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Calibrating galaxy redshifts using absorption by the surrounding intergalactic medium

Rakic, Olivera and Schaye, Joop and Steidel, Charles C. and Rudie, Gwen C. (2011) Calibrating galaxy redshifts using absorption by the surrounding intergalactic medium. Monthly Notices of the Royal Astronomical Society, 414 (4). pp. 3265-3271. ISSN 0035-8711.

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Rest-frame UV spectral lines of star-forming galaxies are systematically offset from the galaxies' systemic redshifts, probably because of large-scale outflows. We calibrate galaxy redshifts measured from rest-frame UV lines by utilizing the fact that the mean H i Lyα absorption profiles around the galaxies, as seen in spectra of background objects, must be symmetric with respect to the true galaxy redshifts if the galaxies are oriented randomly with respect to the lines of sight to the background objects. We use 15 bright QSOs at z≈2.5–3 and more than 600 foreground galaxies with spectroscopic redshifts at z≈1.9–2.5. All galaxies are within 2 Mpc proper from the lines of sight to the background QSOs. We find that Lyα emission and ISM absorption redshifts require systematic shifts of ∆ν_Lyα = -295^(+35)_(-35) and ∆ν_ISM = 145^(+70)_(-35) km s^(-1), respectively. Assuming a Gaussian distribution, we put 1σ upper limits on possible random redshift offsets of <220 km s^(-1) for Lyα and <420 km s^(-1) for ISM redshifts. For the small subset (<10 per cent) of galaxies for which near-IR spectra have been obtained, we can compare our results to direct measurements based on rest-frame optical, nebular emission lines, which we confirm to mark the systemic redshifts. While our ΔvISM agrees with the direct measurements, our Δv_Lyα is significantly smaller. However, when we apply our method to the near-IR subsample which is characterized by slightly different selection effects, the best-fitting velocity offset comes into agreement with the direct measurement. This confirms the validity of our approach, and implies that no single number appropriately describes the whole population of galaxies, in line with the observation that the line offset depends on galaxy spectral morphology. This method provides accurate redshift calibrations and will enable studies of circumgalactic matter around galaxies for which rest-frame optical observations are not available.

Item Type:Article
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URLURL TypeDescription
Steidel, Charles C.0000-0002-4834-7260
Rudie, Gwen C.0000-0002-8459-5413
Additional Information:© 2011 The Authors. Monthly Notices of the Royal Astronomical Society © 2011 RAS. Accepted 2011 February 28. Received 2011 January 20; in original form 2010 November 2. Article first published online: 9 May 2011. 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 NASA, and was made possible by the generous financial support of the W.M. Keck Foundation. We are very grateful to Alice Shapley, Dawn Erb, Naveen Reddy, Milan Bogosavljevíc and Max Pettini for their help with collecting and processing the data. We also thank the anonymous referee whose comments improved the clarity of the paper. This work was supported by an NWO VIDI grant (OR, JS), by the US National Science Foundation through grants AST-0606912 and AST-0908805, and by the David and Lucile Packard Foundation (CCS). CCS acknowledges additional support from the John D. and Catherine T. MacArthur Foundation and the Peter and Patricia Gruber Foundation. We thank the W. M. Keck Observatory staff for their assistance with the observations. We also thank the Hawaiian people, as without their hospitality the observations presented here would not have been possible.
Funding AgencyGrant Number
Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO)UNSPECIFIED
David and Lucile Packard FoundationUNSPECIFIED
John D. and Catherine T. MacArthur FoundationUNSPECIFIED
Peter and Patricia Gruber FoundationUNSPECIFIED
W. M. Keck FoundationUNSPECIFIED
Subject Keywords:galaxies: high-redshift; intergalactic medium; quasars: absorption lines
Issue or Number:4
Record Number:CaltechAUTHORS:20110812-111221376
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Official Citation:Rakic, O., Schaye, J., Steidel, C. C. and Rudie, G. C. (2011), Calibrating galaxy redshifts using absorption by the surrounding intergalactic medium. Monthly Notices of the Royal Astronomical Society, 414: 3265–3271. doi: 10.1111/j.1365-2966.2011.18624.x
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:24829
Deposited By: Jason Perez
Deposited On:12 Aug 2011 20:43
Last Modified:04 Dec 2019 23:29

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