A Caltech Library Service

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

Chen, Yuguang and Steidel, Charles C. and Hummels, Cameron B. and Rudie, Gwen C. and Dong, Bili and Trainor, Ryan F. and Bogosavljević, Milan and Erb, Dawn K. and Pettini, Max and Reddy, Naveen A. and Shapley, Alice E. and Strom, Allison L. and Theios, Rachel L. and Faucher-Giguère, Claude-André and Hopkins, Philip F. and Kereš, Dušan (2020) The Keck Baryonic Structure Survey: using foreground/background galaxy pairs to trace the structure and kinematics of circumgalactic neutral hydrogen at z ∼ 2. Monthly Notices of the Royal Astronomical Society, 499 (2). pp. 1721-1746. ISSN 0035-8711.

PDF - Published Version
See Usage Policy.

[img] PDF - Submitted Version
See Usage Policy.


Use this Persistent URL to link to this item:


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 〈zbg〉 = 2.57 (foreground, background redshifts, respectively.) The ensemble of sightlines and foreground galaxies is used to construct a 2D map of the mean excess HI Lyα 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 HI 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 HI. The best-fitting model suggests that galaxy-scale outflows with initial velocity v_(out) ≃ 600 km s⁻¹ dominate the kinematics of circumgalactic HI out to D_(tran) ≃ 50 kpc, while HI 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 HI 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 D_(tran) ≃ 300 pkpc (∼1 cMpc), the line-of-sight kinematics are dominated by Hubble expansion.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper ItemData
Chen, Yuguang0000-0003-4520-5395
Steidel, Charles C.0000-0002-4834-7260
Hummels, Cameron B.0000-0002-3817-8133
Rudie, Gwen C.0000-0002-8459-5413
Trainor, Ryan F.0000-0002-6967-7322
Bogosavljević, Milan0000-0002-4710-4814
Erb, Dawn K.0000-0001-9714-2758
Pettini, Max0000-0002-5139-4359
Reddy, Naveen A.0000-0001-9687-4973
Shapley, Alice E.0000-0003-3509-4855
Strom, Allison L.0000-0001-6369-1636
Theios, Rachel L.0000-0002-4236-1037
Faucher-Giguère, Claude-André0000-0002-4900-6628
Hopkins, Philip F.0000-0003-3729-1684
Kereš, Dušan0000-0002-1666-7067
Additional Information:© 2020 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model ( Accepted 2020 September 10. Received 2020 August 31; in original form 2020 April 30. Published: 23 September 2020. Based on data obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. This paper has included data obtained using Keck/LRIS (Oke et al. 1995; Steidel et al. 2004; Rockosi et al. 2010) and Keck/MOSFIRE (McLean et al. 2010, 2012). We thank the W. M. Keck Observatory staff for their assistance with the observations over two decades. The following software packages have been crucial to preparing for this paper: the IDL Astronomy User’s Library,17 the Coyote IDL library,18ASTROPY (Astropy Collaboration 2018), EMCEE (Foreman-Mackey et al. 2013), THE YT PROJECT (Turk et al. 2011), and TRIDENT (Hummels, Smith & Silvia 2017). This work was supported in part by grant AST-1313472 from the US NSF, and by a grant from the Caltech/JPL President’s and Director’s Program. CAFG was supported by NSF through grants AST-1517491, AST-1715216, and CAREER award AST-1652522, by NASA through grant 17-ATP17-0067, by STScI through grants HST-GO-14681.011, HST-GO-14268.022-A, and HST-AR-14293.001-A, and by a Cottrell Scholar Award from the Research Corporation for Science Advancement. We would like to thank the anonymous referee for providing valuable feedback. YC would like to thank Hongjie Zhu, for her continuous encouragements when preparing for this paper. We would like to acknowledge Yiqiu Ma, E. Sterl Phinney, and Mateusz Matuszewski, for their constructive discussions. Finally, we thank collaborators Kurt L. Adelberger, Matthew P. Hunt, David R. Law, Olivera Rakic, and Monica L. Turner for their contributions to the KBSS survey over the course of nearly two decades. Data Availability: The processed data underlying this article are available on the KBSS website ( with DOI: 10.22002/D1.1458.
Group:Astronomy Department, TAPIR
Funding AgencyGrant Number
W. M. Keck FoundationUNSPECIFIED
JPL President and Director's FundUNSPECIFIED
Cottrell Scholar of Research CorporationUNSPECIFIED
Subject Keywords:galaxies: evolution – galaxies: high-redshift – galaxies: ISM– galaxies: intergalatic medium
Issue or Number:2
Record Number:CaltechAUTHORS:20200626-150116926
Persistent URL:
Official Citation:Yuguang Chen(陈昱光), Charles C Steidel, Cameron B Hummels, Gwen C Rudie, Bili Dong(董比立), 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, The Keck Baryonic Structure Survey: using foreground/background galaxy pairs to trace the structure and kinematics of circumgalactic neutral hydrogen at z ∼ 2, Monthly Notices of the Royal Astronomical Society, Volume 499, Issue 2, December 2020, Pages 1721–1746,
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:104099
Deposited By: George Porter
Deposited On:29 Jun 2020 15:45
Last Modified:04 Dec 2020 20:12

Repository Staff Only: item control page