Searches for the galaxies associated with low-metallicity circumgalactic clouds reveal one population of clouds associated with galactic halos and another population that appears to be more distant from detectable galaxies
The figure above, from Berg et al. (2022) , shows how the metallicities of gas clouds outside of galaxies are related to the stellar masses of the associated galaxies.
It comes from a program to identify and characterize the host galaxies associated with a sample of circumgalactic gas clouds (all are partial Lyman limit systems; pLLSs) at z < 1 using integral field unit (IFU) observations. That program is part of the Bimodal Absorption System Imaging Campaign (BASIC), which aims to understand how the metallicities of circumgalactic gas clouds at z < 1 are related to their environments.
To that end, we compared circumgalactic cloud metallicity to galactic stellar mass in our BASIC-IFU sample of host galaxies (green diamonds) and added the COS-Halos  sample of galaxies associated with pLLSs and Lyman limit systems. The COS-Halos sample includes both star-forming (blue stars) and quiescent (pink pentagons) host galaxies. Dashed and dotted lines in the figure represent solar and 4% solar metallicity, respectively. As the figure shows, the BASIC-IFU sample extends to significantly lower cloud metallicity than COS-Halos.
The BASIC-IFU sample also extends to greater distances from the associated galaxy. Green shading of the BASIC-IFU points shows the ratio of absorption-line impact parameter to virial radius (Rvir). Furthermore, some of the BASIC-IFU absorbers lack identifiable host galaxies and are plotted as field limits representing the minimum stellar mass we would have been able to detect at the cloud’s redshift in our observations. All of the COS-Halos absorbers are within 0.5 Rvir of the associated galaxy.
Our results reveal why the COS-Halos survey was unable to detect clouds with metallicities as low as the lowest metallicity gas identified in the COS CGM Compendium . Because of its survey design, COS-Halos did not probe the surroundings of galaxies with , nor did it explore large impact parameters around more massive galaxies. Consequently, COS-Halos missed the types of low-metallicity clouds that our program detected. They would also not have included the clouds for which we are unable to identify host galaxies.
Taken together, these findings indicate that there are two populations of low-metallicity gas clouds at z < 1. One is associated with the halos of galaxies. Another is associated with overdense regions of the universe lying outside the halos of galaxies.