:11.984 | 1.703 | 4.80E-12
1RXS J151747.3+652522 | 15:17:47.600 | +65:25:23.90 | 0.702 | 6.34E-12
1RXS J153501.1+532042 | 15:35:00.801 | +53:20:37.33 | 0.890 | 8.52E-12
PKS 2155-304 | 21:58:52.0651 | -30:13:32.118 | 0.116 | 2.14E-11
3C 454.3 | 22:53:57.7479 | +16:08:53.560 | 0.859 | 4.93E-12
H2356-309 | 23:59:07.910 | -30:37:40.60 | 0.165 | 3.83E-12

Note. — Merit taken from (Bregman et al., 2015). Targets 3C 273, Mrk 421, PG
1553+113, and PKS 2155-304 will not be discussed in the results section this
paper as the surrounding fields have already been studied in detail. We
mention them in this table because they are included in the primary targets of
future X-ray observatories. Information about these fields can be found at (in
order): Prochaska et al. (2011), Abdo et al. (2011), Johnson et al.
(2019),Chevalier et al. (2019).

We gather information about foreground objects by conducting searches in the
NASA/IPAC Extragalactic Database (NED) for all objects within a radius of 10
arcminutes with the AGN at the center. The cosmological parameters are set
according to the Planck 2015 results (Planck Collaboration et al., 2013) with
redshift corrected to the reference frame defined by the 3K CMB. We create
tables of NED values for the redshift, location (RA and DEC), apparent
magnitude and angular separation of each object. The completeness of the NED
data varies with each AGN field, and we do not include magnitudes in the
results for fields which have incomplete photometry. There are multiple
objects in every field that do not have available redshifts in NED, so we
discuss those objects separately. We also correct our lists for objects that
are double counted or false positives (i.e. listing a source at a location
where there was not one visible in the optical image). Galaxies and stars are
differentiated by NED based on their instrumental point spread function (PSF).
In the survey data we obtained, galaxies are distinguished by an extended PSF,
whereas stars are point-like. We also confirm NED object classifications by
inspection of fields and objects with a combination of SAOImage DS9 (Joye &
Mandel, 2003), ALADIN (Bonnarel et al., 2000), and the SDSS Navigation Tool.

When we have information from NED, we use the ESO Online Digitized Sky Survey
(DSS) to obtain optical images of these regions from the Second Generation DSS
in the red waveband. This survey uses the Oschin (Palomar Schmidt) and UK
Schmidt telescopes with a limiting magnitude of approximately 22.5 mag (BJ)
(Reid et al., 1991). Using a limit 1.5 mag lower than this value, we filter
the list to exclude all objects with a magnitude greater than 21 mag (BJ) for
fields that had over 100 objects in NED. We set the limit to be lower than
that of the Oschin and UK Schmidt telescopes so our primary sources are
accounted for in other optical surveys with lower limits (i.e., DES, SDSS,
Pan-STARRS, VLT-ATLAS). The image size is 20′x20′ by default, with some using
different angular sizes to better show the distribution of objects in the
field. We indicate when the image size deviates from the default in the figure
captions. To complete these images, we make a region file for each field,
marking the location of objects of interest, color coded by redshift if
applicable. We superimpose these regions on the optical DSS images using
SAOImage DS9, and we use these figures as a visual guide to select potential
absorbers. We mark the most likely absorbers (criteria for this listed below)
with a number and discuss the numbered objects in the results. We use standard
cardinal directions with the AGN at the center to refer to the location of an
object.

We also create plots of angular separation in arcminutes as a function of
redshift, marking the redshift of the AGN as a vertical line. This is only
done for objects that have available redshift values. To make an estimate of
the physical distance to the AGN, we use Cosmology Calculator (Wright, 2006)
with flat universe parameters to find separation as a function of redshift for
physical distances of 1 and 0.5 Mpc. We plot these functions on the graph to
gauge an object’s physical distance to the AGN. For example, if the object is
closer than 0.5 Mpc to the AGN it lies below the line corresponding to that
distance.

The NED data, optical images, and separation vs. redshift graphs allow us to
select the most likely objects to cause absorption lines. The main criteria we
look for are objects that are brightest (lowest magnitude) and have the
closest proximity to the AGN. The objects we selected to discuss in the brief
paragraphs for each field minimize both the magnitude and separation. We
calculate the impact parameter for each object by dividing the physical
diameter by the angular diameter then multipyling by the separation. The
resulting list of objects provides a good foundation upon which we expand to
include other objects of interest. Using the separation vs. redshift graphs,
we identify and pick objects that are possible virialized galaxy groups or lie
at the redshift of the AGN.

##  3 Potential absorption hosts

Not all galaxies are expected to have extended hot halos that would produce
X-ray absorption lines, such as O VII resonance lines from K-alpha emission,
which should be the strongest. Halo mass calculations for an O VII column
density $\geq 10^{15}$ cm-2 peak at a stellar mass of $M_{star}\approx
10^{10.9}$ M⊙, decreasing to higher and lower stellar masses (Qu & Bregman,
2018). In the following, we choose to identify the galaxies more massive than
M(r) = -20, which is about 1.5 magnitudes below L* and corresponds to
$M_{star}\approx 10^{10.4}$ M⊙. We can convert this into a magnitude as a
function of redshift, including a K correction for a typical late-type galaxy,
which corresponds to r magnitudes of: 18.4 at z = 0.1; 20.2 at z = 0.2; 21.2
at z = 0.3; 22.0 at z = 0.4; and 22.7 at z = 0.5. When z $<0.2$, SDSS provides
photo-z values that are generally accurate and a number of spectra as well.
For z $\approx 0.3$, photo-z values become less reliable, while spectra are
lacking for all but the most luminous galaxies. However, galaxies can be
identified to r $\approx 22$ with SDSS and fainter with Pan-STARRS and DES
(magnitude limit of 23.2 - 23.5), so potential M(r) = -20 target galaxies can
be identified to z $\approx 0.4$, beyond which dedicated imaging programs
would be required.

We also can apply a characteristic search radius and use 1 Mpc, which is $\sim
5$ Rvir of an L* galaxy and about 2R500 for a galaxy group or poor cluster. At
a fixed metric radius, the angular search radii decreases until $z\sim 0.5$,
after which the angular radius changes slowly. The implication is that at
lower redshift, such as z = 0.1, the search radius is 9′ and with m(r) $<$
18.38 – the target galaxies are relatively bright and are easy to identify.
The 1 Mpc radius corresponds to 5′ at z = 0.2, 3.7′ at z = 0.3, 3.1′ at z =
0.4, 2.7′ at z = 0.5, and 2.0′ at z = 1. This leads to a decreasing search
area with increasing z, but with fainter galaxy candidates. This aspect is
included in our analysis of the images.

###  3.1 1ES 1028+511

####  3.1.1 Objects with known redshifts

(a) Figure 1a

(b) Figure 1b

Figure 1:  Cardinal directions in the figure are standard: N is up, S is down
and E is to the left in the direction of increasing Right Ascension. (a) The
box at the center marks the location of the AGN. Objects within a radius of
10′ from the AGN are marked by a circle with color corresponding to their
redshift. (b) Separation of the object from the AGN in arcminutes versus the
redshift. Dashed lines show 1 Mpc, 500 kpc. 500 kpc is about 2$R_{200}$ of a
L* galaxy while 1 Mpc is similar to $R_{200}$ of a medium mass galaxy cluster.
A vertical line indicates the redshift of the AGN.

For the objects with known redshifts surrounding 1ES 1028+511 (z=0.36), we
found four candidates for absorption. 2MASS J10312764+5052394 (Object 1:
10:31:27.6,+50:52:39) is the closest object, located 1.7′ SE from the AGN in
Figure 1 with a spectroscopic redshift of 0.36 and magnitude 21.0 (g SDSS
Model AB). It’s estimated physical distance is 0.5 Mpc with a line-of-sight
impact parameter of 795 kpc. 2MASS J10311034+5052107 (Object 2:
10:31:10.3,+50:52:11) is the next closest object with a spectroscopic redshift
of 0.14, magnitude 18.3 and separation 1.9′ SW. It has a physical distance of
less than 0.5 Mpc and an impact parameter of 339 kpc. An additional prominent
object in the field is SDSS J103108.88+504708.7 (Object 3:
10:31:08.9,+50:47:09) has a spectroscopic redshift of 0.0029, magnitude 16.3
and separation 6.6′ SW. This object has a physical distance of less than 0.5
Mpc with an impact parameter of 37.5 kpc.

####  3.1.2 Objects with unknown redshifts

Figure 2:  Potential absorption line host galaxies without known redshifts.
The angular size of the image (10′) was narrowed from the 20′ (angular size of
Figure 2a) in order to narrow the number surrounding objects from 197 to 35.

