A population of faint extended line emitters and the host galaxies of optically thick QSO absorption systems

We have conducted a long-slit search for low surface brightness Ly alpha emitters at redshift 2: 67 < z < 3.75. A 92 hr long exposure with the ESO VLT FORS2 instrument down to a 1 sigma surface brightness detection limit of 8 x 10(-20) erg cm(-2) s(-1) arcsec(-2) per arcsec(2) aperture yielded a sample of 27 single line emitters with fluxes of a few x 10(-18) erg s(-1) cm(-2). We present arguments that most objects are indeed Ly alpha. The large comoving number density, 3 x 10(-2) h(70)(3) Mpc(-3), the large covering factor, dN/dz similar to 0.2-1, and the often extended Ly alpha emission suggest that the emitters can be identified with the elusive host population of damped Ly alpha systems (DLAS) and high column density Lyman limit systems (LLS). A small inferred star formation rate, perhaps supplemented by cooling radiation, appears to energetically dominate the Ly alpha emission, and is consistent with the low metallicity, low dust content, and theoretically inferred low masses of DLAS, and with the relative lack of success of earlier searches for their optical counterparts. Some of the line profiles show evidence for radiative transfer in galactic outflows. Stacking surface brightness profiles, we find emission out to at least 4 ''. The centrally concentrated emission of most objects appears to light up the outskirts of the emitters (where LLS arise) down to a column density where the conversion from UV to Ly alpha photon becomes inefficient. DLAS, high column density LLS, and the emitter population discovered in this survey appear to be different observational manifestations of the same low-mass, protogalactic building blocks of present-day L-* galaxies.