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 (KBSS), we assemble a sample of more than 200,000 distinct foreground-background pairs with projected angular separations of 3 " − 500 " and spectroscopic redshifts, with 〈zfg〉 = 2.23 and 〈zbg〉 = 2.57 (foreground, background redshifts, respectively.) The ensemble of sightlines and foreground galaxies is used to construct a 2-D map of the mean excess H I Lyα optical depth relative to the intergalactic mean as a function of projected galactocentric distance (20 ≲ Dtran / pkpc ≲ 4000) and line-of-sight velocity.Careful attention to accurate galaxy systemic redshifts, coupled with detailed knowledge of the effective spectral resolution of background-galaxy composite spectra, provides significant information on the line-of-sight kinematics of H I gas as a function of projected distance Dtran. 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 H I. The best-fitting model suggests that galaxy-scale outflows with initial velocity vout ≃ 600 km s−1 dominate the kinematics of circumgalactic H i out to Dtran ≃ 50 kpc, while H I at Dtran ≳ 100 kpc is dominated by infall with characteristic vin ≲ vc, where vc is the circular velocity of the host halo (Mh ∼ 1012 M). Over the impact parameter range 80 ≲ Dtran/pkpc ≲ 200, the H I line-of-sight velocity range reaches a minimum, with a corresponding flattening in the rest-frame Lyα equivalent width. These observations can be naturally explained as the transition between outflow-dominated and accretion-dominated flows. Beyond Dtran ≃ 300 kpc, the line of sight kinematics are dominated by Hubble expansion.
|Original language||English (US)|
|State||Published - Jun 23 2020|
- Galaxies: evolution
- Galaxies: high-redshift
- Galaxies: ISM
ASJC Scopus subject areas