The Building the Bridge survey for z = 3 Lyα emitting galaxies

Abstract

Context. We have substantial information about the kinematics and abundances of galaxies at z ≈ 3 studied in absorption against the light of background QSOs. At the same time we have already studied 1000s of galaxies detected in emission mainly through the Lyman-break selection technique; however, we know very little about how to make the connection between the two data sets. Aims. We aim at bridging the gap between absorption-selected and emission-selected galaxies at z ≈ 3 by probing the faint end of the luminosity function of star-forming galaxies at z ≈ 3. Methods. Narrow-band surveys for Lyman-α (Lyα) emitters have proven to be an efficient probe of faint, star-forming galaxies in the high-redshift universe. We performed narrow-band imaging in three fields with intervening QSO absorbers (a damped Lyα absorber and two Lyman-limit systems) using the VLT. We target Lyα at redshifts 2.85, 3.15, and 3.20. Results. We find a consistent surface density of about 10 Lyα-emitters per square arcmin per unit redshift in all three fields down to our detection limit of about 3 × 10 41 erg s −1 . The luminosity function is consistent with what has been found by other surveys at similar redshifts. About 85% of the sources are fainter than the canonical limit of R = 25.5 for most Lyman-break galaxy surveys. In none of the three fields do we detect the emission counterparts of the QSO absorbers. In particular we do not detect the counterpart of the z = 2.85 damped Lyα absorber towards Q2138−4427. This implies that the DLA galaxy is either not a Lyα emitter or is fainter than our flux limit. Conclusions. Narrow-band surveys for Lyα emitters are excellent for probing the faint end of the luminosity function at z ≈ 3. There is a very high surface density of this class of objects; yet, we only detect galaxies with Lyα in emission, so the density of galaxies with similar broad band magnitudes will be substantially higher. This is consistent with a very steep slope of the faint end of the luminosity function as has been inferred by other studies. This faint population of galaxies is playing a central role in the early Universe. There is evidence that this popualtion is dominating the intergrated star-formation activity, responsible for the bulk of the ionising photons at z 3 and likely also responsible for the bulk of the enrichment of the intergalactic medium.