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

Abstract

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 $8\le \mathrm{logM}\ast /{\mathrm{M}}_{\odot }\le 11.6$. 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: $\log W_{HI1215}^{\mathrm{s}}=(0.34\pm 0.02)-(0.0026\pm 0.0005)\times (R)+(0.286\pm 0.002)\times \log (M\ast /M_{\odot })$. 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.

8 pages, 4 figures, Accepted for publication in ApJ

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