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

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 (\(R_{\rm vir} \sim R_{200m}\)). 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, \(R^{14}_{\rm CGM}\), as a function of galaxy mass. \(R^{14}_{\rm CGM}\) 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 \(R^{14}_{\rm CGM} = 346^{+57}_{-53}\) kpc or \(R^{14}_{\rm CGM} \simeq 1.2R_{\rm vir}\). Intermediate-mass galaxies with \(10^{9.2} < M_{\star}/M_{\odot} < 10^{9.9}\) have an extent of \(R^{14}_{\rm CGM} = 353^{+64}_{-50}\) kpc or \(R^{14}_{\rm CGM} \simeq 2.4R_{\rm vir}\). Low-mass galaxies, \(M_{\star} < 10^{9.2} M_{\odot}\), show a smaller physical scale \(R^{14}_{\rm CGM} = 177_{-65}^{+70}\) kpc and extend to \(R^{14}_{\rm CGM} \simeq 1.6R_{\rm vir}\). Our analysis suggests that using \(R_{\rm vir}\) as a proxy for the characteristic radius of the CGM likely underestimates its extent.