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

We analyze the equivalent widths of HI Ly-$\alpha$ ($W_{Ly\alpha }$) absorption from the inner (R < 160 kpc) circumgalactic medium (CGM) of 85 galaxies at $z\sim 0$ with stellar masses $M\ast$ ranging $\rm{8 \leq log M* / M_{\odot} \leq 11.6}$. Across three orders of magnitude in stellar mass, the CGM of present-day galaxies exhibits a very high covering fraction of cool hydrogen gas ($f_C=87\pm 4$%) indicating that the CGM is ubiquitous in modern, isolated galaxies. When HI Ly-$\alpha$ is detected, its equivalent width declines with increasing radius regardless of the galaxy mass, but the scatter in this trend correlates closely with $M\ast$. Using the radial and stellar mass correlations, we construct a planar surface describing the cool CGM of modern galaxies: $\log W^{\rm{s}}{HI 1215} ; = ; (0.34 \pm 0.02) -( 0.0026 \pm 0.0005)\times ® + (0.286 \pm 0.002) \times \log (M*/M{\odot})$. The RMS scatter around this bivariate relation is $\sim$0.2 dex. We interpret the explicit correlation between $W_{Ly\alpha }$ and $M\ast$ to arise from the underlying dark matter halo mass ($M_{halo}$), thereby suggesting a CGM fundamental plane between $W_{Ly\alpha }$, $R$ and $M_{halo}$. This correlation can be used to estimate the underlying dark matter halo mass from observations of saturated HI Ly-$\alpha$ in the CGM of a modern galaxy.