CGM² I: The Extent of the Circumgalactic Medium Traced by Neutral Hydrogen

Abstract We present initial results from the Cosmic Origins Spectrograph (COS) and Gemini Mapping the Circumgalactic Medium (CGMCGM ≡ CGM 2 ) survey. The CGM 2 survey consists of 1689 galaxies, all with high-quality Gemini-GMOS spectra, within 1 Mpc of 22 z ≲ 1 quasars, all with a signal-to-noise ratio of ∼10 Hubble Space Telescope/COS G130M+G160M spectra. For 572 of these galaxies with stellar masses 10 7 M ⊙ < M ⋆ < 10 11 M ⊙ and z ≲ 0.5, we show that the H i covering fraction above a threshold of N HI > 10 14 cm −2 is ≳0.5 within 1.5 virial radii ( R vir ∼ R 200m ). We examine the H i kinematics and find that the majority of absorption lies within ±250 km s −1 of the galaxy systemic velocity. We examine H i covering fractions over a range of impact parameters to infer a characteristic size of the CGM, , as a function of galaxy mass. is the impact parameter at which the probability of observing an absorber with N HI >10 14 cm −2 is >50%. In this framework, the radial extent of the CGM of M ⋆ > 10 9.9 M ⊙ galaxies is kpc or . Intermediate-mass galaxies with 10 9.2 < M ⋆ / M ⊙ < 10 9.9 have an extent of kpc or . Low-mass galaxies, M ⋆ < 10 9.2 M ⊙ , show a smaller physical scale of kpc and extend to . Our analysis suggests that using R vir as a proxy for the characteristic radius of the CGM likely underestimates its extent.