On the CGM Fundamental Plane: The Halo Mass Dependency of Circumgalactic H i

We analyze the equivalent widths of H i ( ) absorption from the inner (R < 160 kpc) circumgalactic medium (CGM) of 85 galaxies at z ∼ 0 with stellar masses M* ranging . Across three orders of magnitude in stellar mass, the CGM of present-day galaxies exhibits a very high covering fraction of cool hydrogen gas (fC = 87 4%) indicating that the CGM is ubiquitous in modern, isolated galaxies. When H i is detected, its equivalent width declines with increasing radius regardless of the galaxy mass, but the scatter in this trend correlates closely with M*. Using the radial and stellar mass correlations, we construct a planar surface describing the cool CGM of modern galaxies: . The rms scatter around this bivariate relation is ∼0.2 dex. We interpret the explicit correlation between and M* to arise from the underlying dark matter halo mass (Mhalo), thereby suggesting a CGM fundamental plane between , R, and Mhalo. This correlation can be used to estimate the underlying dark matter halo mass from observations of saturated H i in the CGM of a modern galaxy.