The objects with unknown redshifts surrounding 1ES 1028+511 were selected
between magnitude 19.0 and 22.0 (SDSS g) to accommodate the magnitude limits
of observational telescopes, taking in all absorbers besides objects below the
flux threshold of the survey at the redshift of the AGN (Magnitude = -19.0).
There is one object and another grouping of 4 objects shown in Figure 2 within
2.0′ of the AGN that are likely candidates. The object WISEA
J103121.30+505317.8 (Object 1: 10:31:21.31,+50:53:17.84) is 0.53′ E is closest
to the AGN with magntiude 20.0 (SDSS g).

There are also four objects (Group 2: WISEA J103129.44+505310.5, WISEA
J103129.87+505337.3, WISEA J103129.38+505406.9,

WISEA J103131.50+505330.2) located 1.8′ E of the AGN around RA 10:31:29.5 and
DEC 50:53:37.8 with magnitudes ranging from 19.9 to 20.7. There is a
possibility that these four objects are part of a virialized galaxy group.
Until redshifts are obtained for the objects in this field we will not be able
to determine the physical distance between the objects and the AGN.
Information on all objects in this field can be found in Table 3.

###  3.2 1RXS J003334.6-192130

####  3.2.1 Objects with unknown redshifts

Figure 3:

This field lies outside the SDSS footprint, resulting zero objects with known
photo-z redshift values within 10′ of 1RXS J003334.6-192130. There are 108
objects found with unknown redshift and optical magnitude. One source is WISEA
J003330.93-192108.8 (Object 1: 00:33:30.94, -19:21:9.04) located 0.87′ NW of
the AGN in Figure 3. WISEA J003328.74-192053.4 (Object 2:
00:33:28.73,-19:20:53.92) located 1.5′ NW of the AGN. The third object of
interest is WISEA J003339.61-192138.0 (Object 3: 00:33:39.65,-19:21:37.91)
with separation 1.3 E. Further observations of this field will be necessary to
locate more galaxies and finding observational redshifts to obtain a physical
distance from the AGN. Information on all objects in this field can be found
in Table 4.

###  3.3 1RXS J022716.6+0201541

####  3.3.1 Objects with known redshifts

(a) Figure 4a

(b) Figure 4b

Figure 4:  (b) There is incomplete optical photometry for these sources, so
magnitudes are not plotted in Figure 5.2.

Within a 10′ radius of the AGN, there are 12 objects selected that are
candidates for absorption in Figure 4. The physically closest objects to the
AGN include WISEA J022712.68+020111.6 (Object 1: 02:27:12.62, 02:01:11.28), at
spectroscopic redshift 0.19, separation 1.2′ SW, and physical distance of less
than 0.5 Mpc. WISEA J022707.38+020019.7 (Object 2: 02:27:7.35, 02:00:19.69) is
at spectroscopic redshift 0.19, separation 2.9′ SW and physical distance of
approximately 0.5 Mpc. These two objects may belong to a galaxy group. UGC
01923 (Object 3: 02:27:25.55, 02:06:18.11) is another ideal candidate at
spectroscopic redshift 0.0095 and 4.8′ (less than 0.5 Mpc for the redshift of
1RXS J022716.6+0201541, which is z=0.457) NE. The impact parameter for this
object is 57.4 kpc. WISEA J022720.93+020614.6 (Object 4:
02:27:20.94,+02:06:14.51) is located 4.4′ NE at a spectroscopic redshift of
0.39. It has a physical distance greater than 1 Mpc.

####  3.3.2 Objects with unknown redshifts

Figure 5:

There are 25 objects selected within a 10′ range of the AGN with unknown
redshifts, between magnitude 18.1 and 20.5. The object with the closest
angular distance to the AGN is WISEA J022716.94+015948.9 (Object 1:
02:27:16.9,+01:59:49) with magnitude 18.5 (J) located 2.2′ S of the AGN in
Figure 5. The next closest object is WISEA J022705.29+020006.7 (Object 2:
02:27:05.2,+02:00:06) at magnitude 20.4 (J) and 3.4′ SW of the AGN. Another
possible candidate for absorption is WISEA J022710.50+015645.5 (Object 3:
02:27:10.45,+01:56:45.46) at magnitude 20.4 located 5.5′ SE. Information on
all objects in this field can be found in Table 5.

###  3.4 1RXS J110337.7-232931

####  3.4.1 Objects with known redshifts

(a) Figure 6a

(b) Figure 6b

Figure 6:

Like 1RXS J003334.6-192130, this field lies outside the SDSS footprint. Two
objects were found within 10′ of the AGN which are located 2.0′ SW in Figure
6, H 1101-232G1 (Object 1: 11:03:32.5,-23:30:44) with spectroscopic redshift
0.187 with physical distance less than 0.5 Mpc. GALEXASC J110330.49-233059.6
(Object 2: 11:03:30.5,-23:30:58) has a spectroscopic redshift 0.14 and
corresponding physical distance of less than 0.5 Mpc.

####  3.4.2 Objects with unknown redshifts

Figure 7:

There are four objects that were found within 10′ of the AGN, and all lack
magnitudes. The object closest in angular separation WISEA J110335.26-233214.2
(Object 1: 11:03:35.2,-23:32:14) is located 2.8′ S of the AGN in Figure 7.
WISEA J110331.98-232617.9 (Object 2: 11:03:32.0,-23:26:18) is also close to
the AGN, located 3.5′ NW. Due to the poor survey coverage of this region, we
may need further observations so that a physical distance from the AGN can be
determined. We need followup photometry if this object is a potential target
for X-ray spectroscopy missions. Information on all objects in this field can
be found in Table 6.

###  3.5 1RXS J111706.3+201410

####  3.5.1 Objects with known redshifts

(a) Figure 8a

(b) Figure 8b

Figure 8:

10 objects were found in a search within 10′ of the AGN, all between magnitude
15.4-18.6 (SDSS g). There are 8 objects that are located at the same redshift
as the AGN (0.15) in Figure 8 (Group 1: 2MASS J11164872+2013412, 2MASX
J11173447+2015106, 2MASS J11174694+2015379, 2MASX J11173121+2015417, 2MASS
J11170667+2017158, 2MASX J11170982+2015186, 2MASX J11170842+2015296, 2MASX
J11174157+2018572). This suggests that these objects form a group with the
AGN. The objects in this potential group are at a projected physical distance
of less than 1 Mpc from the AGN. 2MASX J11172360+2014247 (Object 2:
11:17:23.6,+20:14:25) located 4.1′ E of the AGN with a photo-z redshift 0.12.
It has a physical distance of approximately 0.5 Mpc from the AGN and an impact
parameter of 633 kpc. 2MASX J11164468+2020557 (Object 3: 11:16:44.7,+20:20:56)
is located 8.5′ NW of the AGN at photo-z redshift 0.09 and an impact parameter
of 945 kpc.

####  3.5.2 Objects with unknown redshifts

Figure 9: There are more than 100 objects in this field, so the search radius
was decreased to 5′.

Within a 5′ radius of the AGN, there are 55 objects with unknown redshifts.
There is a group of 6 objects (Group 1) with an angular separation of
0.15-0.91′ E in Figure 9 between magnitude 19.9-20.0. There are a pair of
objects close to this group (Group 2), located 0.60-0.70′ N of the AGN. There
is another object (Object 3: 11:17:09.78,+20:15:01.8) of magnitude 18.0 1.2′
NE of the AGN. Information on all objects in this field can be found in Table
7.

###  3.6 1RXS J151747.3+652522

####  3.6.1 Objects with known redshifts

(a) Figure 10a

(b) Figure 10b

Figure 10:  (a) Figure 10a has an angular size of 20′, as There are no objects
with available redshifts less than or equal to 10′ from the AGN.(b) There is
incomplete optical photometry for these sources, so magnitudes are not plotted
in Figure 10b.

Five objects were found with known redshifts within 15′ of the AGN. MCG
+11-19-005 (Object 1: 15:19:21.8,+65:34:40) at spectroscopic redshift 0.044
and magnitude 13.6 is located 13.5′ NE in Figure 10, corresponding to a
physical distance of 0.5 Mpc and impact parameter of 786 kpc. WHL
J151824.2+653816 (Object 2: 15:18:24.1,+65:38:16) closest to the redshift of
the AGN at photo-z redshift 0.6582, located 13.4′ NE of the AGN and greater
than 1 Mpc. 2MASX J15175301+6523256 (Object 3: 15:17:53.07,+65:23:26.52) is
located 2.0′ SW at magnitude 16.5, physical distance of 1 Mpc, and photo-z
redshift 0.062.

####  3.6.2 Objects with unknown redshifts

Figure 11:

There are 93 objects between 16.5 and 20.9 magnitude with unknown redshifts
found within 10′ of the AGN. The closest in separation is WISEA
J151748.75+652456.8 (Object 1: 15:17:48.93,+65:24:58.0) located 0.45′ SE of
the AGN in Figure 11 at magnitude 20.4. WISEA J151810.21+652529.7 (Object 2:
15:18:10.2,+65:25:29.68) is located 2.4′ NE of the AGN at magnitude 20.3.
There are two objects (WISEA J151700.10+652618.1 and 2MASX J15165360+6525518,
Group 3) located 5.0′ W of the AGN at magnitudes 17.6 and 17.3 respectively.
Information on all objects in this field can be found in Table 8.

