The Lyα forest of a Lyman break galaxy: very large telescope spectra of MS 1512-cB58 at z=2.724.
SAVAGLIO S., PANAGIA N. and PADOVANI P.
Abstract (from CDS):
The high-redshift galaxy MS 1512-cB58 (z=2.724, mV=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 DA 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 ∼150h–165 Mpc comoving (Ωm=0.3, ΩΛ=0.7) from the galaxy is DA=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 1014 atoms.cm–2 is also ∼3 σ larger than expected. This high-density region is at least 60 h–165 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.