The quasar in the Hubble Deep Field South (HDFS), J2233–606 (zem=2.23) has been exhaustively observed by ground based telescopes and by the STIS spectrograph on board the Hubble Space Telescope (HST) at low, medium and high resolution in the spectral interval from 1120 °A to 10000 °A. The combined data give continuous coverage of the Lyman–alpha forest from redshift 0.9 to 2.24. This very large base–line represents a unique opportunity to study in detail the distribution of clouds associated with emitting structures in the field of the quasar and in nearby fields already observed as part of the HDFS campaign. Here we report the main properties obtained from the large spectroscopic dataset available for the Ly-alpha clouds in the intermediate redshift range 1.20−2.20, where our present knowledge has been complicated by the difficulty in producing good data. The number density is shown to be higher than what is expected by extrapolating the results from both lower and higher redshifts: 63±8 lines with logNHI≥14.0 are found (including metal systems) at =1.7, to be compared with ∼40 lines predicted by extrapolating from previous studies. The redshift distribution of the Lyman–alpha clouds shows a region spanning z~=1.383−1.460 (comoving size of 94 h65^−1 Mpc, Ohmo=1) with a low density of absorption lines; we detect 5 lines in this region, compared with the 16 expected from an average density along the line of sight. The two point correlation function shows a positive signal up to scales of about 3h65^−1 Mpc and an amplitude that is larger for larger HI column densities. The average Doppler parameter is about 27 km s^−1, comparable to the mean value found at z>3, thus casting doubts on the temperature evolution of the Ly-alpha clouds (Refer to PDF file for exact formulas).

Publication Date



Also archived in: arXiv: astro-ph/9901022 v3 5 Feb 1999 It is a pleasure to thank L. Amendola, B. Carswell, S. Casertano, G. Ganis, B. Jannuzi, M. Livio, P. Outram and R. Weymann for useful suggestions.ISSN:1538-4365 Note: imported from RIT’s Digital Media Library running on DSpace to RIT Scholar Works in February 2014.

Document Type


Department, Program, or Center

School of Physics and Astronomy (COS)


RIT – Main Campus