###  3.7 1RXS J153501.1+532042

####  3.7.1 Objects with known redshifts

(a) Figure 12a

(b) Figure 12b

Figure 12:

There are six objects with known redshifts found within 10′ of the AGN. WISEA
J153500.26+532000.7 (Object 1: 15:35:00.2,+53:20:00) is the closest object at
spectroscopic redshift 0.029 and magnitude 16.5, located 0.63′ S from the AGN
in Figure 12 corresponding to a physical distance of less than 0.5 Mpc. It has
an estimated impact parameter of 24.9 kpc. The next closest object is WISEA
J153506.70+532029.2 (Object 2: 15:35:06.7,+53:20:29) at spectroscopic redshift
0.47 and magnitude 22, with separation 0.89′ SE corresponding to less than 0.5
Mpc. It has an impact parameter of 537 kpc. SBS 1534+53 (Object 3:
15:35:29.1,+53:21:39) at spectroscopic redshift 0.29 and magnitude 18.0 is
located 4.35′ NE at approximately 1 Mpc, with an impact parameter of 1630 kpc.
WISEA J153514.10+532535.8 (Object 4: 15:35:14.1,+53:25:36) at spectroscopic
redshift 0.21 and magnitude 19.2 is located 5.36′ NE of the AGN. It is at a
physical distance of 1 Mpc with an impact parameter of 1460 kpc.

####  3.7.2 Objects with unknown redshifts

Figure 13:

There are 76 objects with unknown redshifts found within 10′ of the AGN.
Closest in separation is WISEA J153455.37+531952.6 (Object 1:
15:34:55.39,+53:19:52.61) at magnitude 19.8 located 1.1′ S of the AGN in
Figure 13. WISEA J153509.26+531948.7 (Object 2: 15:35:9.27,+53:19:48.9) at
magnitude 20.3 is located 1.5′ SE of the AGN. WISEA J153504.05+531825.2
(Object 3: 15:35:4.06,+53:18:25.16) at magnitude 20.2 is located 2.6′ S of the
AGN. WISEA J153519.25+532634.8 (Object 4: 15:35:19.24,+53:26:35.56) is the
brightest object in the field at magnitude 14.5, located 6.57′ SE of the AGN.
Information on all objects in this field can be found in Table 9.

###  3.8 3C 279

####  3.8.1 Objects with known redshifts

(a) Figure 14a

(b) Figure 14b

Figure 14:  (a) The size of the image was reduced to 10x10′ to obtain better
resolution on the nearest objects to the AGN. Six objects were cut from the
image, all located at a separation of 8′ or greater.

There are 5 objects found within 10′ of the AGN with known redshifts in Figure
14. LCRS B125355.2-053144 (Object 1: 12:56:30.5,-05:47:58) at spectroscopic
redshift 0.11 and magnitude 15.2 (2MASS J) is located 4.85′ E of the AGN,
corresponding to 0.5 Mpc. LCRS B125357.3-052850 (Object 2:
12:56:32.6,-05:45:04) at spectroscopic redshift 0.14 and magnitude 15.8 is
5.02′ NE of the AGN. This corresponds to a physical distance of less than 1
Mpc and an impact parameter of 10420kpc. LCRS B125317.3-053305 (Object 3:
12:55:52.6,-05:49:20) at spectroscopic redshift 0.11 has angular separation
5.0′ SW and magnitude 15.3, and projected physical distance of around 0.5 Mpc.
This object has an impact parameter of 697 kpc. LCRS B125300.5-053604 (Object
4: 12:55:35.8,-05:52:19) at spectroscopic redshift 0.09 is separated 10′ SW of
the AGN at magnitude 15.0. This correspons to a physical ditance of less than
0.5 Mpc.

####  3.8.2 Objects with unknown redshifts

Figure 15:

There are 33 objects found within 10′ of the AGN with unknown redshifts. The
closest in separation is WISEA J125611.81-054535.6 (Object 1:
12:56:11.86,-05:45:35.75) at magnitude 19.8, located 1.8′ N of the AGN in
Figure 15. Next closest is WISEA J125609.77-054458.7 (Object 2:
12:56:9.59,-05:44:56.98) at magnitude 19.55 located 1.8′ N. A possible group
of two closely separated objects is found approximately 7.0′ SE of the AGN
(Object 3). Finally, there is another possible group of three objects (Object
4) located 7.7′ E of the AGN. Information on all objects in this field can be
found in Table 10.

###  3.9 3C 454.3

####  3.9.1 Objects with known redshifts

(a) Figure 16a

(b) Figure 16b

Figure 16:  Angular size of the figure was reduced from 20′ to 10′ to reduce
the number of objects.

There are 33 objects found within 6′ of the AGN with known redshifts in Figure
16. SSTSL2 J225400.41+160906.4 (Object 1: 22:54:0.5,+16:09:6.01) at
spectroscopic redshift 0.35 and magnitude 21.6 (CFHT R) is located 0.693′ E of
the AGN, corresponding to a physical distance less than 0.5 Mpc. SSTSL2
J225401.86+160939.6 (Object 2: 22:54:1.9,+16:09:38.99) at spectroscopic
redshift 0.42 and magnitude 21.1 is 1.25′ NE of the AGN, with a physical
distance close to 0.5 Mpc. 2MASX J22535061+1609434 (Object 3:
22:53:50.6,+16,09,43.6) is the lowest redshift object in the field, at
specroscopic redshift 0.10 and magnitude 18.7. It is located 1.9′ NW of the
AGN at a physical distance less than 0.5 Mpc, with an impact parameter of 244
kpc. SSTSL2 J225352.98+160843.9 (Object 4: 22:53:53.1,+16:08:43.01) at
spectroscopic redshift 0.157 and magnitude 20.5 is located 1.1′ W of the AGN
with physical distance less than 0.5 Mpc. Lastly, SSTSL2 J225353.90+160858.4
(Object 5: 22:53:53.9,+16:08:57.98) at spectroscopic redshift 0.823 and
magnitude 22.1 is separated 0.93′ W, with a physical distance of about 0.5
Mpc.

####  3.9.2 Objects with unknown redshifts

Figure 17:

There are a total of 2,190 objects found within 10′ including unclassified
gamma-ray, x-ray, infrared and visual sources. When the list was narrowed down
to only include galaxies and visual sources, the result was 18 objects of
interest within 10′ of the AGN. The first object is an unclassified visual
source 3C 454.3:[KSS94] 28 (Object 1: 22:53:57.8,+16:08:39.01) at magnitude
23.1 is located 0.243′ S of the AGN in Figure 17. WISEA J225359.94+160953.9
(Object 2: 22:54:0.0,+16:09:52.99) at magnitude 16.2 is located 1.13′ NE of
the AGN. WISEA J225355.89+160938.9 (Object 3: 22:53:55.9,+16:09:38.02) at
magnitude 21.0 is located 0.86′ NW of the AGN. Information on all objects in
this field can be found in Table 11.

###  3.10 H2356-309

####  3.10.1 Objects with known redshifts

(a) Figure 18a

(b) Figure 18b

Figure 18:

There are a total of 24 objects found within 10′ of the AGN. WISEA
J235910.71-303720.6 (Object 1: 23:59:10.73,-30:37:21.61) at spectroscopic
redshift 0.062 and magnitude 18.7 is located 0.68′ NE of the AGN in Figure 18,
corresponding to a physical distance of less than 0.5 Mpc. WISEA
J235910.27-303514.7 (Object 2:23:59:10.32,-30:35:15.61) at spectroscopic
redshift 0.029 and magnitude 17.5 is 2.47′ N, also at a physical distance of
less than 0.5 Mpc. WISEA J235854.35-303214.9 (Object 3:
23:58:54.35,-30:32:15.9) at spectroscopic redshift 0.165 and magnitude 19.5 is
located 6.15′ NW and at a physical distance of approximately 1 Mpc. 6dF
J2359377-303921 (Object 4: 23:59:37.8,-30:39:20.92) at spectroscopic redshift
0.030 and magnitude 14.1 (SDSS r) is located 6.64′ SE of the AGN. It has a
physical distance of less than 0.5 Mpc.

####  3.10.2 Objects with unknown redshifts

Figure 19:

There are a total of 99 objects with unavailable redshifts found within 10′ of
the AGN. WISEA J235913.66-303645.8 (Object 1: 23:59:13.67,-30:36:45.97) at
magnitude 20.5 is located 1.54′ NE in Figure 19. WISEA J235859.11-303557.9
(Object 2: 23:58:59.14,-30:35:58.09) at magnitude 19.9 is 2.546′ NW of the
AGN. WISEA J235903.56-303906.3 (Object 3: 23:59:3.71,-30:39:8.28) at magnitude
19.4 is located 1.72′ SW. WISEA J235908.04-303513.8 (Object 4:
23:59:8.04,-30:35:13.99) at magnitude 18.3 (SDSS r) is 2.44′ N. Information on
all objects in this field can be found in Table 12.

