IMAGING OF A LYMAN-ALPHA ABSORPTION CLOUD IN FRONT OF THE RADIO GALAXY 4C41.17

THE quasar absorption lines known as the Lyman-alpha forest reveal the presence of a population of extragalactic objects, thought to be clouds of gas or gas-rich protogalaxies. The gas seems to be highly ionized (N(HI)/N(HII)) almost-equal-to 10(-4)), with column densities (N(HI)) ranging from 10(12) to 10(18) cm-2 and Doppler parameters of 15-40 km s-1 (refs 1-3). Different types of measurement have yielded strongly discrepant numbers for the typical diameter, from several tens of kiloparsecs4,5 to only approximately 5 parsecs6,7. Nothing is known about the clouds' shape (spheres or sheets), which could provide another clue to their origin8. The observation of huge, luminous Ly-alpha haloes around some high-redshift radio galaxies9-11 provides a new way of obtaining information about Lyman cloud size and shape: the large sizes (many tens of kiloparsecs) and broad emission-line profiles of these haloes make them ideal light screens against which absorbing clouds at similar redshift might be seen as dark shadows. We report here the possible detection of such an absorbing cloud in front of the radio galaxy 4C41.17; in the simplest interpretation, the cloud has an elongated shape, and dimensions of approximately 40 x 10 kiloparsecs.