NARROW-BAND IMAGING OF FIELDS AROUND OPTICALLY-THICK ABSORPTION SYSTEMS - THE LINE-OF-SIGHT TOWARDS Q-0000-2619

In this paper we describe deeper obervations, both imaging and spectro scopy, of the most distant radio-quiet Lyalpha galaxy known so far, at redshift of z = 3.428 (galaxy G2) and of other faint objects in the f ield around the QSO 0000 - 2619. Galaxy G2, which we discovered in the field around the damped absorber of the QSO with narrow-band imaging tuned to detect Lyalpha emission at the redshift of the absorption sys tem, is the best primaeval galaxy candidate identified so far. Here we report measurements of its restframe UV spectral energy distribution in the interval lambdalambda1000, 1700 angstrom, in particular the dep ression of the continuum due to the blanketing of the intervening Lyal pha forest and the Lyalpha emission line, and give better upper limits for the high-ionization emission lines, such as N V and C IV. We conc lude that this object is very likely a primaeval galaxy observed durin g a major episode of star formation and in which dust does not extingu ish the Lyalpha emission. We also present Lyalpha narrow-band imaging of the galaxy which causes the damped absorber in the spectrum of the QSO. No emission is detected, down to a 3sigma upper limit of 1.2 x 10 (-17) ergs-1 CM-2 in an aperture of 2 arcsec in diameter. A correspond ing upper limit to the rest-frame equivalent width is 6 angstrom. Simi lar limits apply also to other 13 flat-spectrum z > 3 galaxy candidate s identified in the same field by Steidel & Hamilton, if their redshif t is such that their Lyalpha emission falls within our narrow-band fil ter. However, from their rest- frame UV continuum luminosity, we estim ate that the star-formation rates for these galaxies are in the range 30 to 100 M. yr-1. This would suggest that radio-quiet primaeval galax ies, though in a star-forming phase, are not characterized by the inte nse Lyalpha emission and large SFRs typical of radio-galaxies. If thes e candidates are confirmed to be at similar redshifts than those of G2 and the galaxy responsible for the damped system of the QSO, then thi s association of galaxies would be the most distant cluster observed s o far, with a total stellar mass of the order of 10(13) M. (throughout this paper we assume H-0 = 50 km s-1 Mpc-1 and OMEGA = 0, unless diff erently stated).