###  3.11 S5 0836+71

####  3.11.1 Objects with known redshifts

(a) Figure 20a

(b) Figure 20b

Figure 20:

There are 31 objects with known redshifts found within 10′ of the AGN. There
is a large grouping of at least 22 galaxies at redshift 0.24 located
approximately 6′ S of the AGN (Group 1) in Figure 20, greater than 1 Mpc in
projected physical distance. UGC 04522 (Object 2: 08:42:26.6,+70:58:05) at
spectroscopic redshift 0.015, physical distance less than 0.5 Mpc, and
magnitude 14.80 is located 6.7′ NE and has an impact parameter of 133 kpc.

####  3.11.2 Objects with unknown redshifts

Figure 21:  Angular size of the image was reduced from 20′ to 15′ due to there
being more than 100 objects returned in a 10′ search radius.

There are a total of 10 objects with unavailable redshifts found within 10′.
WISEA J084140.99+705210.4 (Object 1: 08:41:40.84,+70:52:9.98) at magnitude
18.9 is located 2.04′ SE of the AGN in Figure 21. WISEA J084047.07+705544.2
(Object 2: 08:40:46.95,+70:55:45.01) at magnitude 19.24 is located 3.68′ NW.
WISEA J084216.83+705148.6 and WISEA J084030.58+705117.9 (Group 3) at
magnitudes 19.3 and 18.3 respectively are located 5.0′ SE. Information on all
objects in this field can be found in Table 13.

###  3.12 Ton 116

####  3.12.1 Objects with known redshifts

(a) Figure 22a

(b) Figure 22b

Figure 22:

There are 22 objects with known redshifts found within 10′ of the AGN. WISEA
J124316.96+362713.6 (Object 1: 12:43:16.97,+36:27:13.79) at spectroscopic
redshift 0.485, physical distance less than 0.5 Mpc, and magnitude 21.3 (SDSS
g) is located 0.99′ SE of the AGN in Figure 22. This corresponds to an impact
parameter of 618 kpc. WISEA J124248.22+362517.6 (Object 2:
12:42:48.24,+36:25:17.72) at spectroscopic redshift 0.128 and magnitude 17.8
(SDSS g) is located 5.5′ SW, corresponding to a physical distance less than 1
Mpc and an impact parameter of 915 kpc. WISEA J124302.14+362213.4 (0bject 3:
12:43:2.18,+36:22:13.69) at spectroscopic redshift 0.452, physical distance
greater than 1 Mpc, and magnitude 21.5 (SDSS g) is located 5.9′ S. This object
has an impact parameter of 3440 kpc. WISEA J124305.07+362144.1 (Object 4:
12:43:5.08,+36:21:44.17) at spectroscopic redshift 0.158 and magnitude 18.2
(SDSS g) is located 6.19′ S of the AGN corresponding to a physical distance of
approximately 1 Mpc. This object has an impact parameter of 1270 kpc.

####  3.12.2 Objects with unknown redshifts

Figure 23:

There are 100 objects with unavailable redshifts within 10′ of the AGN.
Redshifts for more of these objects would be vital, as there are several
bright objects in the field that could be possible absorption hosts besides
the 5 objects indicated in Figure 23. WISEA J124314.74+362752.6 (Object 1:
12:43:14.7,+36:27:53.35) at magnitude 20.6 is located 0.424′ E of the AGN.
WISEA J124321.34+362810.8 (Object 2: 12:43:21.33,+36:28:11.03) at magnitude
20.8 is located 1.79′ E of the AGN. WISEA J124321.71+362839.8 (Object 3:
12:43:21.72,+36:28:39.9) at magnitude 20.8 is located 2.03′ NE of the AGN.
WISEA J124315.69+362952.9 (Object 4: 12:43:15.72,+36:29:53.27) at magnitude
19.6 is located 2.23′ N of the AGN. WISEA J124305.68+362654.3 (Object 5:
12:43:5.7,+36:26:54.2) at magnitude 20.8 is located 1.64′ SW. Information on
all objects in this field can be found in Table 14.

###  3.13 1RXS J122121.7+301041

####  3.13.1 Objects with known redshifts

(a) Figure 24a

(b) Figure 24b

Figure 24:

Within a radius of 10′ around the AGN, There are 11 sources identified with
available redshifts in Figure 24. The closest to the AGN is WISEA
J122134.81+301123.1 (Object 1: 12:21:34.84,+30:11:23.39) at spectroscopic
redshift 0.060, physical distance less than 0.5 Mpc, and with magnitude 18.3
(SDSS g) located 3.0′ NE. This corresponds to a physical distance of less than
0.5 Mpc and an impact parameter of 230 kpc. 3.6′ NE of the AGN is WISEA
J122137.29+301211.7 (Object 2: 12:21:37.29,+30:12:11.77) also at spectroscopic
redshift 0.06 and with magnitude 18. This corresponds to a physical distance
less than 0.5 Mpc and an impact parameter of 291 kpc. WISEA
J122138.25+301405.3 (Object 3: 12:21:38.25,+30:14:5.5) at spectroscopic
redshift 0.06 and magnitude 17.7 is 4.9′ NE of the AGN. This object is at a
physical distance of less than 0.5 Mpc and has an impact parameter of 394 kpc.
These three objects are all at the same redshift, so it is possible that they
comprise a galaxy group. SDSS J122053.73+300947.2 (Object 4:
12:20:53.73,+30:09:47.2) is at spectroscopic redshift 0.04 and magnitude 17.7,
located 6.1′ W. This corresponds to a physical distance of less than 0.5 Mpc.
There are also two objects at a redshift of 0.08, the closest in angular
separation being WISEA J122139.10+300356.7 (Object 5:
12:21:39.11,+30:03:56.88). The object is at magnitude 17.9 and 7.6′ SE iwth an
impact parameter of 818 kpc and physical distance less than 1 Mpc.

####  3.13.2 Objects with unknown redshifts

Figure 25:

There are a total of 108 objects below 21 mag found within 10′ of the AGN.
WISEA J122122.35+301021.1 (Object 1: 12:21:22.34,+30:10:20.78) at magnitude
20.6 (SDSS g) has the least angular separation of 0.28′ SE in Figure 25. WISEA
J122125.49+300932.7 (Object 2: 12:21:25.49,+30:09:32.87) is at magnitude 19.6
and separation 1.3′ SE. WISEA J122123.38+300853.6 (Object 3:
12:21:23.39,+30:08:53.7) is at magnitude 19.2 and separation 1.8′ S. WISEA
J122134.44+301149.1 (Object 4: 12:21:34.48,+30:11:49.13) at magnitude 18.5 is
located 2.9′ NE from the AGN. Information on all objects in this field can be
found in Table 15.

###  3.14 1RXS J142239.1+580159

####  3.14.1 Objects with known redshifts

(a) Figure 26a

(b) Figure 26b

Figure 26:

Within a radius of 10′ around the AGN, we found 17 objects. WISEA
J142325.43+575923.7 (Object 1: 14:23:25.43,+57:59:23.5) at spectroscopic
redshift 0.044 and magnitude 17.1 (SDSS g) has an angular separation of 6.7′
SE of the AGN in Figure 26, and a corresponding physical distance of around
0.5 Mpc. This object has an impact parameter of 388 kpc. The next candidate,
SDSS J142214.68+580610.9 (Object 2: 14:22:14.69,+58:06:10.94) at spectroscopic
redshift 0.18 and magnitude 20.3 is 5.3′ NW. This corresponds to a physical
distance of 1 Mpc. WISEA J142300.36+580856.9 (Object 3:
14:23:0.43,+58:08:56.94) at spectroscopic redshift 0.36 and magnitude 17.0 is
7.6′ N. This object has a physical distance greater than 1 Mpc. Finally, WISEA
J142212.41+580537.1 (Object 4: 14:22:12.47,+58:05:37.64) is at spectroscopic
redshift 0.51 and magnitude 21.1 and 5.1 ′ NW. This corresponds to a physical
distance greater than 1 Mpc and an impact parameter of 1940 kpc.

####  3.14.2 Objects with unknown redshifts

Figure 27:

We found 66 objects below 21 mag (SDSS g). The first object is SDSS
J142237.38+580036.1 (Object 1: 14:22:37.38,+58:00:36.14) at magnitude 20 and
separation 1.3′ S in Figure 27. WISEA J142251.82+580324.2 (Object 2:
14:22:51.85,+58:03:24.19) is at magnitude 19.1 and is 2.2′ NE. WISEA
J142245.79+580026.4 (Object 3: 14:22:45.78,+58:00:26.28) is at magnitude 20.8
and located 1.7′ SE. WISEA J142222.64+580122.1 (Object 4:
14:22:22.7,+58:01:21.18) is at magnitude 20.5 and 2.2′ SW. Information on all
objects in this field can be found in Table 16.

