The Lyα Forest of a Lyman Break Galaxy: Very Large Telescope Spectra of MS 1512−cB58 atz = 2.724

The high-redshift galaxy MS 1512-cB58 ( z = 2.724, m V = 20.64) has been observed with the very efficient high-resolution UV-Visual Eschelle Spectograph on the Very Large Telescope. Although this is a very challenging observational program for a Southern Hemisphere telescope (the galaxy is located at +36° declination), high-resolution spectra (FWHM ≃ 26 km s -1 ) have revealed, with unprecedented detail along a galaxy sight line, the Lyα forest due to intervening clouds in the intergalactic medium (IGM). The mean depression D A due to IGM absorption blueward of the galaxy Lyα wavelength and the number density dn / dz of Lyα clouds have been compared with equivalent results obtained for QSO sight lines at similar redshifts. Our results indicate a possible excess of absorption close to the galaxy. The mean depression at ~150 h Mpc comoving (Ω m = 0.3, Ω Λ = 0.7) from the galaxy is D A = 0.36 ± 0.03, to be compared with 0.22 ± 0.04 expected from a best fit to QSO sight lines. In the same region ( z ≃ 2.610), the number density of lines with H I column density in excess of 10 14 atoms cm -2 is also ~3 σ larger than expected. This high-density region is at least 60 h Mpc comoving wide, but the large Lyα absorption of the galaxy itself prevents us from detecting a possible structure extending down to the galaxy. This excess of Lyα clouds is suggestive of two possible scenarios. One is the presence of a supercluster of Lyα clouds not associated with cB58. The other is a high density of gas associated with the environment of cB58. Indeed, a hint of the complexity of cB58 and possibly its environment is given by the huge velocity range (almost 1000 km s -1 ) between the optical emission of star-forming regions and UV absorption of its interstellar medium.