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Science 7 September 2001:
Vol. 293. no. 5536, pp. 1800 - 1802
DOI: 10.1126/science.1063034
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Reports

Hydrogen 21-Centimeter Emission from a Galaxy at Cosmological Distance

M. A. Zwaan,1* P. G. van Dokkum,2 M. A. W. Verheijen34

We have detected the neutral atomic hydrogen (HI) emission line at a cosmologically significant distance [redshift (z) = 0.18] in the rich galaxy cluster Abell 2218 with the Westerbork Synthesis Radio Telescope. The HI emission originates in a spiral galaxy 2.0 h65-1 megaparsecs from the cluster core. No other significant detections have been made in the cluster, suggesting that the mechanisms that remove neutral gas from cluster galaxies are efficient. We infer that fewer than three gas-rich galaxies were accreted by Abell 2218 over the past 109 years. This low accretion rate is qualitatively consistent with low-density cosmological models in which clusters are largely assembled at z > 1.

1 School of Physics, University of Melbourne, Victoria 3010, Australia.
2 California Institute of Technology, Mail Stop 105-24, Pasadena, CA 91125, USA.
3 Department of Astronomy, University of Wisconsin, 475 North Charter Street, Madison, WI 53706, USA.
4 National Radio Astronomical Observatory, Post Office Box 0, Socorro, NM 87801, USA.
*   To whom correspondence should be addressed. E-mail: mzwaan{at}physics.unimelb.edu.au

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Science. ISSN 0036-8075 (print), 1095-9203 (online)