###  3.15 1RXS J150759.8+041511

####  3.15.1 Objects with known redshifts

(a) Figure 28a

(b) Figure 28b

Figure 28:

Within a radius of 10′ around the AGN, we found 30 objects. WISEA
J150800.69+041332.0 (Object 1: 15:08:0.7,+04:13:32.02) at spectroscopic
redshift 0.14 and magnitude 18.5 (SDSS g) is located 1.7′ S of the AGN in
Figure 28. This corresponds to a physical distance less than 0.5 Mpc and an
impact parameter of 307 kpc. WISEA J150756.46+041701.4 (Object 2:
15:07:56.45,+04:17:1.64) is at spectroscopic redshift 0.78 and magnitude 20.0
is 2.0′ N. This object has a physical distance less than 1 Mpc and an impact
parameter of 1010 kpc. There appears to be a grouping of 5 galaxies at
redshifts of 0.13-0.16 (Group 3) about 9′ NE. WISEA J150754.93+041834.7
(Object 4: 15:07:54.94,+04:18:34.7) has a spectroscopic redshift 0.081 and
magnitude 17.8 is 3.6′ N of the AGN. This corresponds to a physical distance
of less than 0.5 Mpc and an impact parameter of 383 kpc.

####  3.15.2 Objects with unknown redshifts

Figure 29:

We found 133 objects below 21 mag (SDSS g). First is SDSS J150755.09+041414.1
(Object 1: 15:07:55.09,+04:14:14.14) at magnitude 19 and separation 1.5′ S in
Figure 29. WISEA J150804.48+041420.7 (Object 2: 15:08:4.51,+04:14:21.05) is at
magnitude 20.0 and 1.4′ SE. WISEA J150801.68+041328.6 (Object 3:
15:08:1.69,+04:13:28.7) at magnitude 20.8 has a separation of 1.8′ SE of the
AGN. Information on all objects in this field can be found in Table 17.

Table 2: Target Fields Needing Deeper Imaging and Spectroscopy

AGN | RA (h:m:s) | DEC (d:m:s) | Redshift | No. known | No. unknown |  $<$ 0.5 Mpc
---|---|---|---|---|---|---
1RXS J003334.6-192130 | 00:33:34.200 | -19:21:33.30 | 0.610 | 0 | 108 | 0
1RXS J022716.6+020154 | 02:27:16.580 | +02:02:00.50 | 0.457 | 12 | 25 | 2
S5 0836+71 | 08:41:24.3652 | +70:53:42.173 | 2.172 | 31 | 10 | 2
1ES 1028+511 | 10:31:18.518 | +50:53:35.82 | 0.360 | 5 | 35 | 3
1RXS J110337.7-232931 | 11:03:37.610 | -23:29:31.20 | 0.186 | 2 | 4 | 2
1RXS J111706.3+201410 | 11:17:06.260 | +20:14:07.40 | 0.139 | 10 | 55 | 4
1RXS J122121.7+301041 | 12:21:21.941 | +30:10:37.11 | 0.184 | 11 | 108 | 6
Ton 116 | 12:43:12.7362 | +36:27:43.999 | 1.065 | 22 | 100 | 1
3C 279 | 12:56:11.1665 | -05:47:21.523 | 0.536 | 5 | 33 | 2
1RXS J142239.1+580159 | 14:22:38.895 | +58:01:55.50 | 0.635 | 17 | 66 | 3
1RXS J150759.8+041511 | 15:07:59.7324 | +04:15:11.984 | 1.703 | 30 | 133 | 8
1RXS J151747.3+652522 | 15:17:47.600 | +65:25:23.90 | 0.702 | 4 | 93 | 0
1RXS J153501.1+532042 | 15:35:00.801 | +53:20:37.33 | 0.890 | 6 | 76 | 2
3C 454.3 | 22:53:57.7479 | +16:08:53.560 | 0.859 | 33 | 18 | 9

Note. — No. known refers to the number of objects in the field with available
redshifts and No. unknown refers to the number without available redshifts.
$<$ 0.5 Mpc refers to the number of objects with known redshifts at a physical
distance less than 0.5 Mpc. This column, in addition to merit in Table 1, is
meant to guide observers while prioritizing which background targets to focus
on.

##  4 Discussion

The lists of identified objects become incomplete when $z\gtrsim 0.4$ due to
the magnitude limit several surveys used such as the SDSS (22.0), when
considering target galaxies brighter than 0.25L*; this applies to 10/19
fields. Imaging and spectroscopy of the 10 fields around $z>0.4$ AGN should be
followed up with deeper imaging plus spectroscopy, which is possible with 8m
class telescopes. The number of objects that need spectroscopy per field makes
this an ideal project for multi-fiber spectrographs that have become common.
We identify fields that need followup imaging and spectroscopy in Table 2.

The results of this study can be applied to guide the process of matching
galaxies and groups to their relevant absorption lines. To narrow possible
matches, we can take velocity measurements of likely absorption hosts to
compare with the centroid velocity of the line. If the values are within 1-2
typical rotation curve velocities, we conclude that the hot halo material of
the object caused the line. There are probably ways to distinguish between
galaxies and groups, such as from ion ratios and column densities.

From the measured absorption line systems, we can extract a variety of
physical properties about the hot gaseous component of galaxy halos and the
intergalactic medium (Bregman et al., 2015). We can distinguish between
density profiles of galaxies (i.e. NFW, $\beta=1/2$, flat), determine the
temperature from relative strengths of O VII and O VIII lines, and use
temperature to determine the metallicity of the X-ray absorbing material. It
is also important that we obtain estimates for the overall gas mass and
dynamics of hot gaseous halos. These measurements might help to narrow the
exact distribution of baryons in the universe, either verifying or ruling out
existing models. Measurements of gas distribution from these absorption line
studies will provide constraints on galaxy structure formation models as well.

For an accurate measurement of these properties, it will be necessary to
achieve greater sensitivity in X-ray spectroscopic technology. The Arcus
satellite is dedicated for such spectroscopy, with a proposed spectral
resolution of $R\approx 3000$ and an effective area of $A_{eff}\approx 300$
cm2 (Smith, 2020), compared to Chandra with $R\approx 500$ and $A_{eff}\approx
3.3$ cm2 and XMM with $R\approx 420$ and $A_{eff}\approx 45$ cm2. This meets
the sensitivity requirements to effectively determine gas dynamics and
relative distances to absorbers. If approved, Arcus would begin observations
late in the 2020-30 decade. The Athena X-ray Observatory (Barcons et al.,
2017) is under development with a planned launch date in the early 2030s. For
spectroscopy, it has a microcalorimeter (X-IFU) with a resolution of about 300
(2-2.5 eV; 1000 km s-1) at low redshift for the O VII and O VIII lines.
Although this is an order of magnitude less than the Arcus specifications, it
has a collecting area of $A_{eff}\approx 1.6$ m2, so the detectability of
lines is similar but at a loss of some velocity information. The Lynx mission,
if chosen, would fly probably in the early 2040s, and carries a higher
resolution grating than Arcus, with an expected resolution of about 5000
(Gaskin et al., 2019) and a proposed collecting area of $A_{eff}=2$ m2.

Until the eventual launch of an advanced X-ray spectroscopic instrument, there
is work that still needs to be done in preparation. It will be important to
obtain the redshift and properties of potential galaxy hosts. For the lower
redshift systems, $z<0.3$, this can be accomplished with 2-4 m class
telescopes, but for more distant targets, 8 m class telescopes will be
required. The priority redshift range is $z<1.3$, because at larger redshifts,
the O VII resonance line is redshifted below the Galactic absorption cutoff.

##  5 Conclusion

In this paper, we attempt to advance our strategy for detecting the missing
baryons in hot X-ray absorbing material. By using NED data, and public optical
DSS images, we produced separation vs. redshift plots, from which we
identified the most likely absorbers in several AGN fields that X-ray
observatories are likely to cover. From the results of our study, we determine
critical steps for completing mission preparation and observing missing
baryons:

• We need further optical and spectroscopic coverage of the fields we
identified. This will give us enough accuracy to recognize the best AGN
targets by the likelihood of observing absorption line systems in their
spectra.

• We can benefit from improving the precision in our technique for matching
absorption lines to their respective hosts. One might gain improvements by
comparing predictions from large scale structure simulations (e.g.,
Oppenheimer 2018, and testing it against the highest ionization UV lines, such
as C IV and O VI.

The most likely absorption hosts identified here probably do not encompass
every object that will cause absorption. There also may be a more detailed set
of criteria, and when we have more complete optical/redshift coverage for
objects this will be better determined.

The future of X-ray absorption studies is arriving soon with high-resolution
spectroscopy. Preparing for this future is crucial for the success of these
missions and for understanding the hot gaseous component of the universe.

