THE Lyα FOREST OF A LYMAN BREAK GALAXY: VERY LARGE TELESCOPE SPECTRA OF MS 1512-cB58 AT z = 2.724

摘要

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 js located at + 36° declination), high-resolution spectra (FWHM approx= 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_(65)~(-1) 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 approx= 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_(65)~(-1) 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 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.