CGM² + CASBaH: The Mass Dependence of H i Lyα-Galaxy Clustering and the Extent of the CGM

We combine data sets from the CGM² and CASBaH surveys to model a transition point, R _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 Lyα. Our best-fitting model is a 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 _cross as a function of galaxy stellar mass, which can be interpreted as the boundary between the CGM and IGM. For galaxies with 10⁸ ≤ M _⋆/M _⊙ ≤ 10^10.5, we find that R _cross(M _⋆) ≈ 2.0 ± 0.6R _vir. Additionally, we find excellent agreement between R _cross(M _⋆) 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 ≈ 10⁴K gas to distances beyond the virial radius.