CGM${}^2$ I: The Extent of the Circumgalactic Medium Traced by Neutral Hydrogen

Abstract

We present initial results from the \textit{COS and Gemini Mapping the Circumgalactic Medium} (\mbox{CGMCGM} $\equiv$ CGM${}^2$) survey. The CGM${}^2$ survey consists of 1689 galaxies, all with high-quality Gemini GMOS spectra, within 1 Mpc of twenty-two $z\lesssim 1$ quasars, all with S/N$\sim$10 {\emph{HST/COS}} G130M$+$G160M spectra. For 572 of these galaxies having stellar masses $10^7{M}_{\odot }<M_{\star }<10^{11}{M}_{\odot }$ and $z\lesssim 0.5$, we show that the \ion{H}{1} covering fraction above a threshold of \NHI$>10^{14}$cm${}^{-2}$ is $\gtrsim 0.5$ within 1.5 virial radii ($R_{\mathrm{vir}}\sim R_{200m}$). We examine the \ion{H}{1} kinematics and find that the majority of absorption lies within $\pm$ 250 km s${}^{-1}$ of the galaxy systemic velocity. We examine \ion{H}{1} covering fractions over a range of impact parameters to infer a characteristic size of the CGM, $R_{\mathrm{CGM}}^{14}$, as a function of galaxy mass. $R_{\mathrm{CGM}}^{14}$ is the impact parameter at which the probability of observing an absorber with \NHI $>$ 10${}^{14}$ cm${}^{-2}$ is $>$ 50\%. In this framework, the radial extent of the CGM of $M_{\star }>10^{9.9}{M}_{\odot }$ galaxies is $R_{\mathrm{CGM}}^{14}=346_{-53}^{+57}$ kpc or $R_{\mathrm{CGM}}^{14}\simeq 1.2R_{\mathrm{vir}}$. Intermediate-mass galaxies with $10^{9.2}<M_{\star }/M_{\odot }<10^{9.9}$ have an extent of $R_{\mathrm{CGM}}^{14}=353_{-50}^{+64}$ kpc or $R_{\mathrm{CGM}}^{14}\simeq 2.4R_{\mathrm{vir}}$. Low-mass galaxies, $M_{\star }<10^{9.2}{M}_{\odot }$, show a smaller physical scale $R_{\mathrm{CGM}}^{14}=177_{-65}^{+70}$ kpc and extend to $R_{\mathrm{CGM}}^{14}\simeq 1.6R_{\mathrm{vir}}$. Our analysis suggests that using $R_{\mathrm{vir}}$ as a proxy for the characteristic radius of the CGM likely underestimates its extent.

30 pages, 14 figures. Submitted to ApJ, Comments welcome

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