##  6 Acknowledgements

We thank the individuals who offered encouragement and insight, including
Chris Miller, Sean Johnson, Mario Mateo, Laura Brennerman, Andrew Ptak, and
Randall Smith. We gratefully acknowledge support from NASA through the
Astrophysics Data Analysis Program, awards NNX15AM93G and 80NSSC19K1013, and
from the University of Michigan. This research has made use of the NASA/IPAC
Extragalactic Database (NED), which is operated by the Jet Propulsion
Laboratory, California Institute of Technology, under contract with NASA. This
research has made use of the SIMBAD database and the Aladin sky atlas, which
were produced and is maintained by the Centre de Données astronomiques de
Strasbourg, France. We also acknowledge the use of several important sky
surveys, which were produced with generous support from four US Federal
agencies, more than a dozen organizations in eight countries, and two private
foundations. Specifically, these surveys include 2MASS, WISE, SDSS, Pan-
STARRS, the Dark Energy Survey, and the VLT-ATLAS.

##  Appendix A Tables

Table 3: Objects within 10′ of 1ES 1028+511, sorted from least to greatest
angular separation.

Object | RA (h:m:s) | DEC (d:m:s) | Redshift | Type | App. mag | Sep. (′) | Impact parameter (kpc)
---|---|---|---|---|---|---|---
WISEA J103121.30+505317.8 | 10:31:21.31 | +50:53:17.84 |  |  | 0.533 | 20 |
WISEA J103113.72+505359.2 | 10:31:13.72 | +50:53:59.46 |  |  | 0.853 | 21.9 |
WISEA J103112.90+505248.8 | 10:31:12.9 | +50:52:49.08 |  |  | 1.179 | 21.2 |
WISEA J103112.41+505419.4 | 10:31:12.45 | +50:54:19.87 |  |  | 1.205 | 21.1 |
SDSS J103113.45+505229.9 | 10:31:13.46 | +50:52:29.93 |  |  | 1.357 | 20.6 |
WISEA J103108.50+505324.2 | 10:31:8.53 | +50:53:25.3 |  |  | 1.585 | 20.7 |
2MASS J10312764+5052394 | 10:31:27.6 | +50:52:39 | 0.359698 | Spec | 1.717 | 21 | 795
WISEA J103129.44+505310.5 | 10:31:29.42 | +50:53:10.39 |  |  | 1.77 | 21.9 |
WISEA J103129.87+505337.3 | 10:31:29.86 | +50:53:37.79 |  |  | 1.789 | 19.9 |
WISEA J103129.38+505406.9 | 10:31:29.41 | +50:54:6.91 |  |  | 1.794 | 20.7 |

Note. — Table 3 is published in its entirety in the machine-readable format.
We give a sample of 10 objects here.

Table 4: Objects within 10′ of 1RXS J003334.6-192130, sorted from least to
greatest angular separation.

Object | RA (h:m:s) | DEC (d:m:s) | Redshift | Type | App. mag | Sep. (′) | Impact parameter (kpc)
---|---|---|---|---|---|---|---
WISEA J003330.93-192108.8 | 00:33:30.94 | -19:21:9.04 |  |  | 0.87 | 19.29 |
WISEA J003339.61-192138.0 | 00:33:39.65 | -19:21:37.91 |  |  | 1.287 | 19.8 |
WISEA J003328.74-192053.4 | 00:33:28.73 | -19:20:53.92 |  |  | 1.448 | 19.5 |
WISEA J003340.18-192238.1 | 00:33:40.23 | -19:22:38.42 |  |  | 1.79 | 19.85 |
WISEA J003339.69-191952.6 | 00:33:39.73 | -19:19:53.44 |  |  | 2.115 | 19.5 |
WISEA J003341.33-192011.9 | 00:33:41.34 | -19:20:11.54 |  |  | 2.166 | 19.51 |
WISEA J003343.67-192104.4 | 00:33:43.72 | -19:21:4.18 |  |  | 2.298 | 19.83 |
WISEA J003326.44-191951.1 | 00:33:26.45 | -19:19:51.1 |  |  | 2.499 | 19.1 |
WISEA J003323.91-192008.9 | 00:33:23.88 | -19:20:9.46 |  |  | 2.807 | 19.73 |
WISEA J003337.85-191847.2 | 00:33:37.96 | -19:18:47.16 |  |  | 2.908 | 19.9 |

Note. — Table 4 is published in its entirety in the machine-readable format.
We give a sample of 10 objects here. This field is outside the SDSS footprint,
with a redshift close to the spectroscopic limit of $\approx 0.5$. These
factors make it a primary candidate for deeper spectroscopy and photometry.

Table 5: Objects within 10′ of 1RXS J022716.6+0201541, sorted from least to
greatest angular separation.

Object | RA (h:m:s) | DEC (d:m:s) | Redshift | Type | App. mag | Sep. (′) | Impact parameter (kpc)
---|---|---|---|---|---|---|---
WISEA J022712.68+020111.6 | 02:27:12.62 | 02:01:11.28 | 0.188566 | Spec | 19.45 | 1.286 |
WISEA J022716.94+015948.9 | 02:27:16.93 | 01:59:48.98 |  |  | 18.5 | 2.193 |
WISEA J022707.38+020019.7 | 02:27:7.35 | 02:00:19.69 | 0.188994 | Spec | 19.15 | 2.853 |
WISEA J022705.29+020006.7 | 02:27:5.22 | 02:00:6.08 |  |  | 20.37 | 3.419 |
WISEA J022720.93+020614.6 | 02:27:20.94 | 02:06:14.51 | 0.387021 | Spec | 16.907 | 4.371 |
APMUKS(BJ) B022432.88+014436.3 | 02:27:7.92 | 01:58:2.39 |  |  | 20.49 | 4.52 |
UGC 01923 | 02:27:25.55 | 02:06:18.11 | 0.009593 | Spec | 17 | 4.843 | 57.4
WISEA J022710.50+015645.5 | 02:27:10.45 | 01:56:45.46 |  |  | 20.4 | 5.469 |
WISEA J022652.73+015937.0 | 02:26:52.73 | 01:59:37.1 | 0.362079 | Spec |  | 6.421 |
WISEA J022739.31+015825.3 | 02:27:39.28 | 01:58:25.82 |  |  | 19.32 | 6.705 |

Note. — Table 5 is published in its entirety in the machine-readable format.
We give a sample of 10 objects here. This source lies outside the SDSS
footprint, and the surrounding objects are not well documented. In addition to
the field described in Table 4, this is another prime candidate for deeper
imaging.

Table 6: Objects within 10′ of 1RXS J110337.7-232931, sorted from least to
greatest angular separation.

Object | RA (h:m:s) | DEC (d:m:s) | Redshift | Type | App. mag | Sep. (′) | Impact parameter (kpc)
---|---|---|---|---|---|---|---
H 1101-232G1 | 11:03:32.5 | -23:30:44 | 0.187 | Spec | 18.1 | 1.675 |
GALEXASC J110330.49-233059.6 | 11:03:30.5 | -23:30:58 | 0.14 | Spec | 21.34 | 2.168 |
WISEA J110335.26-233214.2 | 11:03:35.2 | -23:32:14 |  |  |  | 2.774 |
WISEA J110331.98-232617.9 | 11:03:32.0 | -23:26:18 |  |  |  | 3.471 |
WISEA J110322.59-233521.1 | 11:03:22.6 | -23:35:21 |  |  |  | 6.776 |
WISEA J110330.22-233635.2 | 11:03:30.2 | -23:36:35 |  |  |  | 7.272 |

Note. — Table 6 is published in its entirety in the machine-readable format.
We give a sample of 10 objects here. This source lies outside the SDSS
footprint, and the surrounding objects are not well documented. In addition to
the field described in Table 4, this is another prime candidate for deeper
imaging.

Table 7: Objects within 10′ of 1RXS J111706.3+201410, sorted from least to
greatest angular separation.

Object | RA (h:m:s) | DEC (d:m:s) | Redshift | Type | App. mag | Sep. (′) | Impact parameter (kpc)
---|---|---|---|---|---|---|---
2MASS J11170682+2014039 | 11:17:6.83 | 20:14:3.59 |  |  | 19.9 | 0.148 |
SDSS J111705.91+201417.5 | 11:17:5.92 | 20:14:17.52 |  |  | 20.4 | 0.187 |
WISEA J111705.49+201412.9 | 11:17:5.48 | 20:14:11.58 |  |  | 19.1 | 0.195 |
WISEA J111708.46+201401.7 | 11:17:8.48 | 20:14:1.68 |  |  | 20.7 | 0.529 |
SDSS J111705.25+201440.8 | 11:17:5.26 | 20:14:40.85 |  |  | 20.3 | 0.605 |
WISEA J111704.66+201442.8 | 11:17:4.63 | 20:14:42.83 |  |  | 19.7 | 0.703 |
WISEA J111710.11+201401.1 | 11:17:10.11 | 20:14:0.96 |  |  | 20.8 | 0.909 |
WISEA J111709.83+201337.8 | 11:17:9.86 | 20:13:37.78 |  |  | 21 | 0.977 |
WISEA J111710.91+201355.3 | 11:17:10.91 | 20:13:55.02 |  |  | 19.5 | 1.109 |
SDSS J111711.15+201359.2 | 11:17:11.16 | 20:13:59.3 |  |  | 20.5 | 1.157 |

Note. — Table 7 is published in its entirety in the machine-readable format.
We give a sample of 10 objects here. The objects in this field have mostly
photo-z redshift values, so spectroscopic followup will be necessary.

