The Lyman α emission of high-z damped Lyman α systems

Using a spectral stacking technique we searched for the average Lyman alpha (Ly alpha) emission from high-z damped Ly alpha (DLA) galaxies detected in the Sloan Digital Sky Survey quasi-stellar object (QSO) spectra. We used a sample of 341 DLA galaxies of mean redshift < z > = 2.86 and log N(H I) >= 20.62 to place a 3 sigma upper limit of 3.0 x 10(-18) erg s(-1) cm(-2) on the Ly alpha flux emitted within similar to 1.5 arcsec (or 12 kpc) from the QSO line of sight. This corresponds to an average Lya luminosity of <= 2 x 10(41) erg s(-1) or 0.03L(*)(Ly alpha). This limit is deeper than the limit of most surveys for faint Ly alpha emitters. The lack of Ly alpha emission in DLA galaxies is consistent with the in situ star formation, for a given N(H I), being less efficient than what is seen in local galaxies. Thus, the overall DLA population seems to originate from the low-luminosity end of the high-redshift Ly alpha-emitting galaxies and/or to be located far away from the star-forming regions. The latter may well be true since we detect strong OVI absorption in the stacked spectrum, indicating that DLA galaxies are associated with a highly ionized phase, possibly the relics of galactic winds and/or originating from cold accretion flows. We find the contribution of DLA galaxies to the global star formation rate density to be comparatively lower than that of Lyman break galaxies.