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

The high redshift galaxy MS1512-cB58 (z=2.724, m_V=20.64) has been observedwith the very efficient high resolution echelle spectrograph VLT/UVES. Althoughthis is a very challenging observational program for a Southern hemispheretelescope (the galaxy is located at +36 deg declination), high resolutionspectra (FWHM ~ 26 km/s) have revealed, with unprecedented detail along agalaxy sight line, the Lyman-alpha forest due to intervening clouds in theintergalactic medium (IGM). The mean depression D_A due to IGM absorptionblueward of the galaxy Ly-alpha wavelength and the number density dn/dz ofLy-alpha clouds have been compared with equivalent results obtained for QSOsight lines at similar redshifts. Our results indicate a possible excess ofabsorption close to the galaxy. The mean depression at ~ 150 h_65^-1 Mpccomoving (Omega_m=0.3, Omega_Lambda=0.7) from the galaxy is D_A=0.36+/-0.03, tobe compared with 0.22+/-0.04, expected from a best fit to QSO sight lines. Inthe same region (z=2.610), the number density of lines with HI column densityin excess of 10^14 atoms/cm2 is also about 3 sigma larger than expected. Thishigh density region is at least 60 h_65^-1 Mpc comoving wide, but the largeLy-alpha absorption of the galaxy itself prevents us from detecting a possiblestructure extending down to the galaxy. This excess of Ly-alpha clouds issuggestive of two possible scenarios. One is the presence of a super cluster ofLy-alpha clouds not associated with cB58. The other is a high density of gasassociated with the environment of cB58. Indeed, a hint of the complexity ofcB58 and possibly its environment is given by the huge velocity range (almost1000 km/s) between the optical emission of star forming regions and UVabsorption of its interstellar medium.Comment: Accepted for publication in Ap