Table 8: Objects within 15′ of 1RXS J151747.3+652522, sorted from least to
greatest angular separation.

Object | RA (h:m:s) | DEC (d:m:s) | Redshift | Type | App. mag | Sep. (′) | Impact parameter (kpc)
---|---|---|---|---|---|---|---
WISEA J151748.75+652456.8 | 15:17:48.93 | 65:24:58.0 |  |  | 20.40 | 0.454 |
WISEA J151754.35+652342.2 | 15:17:54.37 | 65:23:42.22 |  |  | 20.80 | 1.835 |
SDSS J151729.52+652543.6 | 15:17:29.52 | 65:25:43.68 |  |  | 20.40 | 1.908 |
2MASX J15175301+6523256 | 15:17:53.07 | 65:23:26.52 | 0.061792 | Phot | 16.47 | 2.038 |
WISEA J151759.90+652348.8 | 15:17:59.96 | 65:23:48.84 |  |  | 20.30 | 2.041 |
WISEA J151810.21+652529.7 | 15:18:10.2 | 65:25:29.68 |  |  | 20.30 | 2.352 |
WISEA J151721.63+652409.8 | 15:17:21.8 | 65:24:10.94 |  |  | 20.90 | 2.946 |
SDSS J151813.00+652658.9 | 15:18:13.01 | 65:26:58.96 |  |  | 20.10 | 3.079 |
WISEA J151812.77+652706.4 | 15:18:12.72 | 65:27:6.41 |  |  | 20.40 | 3.12 |

Note. — Table 8 is published in its entirety in the machine-readable format.
We give a sample of 10 objects here. The only objects with available redshift
values lie outside a 10′ radius of the source. Spectroscopic followup with
objects in this field within 10′ are necessary to complete this field.

Table 9: Objects within 10′ of 1RXS J153501.1+532042, sorted from least to
greatest angular separation.

Object | RA (h:m:s) | DEC (d:m:s) | Redshift | Type | App. mag | Sep. (′) | Impact parameter (kpc)
---|---|---|---|---|---|---|---
WISEA J153500.26+532000.7 | 15:35:00.2 | +53:20:00 | 0.0293361 | Spec | 16.5 | 0.631 | 24.895
WISEA J153506.70+532029.2 | 15:35:06.7 | +53:20:29 | 0.466693 | Spec | 22 | 0.893 | 537
WISEA J153455.37+531952.6 | 15:34:55.39 | 53:19:52.61 |  |  | 19.8 | 1.1 |
WISEA J153509.26+531948.7 | 15:35:9.27 | 53:19:48.9 |  |  | 20.3 | 1.5 |
WISEA J153504.05+531825.2 | 15:35:4.06 | 53:18:25.16 |  |  | 20.2 | 2.256 |
WISEA J153515.89+532051.4 | 15:35:15.91 | 53:20:51.54 |  |  | 20.7 | 2.267 |
SDSS J153447.51+531914.6 | 15:34:47.51 | 53:19:14.7 |  |  | 19.6 | 2.415 |
WISEA J153458.46+532305.5 | 15:34:58.44 | 53:23:5.32 |  |  | 20.7 | 2.491 |
WISEA J153456.60+531813.1 | 15:34:56.58 | 53:18:12.46 |  |  | 19.9 | 2.495 |
SDSS J153510.47+531810.8 | 15:35:10.48 | 53:18:10.84 |  |  | 20.7 | 2.837 |

Note. — Table 9 is published in its entirety in the machine-readable format.
We give a sample of 10 objects here.

Table 10: Objects within 10′ of 3C 279, sorted from least to greatest angular
separation.

Object | RA (h:m:s) | DEC (d:m:s) | Redshift | Type | App. mag | Sep. (′) | Impact parameter (kpc)
---|---|---|---|---|---|---|---
WISEA J125611.81-054535.6 | 12:56:11.86 | -05:45:35.75 |  |  | 19.77 | 1.771 |
WISEA J125609.77-054458.7 | 12:56:9.59 | -05:44:56.98 |  |  | 19.55 | 2.441 |
LCRS B125323.5-052906 | 12:55:58.73 | -05:45:21.56 |  |  | 17.94 | 3.684 |
WISEA J125607.81-055115.7 | 12:56:7.85 | -05:51:15.73 |  |  | 19.77 | 3.99 |
LCRS B125328.2-053447 | 12:56:3.6 | -05:51:1.19 |  |  | 17.99 | 4.117 |
WISEA J125617.41-055117.2 | 12:56:17.42 | -05:51:17.46 |  |  | 19.2 | 4.229 |
LCRS B125328.0-052716 | 12:56:3.3 | -05:43:30.5 |  |  | 18.44 | 4.319 |
WISEA J125611.51-055206.2 | 12:56:11.52 | -05:52:6.35 |  |  | 20.36 | 4.748 |
WISEA J125557.93-054347.6 | 12:55:57.94 | -05:43:48.11 |  |  | 20.09 | 4.845 |
LCRS B125355.2-053144 | 12:56:30.5 | -05 47 58 | 0.106774 | Spec | 15.242 | 4.847 |

Note. — Table 10 is published in its entirety in the machine-readable format.
We give a sample of 10 objects here. This field lies outside the SDSS
footprint has poor spectroscopic coverage. Deeper imaging is needed to
complete the information about this field

Table 11: Objects within 10′ of 3C 454.3, sorted from least to greatest
angular separation.

Object | RA (h:m:s) | DEC (d:m:s) | Redshift | Type | App. mag | Sep. (′) | Impact parameter (kpc)
---|---|---|---|---|---|---|---
3C 454.3:[KSS94] 28 | 22:53:57.8 | 16:08:39.01 |  |  | 23.1 | 0.243 |
WISEA J225355.74+160850.7 | 22:53:55.7 | 16:08:49.99 |  |  | 22.2 | 0.495 |
[CLW2001] 2251+1552 +0322-0003 | 22:53:59.94 | 16:08:53.3 | 0.3529 | Spec |  | 0.525 |
RX J2253.9+1608:[ZEH2003] 04 | 22:54:0.0 | 16:08:30.98 |  |  |  | 0.659 |
SSTSL2 J225400.41+160906.4 | 22:54:0.5 | 16:09:6.01 | 0.3526 | Spec | 21.6 | 0.693 |
3C 454.3:[KSS94] 17 | 22:53:55.4 | 16:09:25.99 |  |  | 22 | 0.781 |
GALEXMSC J225356.94+160944.6 | 22:53:57.1 | 16:09:42.98 |  |  | 21.7 | 0.839 |
3C 454.3:[KSS94] 05 | 22:53:58.2 | 16:09:43.99 |  |  | 22.8 | 0.848 |
SSTSL2 J225400.57+160925.5 | 22:54:0.6 | 16:09:24.98 |  |  | 22.1 | 0.862 |
WISEA J225355.89+160938.9 | 22:53:55.9 | 16:09:38.02 |  |  | 21 | 0.863 |

Note. — Table 11 is published in its entirety in the machine-readable format.
We give a sample of 10 objects here.

Table 12: Objects within 10′ of H2356-309, sorted from least to greatest
angular separation.

Object | RA (h:m:s) | DEC (d:m:s) | Redshift | Type | App. mag | Sep. (′) | Impact parameter (kpc)
---|---|---|---|---|---|---|---
WISEA J235910.71-303720.6 | 23:59:10.73 | -30:37:21.61 | 0.0627 | Spec | 18.7 | 0.684 |
MRSS 409-104402 | 23:59:14.62 | -30:37:24.67 |  |  | 20 | 1.467 |
WISEA J235913.66-303645.8 | 23:59:13.67 | -30:36:45.97 |  |  | 20.47 | 1.537 |
WISEA J235911.47-303908.4 | 23:59:11.37 | -30:39:7.09 |  |  | 18.8 | 1.622 |
WISEA J235903.56-303906.3 | 23:59:3.71 | -30:39:8.28 |  |  | 19.43 | 1.718 |
MRSS 409-105369 | 23:59:6.35 | -30:35:49.7 |  |  | 18.6 | 1.878 |
MRSS 409-104126 | 23:59:17.26 | -30:37:43.32 |  |  | 20 | 2.011 |
WISEA J235901.67-303926.5 | 23:59:1.67 | -30:39:25.99 |  |  | 19.8 | 2.21 |
WISEA J235917.55-303651.0 | 23:59:17.49 | -30:36:50.8 |  |  | 19.4 | 2.222 |
WISEA J235917.89-303833.5 | 23:59:17.94 | -30:38:33.86 |  |  | 18.3 | 2.332 |

Note. — Table 12 is published in its entirety in the machine-readable format.
We give a sample of 10 objects here.

