The Keck Baryonic Structure Survey: using foreground/background galaxy pairs to trace the structure and kinematics of circumgalactic neutral hydrogen at z ∼ 2. Issue 2 (23rd September 2020) - Chen, Yuguang
ABSTRACT: We present new measurements of the spatial distribution and kinematics of neutral hydrogen in the circumgalactic and intergalactic medium surrounding star-forming galaxies at z ∼ 2. Using the spectra of ≃3000 galaxies with redshifts 〈 z 〉 = 2.3 ± 0.4 from the Keck Baryonic Structure Survey, we assemble a sample of more than 200 000 distinct foreground-background pairs with projected angular separations of 3–500 arcsec and spectroscopic redshifts, with 〈 z fg 〉 = 2.23 and 〈 z bg 〉 = 2.57 (foreground, background redshifts, respectively.) The ensemble of sightlines and foreground galaxies is used to construct a 2D map of the mean excess $\rm{H\, {\small I}}$ $\rm Ly\, \alpha$ optical depth relative to the intergalactic mean as a function of projected galactocentric distance (20 ≲ D tran /pkpc ≲ 4000) and line-of-sight velocity. We obtain accurate galaxy systemic redshifts, providing significant information on the line-of-sight kinematics of $\rm{H\, {\small I}}$ gas as a function of projected distance D tran . We compare the map with cosmological zoom-in simulation, finding qualitative agreement between them. A simple two-component (accretion, outflow) analytical model generally reproduces the observed line-of-sight kinematics and projected spatial distribution of $\rm{H\, {\small I}}$ . The best-fitting model suggests that galaxy-scale outflows with initial velocity v out ≃ 600 km s $^{-1}\, $ dominate the kinematics of circumgalactic $\rm{H\, {\small I}}$ out to D tran ≃ 50 kpc, while $\rm{H\, {\small I}}$ at D tran ≳ 100 kpc is dominated by infall with characteristic v in ≲ circular velocity. Over the impact parameter range 80 ≲ D tran /pkpc ≲ 200, the $\rm{H\, {\small I}}$ line-of-sight velocity range reaches a minimum, with a corresponding flattening in the rest-frame $\rm Ly\, \alpha$ equivalent width. These observations can be naturally explained as the transition between outflow-dominated and accretion-dominated flows. Beyond D tran ≃ 300 pkpc (∼1 cMpc), the line-of-sight kinematics are dominated by Hubble expansion.