ar X iv : a st ro - p h / 01 11 12 5 v 1 6 N ov 2 00 1 The Lyman – α forest of a Lyman Break Galaxy : VLT Spectra of MS 1512

The high redshift galaxy MS1512–cB58 (z = 2.724, m V = 20.64) has been observed with the very efficient high resolution echelle spectrograph VLT/UVES. 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 Lyman–α forest due to intervening clouds in the intergalac-tic 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 −1 65 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 HI column density in excess of 10 14 atoms cm −2 is also ∼ 3σ larger than expected. This high density region is at least 60 h −1 65 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 super cluster 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 – 2 – 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.