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

Abstract

We analyze the equivalent widths of H I {{Ly}}α ({W}_Lyα }) absorption from the inner (R < 160 kpc) circumgalactic medium (CGM) of 85 galaxies at z ∼ 0 with stellar masses M _* ranging 8≤slant {log} {M}_* /{M}_⊙ ≤slant 11.6. Across three orders of magnitude in stellar mass, the CGM of present-day galaxies exhibits a very high covering fraction of cool hydrogen gas (f _C = 87 ± 4%) indicating that the CGM is ubiquitous in modern, isolated galaxies. When H I {{Ly}}α 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: {log} {W}_{{H}{{I}} 1215}^{{s}}=(0.34+/- 0.02)-(0.0026+/- 0.0005) × (R)+(0.286+/- 0.002)× {log}({M}_* /{M}_⊙ ). The rms scatter around this bivariate relation is ∼0.2 dex. We interpret the explicit correlation between {W}_Lyα } and M* to arise from the underlying dark matter halo mass (M _halo), thereby suggesting a CGM fundamental plane between {W}_Lyα }, R, and M _halo. This correlation can be used to estimate the underlying dark matter halo mass from observations of saturated H I {{Ly}}α in the CGM of a modern galaxy.