CGM² I: The extent of the circumgalactic medium traced by neutral hydrogen

We present initial results from the Cosmic Origins Spectrograph (COS) and Gemini Mapping the Circumgalactic Medium (CGMCGM ≡ CGM²) survey. The CGM² 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 10⁷ M_e < M_å < 10¹¹ M_e and z ≲ 0.5, we show that the H I covering fraction above a threshold of N_HI > 10¹⁴cm⁻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, R_CGM¹⁴ , as a function of galaxy mass. R_CGM¹⁴ is the impact parameter at which the probability of observing an absorber with N_HI >10¹⁴ cm⁻² is >50%. In this framework, the radial extent of the CGM of M_å > 10^9.9 M_e galaxies is R_CGM¹⁴ = 346_-⁺₅₃⁵⁷ kpc or R_CGM¹⁴ ~ 1.2R_vir. Intermediate-mass galaxies with 10^9.2 < M_å/M_e < 10^9.9 have an extent of R_CGM¹⁴ = 353_-⁺₅₀⁶⁴ kpc or R_CGM¹⁴ ~ 2.4R_vir. Low-mass galaxies, M_å < 10^9.2 M_e, show a smaller physical scale of R_CGM¹⁴ = 177_-⁺₆₅⁷⁰ kpc and extend to R_CGM¹⁴ ~ 1.6R_vir. Our analysis suggests that using R_vir as a proxy for the characteristic radius of the CGM likely underestimates its extent.