The Ly alpha forest of the quasar in the Hubble Deep Field South

The quasar in the Hubble Deep Field South (HDF-S), J2233-606 (z(em) = 2.23) , has been observed exhaustively by ground-based telescopes and by the Spac e Telescope Imaging Spectrograph on board the Hubble Space Telescope at low , medium, and high resolution in the spectral interval from 1120 to 10000 A ngstrom. The combined data give continuous coverage of the Ly alpha forest from redshift 0.9 to 2.24. This very large baseline represents a unique opp ortunity to study in detail the distribution of clouds associated with emit ting structures in the field of the quasar and in nearby fields already obs erved as part of the HDF-S campaign. Here we report on the main properties obtained from the large spectroscopic data set that is available for the Ly alpha clouds in the intermediate-redshift range of 1.20-2.20, where our pr esent knowledge has been complicated by the difficulty in producing good da ta. The number density is shown to be higher than what is expected by extra polating the results from both lower and higher redshifts: 63 +/- 8 lines w ith logN(HI) greater than or equal to 14.0 are found (including metal syste ms) at (z) = 1.7, compared with the similar to 40 lines predicted by extrap olating from previous studies. The redshift distribution of the Ly alpha cl ouds shows a region spanning z similar or equal to 1.383-1.460 (comoving si ze of 94 h(65)(-1) Mpc, Omega(0) = 1) with a low density of absorption line s; we detect five lines in this region, compared with the 16 expected from an average density along the line of sight. The two-point correlation funct ion shows a positive signal up to scales of about 3 h(65)(-1) Mpc and an am plitude that is larger for larger H I column densities. The average Doppler parameter is about 27 km s(-1), which is comparable to the mean value foun d at z > 3, thus casting doubts on the temperature evolution of the Ly alph a clouds.