CGM$^2$ $+$ CASBaH: The Mass Dependence of H~I Ly$\alpha$-Galaxy Clustering and the Extent of the CGM

TL;DR

This study models the transition between circumgalactic and intergalactic media, revealing how the boundary radius depends on galaxy mass and aligns with the splashback radius, informing galaxy evolution theories.

Contribution

It introduces a new model combining absorber-galaxy correlation with a Gaussian profile to determine the CGM-IGM boundary as a function of galaxy mass.

Findings

$R_{cross}$ scales with galaxy mass as approximately 2 times the virial radius.

$R_{cross}$ agrees with the splashback radius for galaxies with $10^8$ to $10^{10.5} M_{\odot}$.

Results support models where $10^4$K gas extends beyond the virial radius.

Abstract

We combine datasets from the CGM$^{2}$ and CASBaH surveys to model a transition point, $R_{\rm cross}$, between circumgalactic and intergalactic media (CGM and IGM, respectively). In total, our data consist of 7244 galaxies at z < 0.5 with precisely measured spectroscopic redshifts, all having impact parameters of 0.01 - 20 comoving Mpc from 28 QSO sightlines with high-resolution UV spectra that cover H I Ly$\alpha$. Our best-fitting model is an exclusionary two-component model that combines a 3D absorber-galaxy cross correlation function with a simple Gaussian profile at inner radii to represent the CGM. By design, this model gives rise to a determination of $R_{\rm cross}$ as a function of galaxy stellar mass, which can be interpreted as the boundary between the CGM and IGM. For galaxies with $10^8 \leq M_{\star}/M_{\odot} \leq 10^{10.5}$, we find that $R_{\rm cross}(M_{\star}) \approx 2 \pm 0.6 R_{\rm vir}$. Additionally, we find excellent agreement between $R_{\rm cross}(M_{\star})$ and the theoretically-determined splashback radius for galaxies in this mass range. Overall, our results favor models of galaxy evolution at z < 0.5 that distribute $T \approx 10^{4}$K gas to distances beyond the virial radius.