  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.
