The Keck Baryonic Structure Survey: Using foreground/background galaxy pairs to trace the structure and kinematics of circumgalactic neutral hydrogen at z ∼ 2

Yuguang Chen*, Charles C. Steidel, Cameron B. Hummels, Gwen C. Rudie, Bili Dong, Ryan F. Trainor, Milan Bogosavljević, Dawn K. Erb, Max Pettini, Naveen A. Reddy, Alice E. Shapley, Allison L. Strom, Rachel L. Theios, Claude André Faucher-Giguère, Philip F. Hopkins, Dušan Kereš

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

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 (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 s1 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 languageEnglish (US)
JournalUnknown Journal
StatePublished - Jun 23 2020

Keywords

  • Galaxies: evolution
  • Galaxies: high-redshift
  • Galaxies: ISM

ASJC Scopus subject areas

  • General

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