The Bimodal Absorption System Imaging Campaign

To understand the connection between the galaxy and the circumgalactic medium, I identified and characterized the host galaxies of 19 partial Lyman limit systems at z<1 in Berg et al. (2023). These absorbers (identified in QSO sightlines) were selected based on their HI content only, so I had no a priori knowledge of the gas metallicity or host galaxies. I used integral field spectroscopy observations from VLT/MUSE and Keck/KCWI for this project. There were no strong correlations between the galaxy and gas properties, except for stellar mass. I determined metal-enriched absorbers have a ~78% probability of being associated with galaxies of log(M*) = 9.0 and above, whereas low-metallicity absorbers only have a ~39% probability. From this, we infer 3 absorber populations: a metal-enriched population in the circumgalactic medium of galaxies, a low-metallicity population in the circumgalactic medium of galaxies, and another low-metallicity population associated with overdense regions of the universe.

Here is the absorber metallicity versus normalized impact parameter and normalized velocity offset of the absorber-associated galaxies. This is the full sample where the most probable host galaxy, other potential associated galaxies, and galaxies that were cut are shown for completeness. Many absorbers have several potential associated galaxies (perhaps associated with small galaxy groups) and the majority of the galaxies are star-forming.

This is the absorber metallicity versus the host galaxy metallicity for those galaxies that had the necessary spectral coverage. The gray line shows the one-to-one correspondance between the two quantities. I found that the metal-enriched absorbers could not be due to recent outflows, since their metallicity was not the same as their host galaxy metallicity or greater.

Our main result is summarized in this plot of absorber metallicity versus host galaxy stellar mass. I was unable to identify host galaxies for several of the absorbers. When I compared our results to those from COS-Halos, a dividing line around log(M*) ~ 9.0 became apparent. The majority of metal-enriched absorbers are found in the circumgalactic medium of galaxies with log(M*) = 9.0 or above, while the low-metallicity absorbers have two populations. One is associated with the circumgalactic medium of galaxies with log(M*) = 9.0 or above, and the other is associated with overdense regions of the universe.