Hydrogen 21-centimeter emission from a galaxy at cosmological distance

We have detected the neutral atomic hydrogen (HI) emission line at a cosmol ogically significant distance [redshift (z) = 0.18] in the rich galaxy clus ter Abell 2218 with the Westerbork Synthesis Radio Telescope. The HI emissi on originates in a spiral galaxy 2.0 h(65)(-1) megaparsecs from the cluster core. No other significant detections have been made in the cluster, sugge sting 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 10(9) years. This low accretion rate is qualita tively consistent with low-density cosmological models in which clusters ar e largely assembled at z > 1.