Table 13: Objects within 10′ of S5 0836+71, sorted from least to greatest
angular separation.

Object | RA (h:m:s) | DEC (d:m:s) | Redshift | Type | App. mag | Sep. (′) | Impact parameter (kpc)
---|---|---|---|---|---|---|---
WISEA J084140.99+705210.4 | 08:41:40.84 | 70:52:9.98 |  |  | 18.86 | 2.044 |
WISEA J084047.07+705544.2 | 08:40:46.95 | 70:55:45.01 |  |  | 19.24 | 3.681 |
GALEXASC J084150.37+705009.3 | 08:41:50.4 | +70:50:09 | 1.242 | Phot | 18.3 | 4.147 |
WISEA J084213.74+705146.0 | 08:42:13.56 | 70:51:46.01 |  |  | 18.75 | 4.47 |
WISEA J084134.24+704906.8 | 08:41:34.01 | 70:49:7.0 |  |  | 19.32 | 4.654 |
WISEA J084216.83+705148.6 | 08:42:16.64 | 70:51:47.99 |  |  | 19.31 | 4.685 |
WISEA J084030.58+705117.9 | 08:40:30.4 | 70:51:19.01 |  |  | 18.3 | 5.023 |
[VMF98] 057:[BBS2002] 051 | 08:40:52.3 | +70:49:17 | 0.2418 | Spec | 19.5 | 5.138 |
2MASS J08415508+7048592 | 08:41:54.7 | +70:48:59 | 0.2387 | Spec | 19.13 | 5.337 |
WISEA J084124.44+704820.1 | 08:41:24.21 | 70:48:20.02 |  |  | 19.02 | 5.37 |

Note. — Table 13 is published in its entirety in the machine-readable format.
We give a sample of 10 objects here.

Table 14: Objects within 10′ of Ton 116, sorted from least to greatest angular
separation.

Object | RA (h:m:s) | DEC (d:m:s) | Redshift | Type | App. mag | Sep. (′) | Impact parameter (kpc)
---|---|---|---|---|---|---|---
WISEA J124314.74+362752.6 | 12:43:14.7 | 36:27:53.35 |  |  | 20.6 | 0.424 |
RX J1243.2+3627:[BEV98] 002 | 12:43:14.8 | 36:27:51.01 |  |  | 18.7 | 0.431 |
WISEA J124316.96+362713.6 | 12:43:16.97 | 36:27:13.79 | 0.485129 | Spec | 21.3 | 0.989 | 618
WISEA J124305.68+362654.3 | 12:43:5.7 | 36:26:54.2 |  |  | 20.8 | 1.641 |
WISEA J124321.34+362810.8 | 12:43:21.33 | 36:28:11.03 |  |  | 20.8 | 1.787 |
SDSS J124312.06+362544.1 | 12:43:12.06 | 36:25:44.15 |  |  | 20.9 | 2.002 |
WISEA J124321.71+362839.8 | 12:43:21.72 | 36:28:39.9 |  |  | 20.8 | 2.033 |
WISEA J124304.77+362617.8 | 12:43:4.81 | 36:26:17.74 |  |  | 20.4 | 2.147 |
WISEA J124315.69+362952.9 | 12:43:15.72 | 36:29:53.27 |  |  | 19.6 | 2.236 |
WISEA J124323.65+362635.4 | 12:43:23.6 | 36:26:35.23 |  |  | 20.9 | 2.467 |

Note. — Table 13 is published in its entirety in the machine-readable format.
We give a sample of 10 objects here.

Table 15: Objects within 10′ of 1RXS J122121.7+301041, sorted from least to
greatest angular separation.

Object | RA (h:m:s) | DEC (d:m:s) | Redshift | Type | App. mag | Sep. (′) | Impact parameter (kpc)
---|---|---|---|---|---|---|---
WISEA J122122.35+301021.1 | 12:21:22.34 | 30:10:20.78 |  |  | 20.2 | 0.285 |
WISEA J122125.49+300932.7 | 12:21:25.49 | 30:09:32.87 |  |  | 19.6 | 1.317 |
SDSS J122129.52+301103.6 | 12:21:29.52 | 30:11:3.66 |  |  | 20.9 | 1.697 |
WISEA J122123.38+300853.6 | 12:21:23.39 | 30:08:53.7 |  |  | 19.2 | 1.751 |
SDSS J122115.97+300924.9 | 12:21:15.97 | 30:09:24.98 |  |  | 20.7 | 1.763 |
WISEA J122114.21+300925.5 | 12:21:14.18 | 30:09:25.96 |  |  | 20.7 | 2.054 |
WISEA J122132.81+301145.6 | 12:21:32.78 | 30:11:44.77 |  |  | 20.1 | 2.6 |
WISEA J122134.81+301123.1 | 12:21:34.84 | 30:11:23.39 | 0.060387 | Spec | 18.3 | 2.892 | 230
WISEA J122111.10+301218.6 | 12:21:11.09 | 30:12:18.79 |  |  | 20.9 | 2.893 |
WISEA J122134.44+301149.1 | 12:21:34.48 | 30:11:49.13 |  |  | 18.49 | 2.963 |

Note. — Table 14 is published in its entirety in the machine-readable format.
We give a sample of 10 objects here.

Table 16: Objects within 10′ of 1RXS J142239.1+580159, sorted from least to
greatest angular separation.

Object | RA (h:m:s) | DEC (d:m:s) | Redshift | Type | App. mag | Sep. (′) | Impact parameter (kpc)
---|---|---|---|---|---|---|---
WISEA J142325.43+575923.7 | 14:23:25.43 | 57:59:23.5 | 0.04411 | Spec | 17.1 | 6.663 | 388
SDSS J142237.38+580036.1 | 14:22:37.38 | 58:00:36.14 |  |  | 20 | 1.338 |
WISEA J142245.79+580026.4 | 14:22:45.78 | 58:00:26.28 |  |  | 20.8 | 1.744 |
WISEA J142222.64+580122.1 | 14:22:22.7 | 58:01:21.18 |  |  | 20.5 | 2.218 |
WISEA J142251.82+580324.2 | 14:22:51.85 | 58:03:24.19 |  |  | 19.1 | 2.263 |
WISEA J142245.79+580408.5 | 14:22:45.81 | 58:04:8.98 |  |  | 20.6 | 2.405 |
SDSS J142224.44+575959.6 | 14:22:24.45 | 57:59:59.64 |  |  | 20.9 | 2.718 |
WISEA J142300.70+580206.3 | 14:23:0.72 | 58:02:5.82 |  |  | 20.4 | 2.894 |
WISEA J142232.79+575905.3 | 14:22:32.85 | 57:59:5.57 |  |  | 20.3 | 2.943 |
WISEA J142216.63+580211.3 | 14:22:16.65 | 58:02:11.83 |  |  | 20.2 | 2.957 |

Note. — Table 15 is published in its entirety in the machine-readable format.
We give a sample of 10 objects here.

Table 17: Objects within 10′ of 1RXS J150759.8+041511, sorted from least to
greatest angular separation.

Object | RA (h:m:s) | DEC (d:m:s) | Redshift | Type | App. mag | Sep. (′) | Impact parameter (kpc)
---|---|---|---|---|---|---|---
WISEA J150800.69+041332.0 | 15:08:0.7 | 04:13:32.02 | 0.140261 | Spec | 18.5 | 1.684 | 307
WISEA J150804.48+041420.7 | 15:08:4.51 | 04:14:21.05 |  |  | 20 | 1.462 |
SDSS J150755.09+041414.1 | 15:07:55.09 | 04:14:14.14 |  |  | 19 | 1.506 |
SDSS J150754.03+041438.8 | 15:07:54.04 | 04:14:38.9 |  |  | 20.3 | 1.523 |
SDSS J150804.76+041413.7 | 15:08:4.77 | 04:14:13.74 |  |  | 20.7 | 1.587 |
WISEA J150754.07+041602.8 | 15:07:54.07 | 04:16:2.78 |  |  | 20.8 | 1.646 |
WISEA J150801.68+041328.6 | 15:08:1.69 | 04:13:28.7 |  |  | 20.8 | 1.789 |
WISEA J150753.76+041411.6 | 15:07:53.8 | 04:14:9.82 |  |  | 20.5 | 1.805 |
WISEA J150805.58+041618.6 | 15:08:5.58 | 04:16:19.31 |  |  | 19.5 | 1.839 |
WISEA J150756.46+041701.4 | 15:07:56.45 | 04:17:1.64 | 0.775 | Spec | 20.047 | 2.002 | 1010

Note. — Table 17 is published in its entirety in the machine-readable format.
We give a sample of 10 objects here.

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