The Hosts of X-ray Absorption Lines Toward AGNs

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-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.


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 . It maghuber@umich.edu, jbregman@umich.edu is important to note the temperature range of these studies are below the virial temperature of L* galaxies (≈ 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 10 4 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(Prochaska et al. , 2019Johnson 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.

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.
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 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 ( 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 (B J ) (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 (B J ) 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.

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 Kalpha emission, which should be the strongest. Halo mass calculations for an O VII column density ≥ 10 15 cm −2 peak at a stellar mass of M star ≈ 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 ≈ 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 ≈ 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 ≈ 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 ≈ 0.4, beyond which dedicated imaging programs would be required.
We also can apply a characteristic search radius and use 1 Mpc, which is ∼ 5 R vir of an L* galaxy and about 2R 500 for a galaxy group or poor cluster. At a fixed metric radius, the angular search radii decreases until z ∼ 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. 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.    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).
These two objects may belong to a galaxy group.
3.4.2. Objects with unknown redshifts 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   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    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.
3.9.2. Objects with unknown redshifts 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     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  Table  12. 3.11. S5 0836+71 3.11.1. Objects with known redshifts   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.

DISCUSSION
The lists of identified objects become incomplete when z 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, β = 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 spec- 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.
troscopy, with a proposed spectral resolution of R ≈ 3000 and an effective area of A ef f ≈ 300 cm 2 (Smith 2020), compared to Chandra with R ≈ 500 and A ef f ≈ 3.3 cm 2 and XMM with R ≈ 420 and A ef f ≈ 45 cm 2 . 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 ef f ≈ 1.6 m 2 , 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 ef f = 2 m 2 . Until the eventual launch of an advanced Xray 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.

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.

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. Note- Table 3 is published in its entirety in the machine-readable format. We give a sample of 10 objects here. 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 ≈ 0.5. These factors make it a primary candidate for deeper spectroscopy and photometry. 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. 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. 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. 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. Note- Table 9 is published in its entirety in the machine-readable format. We give a sample of 10 objects here. 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 Note- Table 11 is published in its entirety in the machine-readable format. We give a sample of 10 objects here.

2.332
Note- Table 12 is published in its entirety in the machine-readable format. We give a sample of 10 objects here.

5.37
Note- Table 13 is published in its entirety in the machine-readable format. We give a sample of 10 objects here.