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

Abstract

Using a spectral stacking technique we searched for the average Lyman (Lyα) emission from high-z damped Lyα (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 3σ upper limit of 3.0 × 10^-18 erg s^-1 cm^-2 on the Lyα flux emitted within ∼1.5 arcsec (or 12 kpc) from the QSO line of sight. This corresponds to an average Lyα luminosity of ≤2 × 10⁴¹ erg s^-1 or 0.03L_∗ (Lyα). This limit is deeper than the limit of most surveys for faint Lyα emitters. The lack of Lyα 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 Lyα-emitting galaxies and/or to be located far away from the star-forming regions. The latter may well be true since we detect strong O vi 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.