Directly imaging damped Lyman α galaxies at z > 2 - I. Methodology and first results

We present the methodology for, and the first results from, a new imaging programme aimed at identifying and characterizing the host galaxies of damped Lyman α absorbers (DLAs) at z≥ 2. We target quasar sightlines with multiple optically thick H i absorbers and use the higher redshift system as a 'blocking filter' (via its Lyman-limit absorption) to eliminate all far-ultraviolet (FUV) emission from the quasar. This allows us to directly image the rest-frame FUV continuum emission of the lower redshift DLA, without any quasar contamination and with no bias towards large impact parameters. We introduce a formalism based on galaxy number counts and Bayesian statistics with which we quantify the probability that a candidate is the DLA host galaxy. This method will allow the identification of a bona fide sample of DLAs that are too faint to be spectroscopically confirmed. The same formalism can be adopted to the study of other quasar absorption-line systems (e.g. Mg ii absorbers). We have applied this imaging technique to two quasi-stellar object sightlines. For the z~ 2.69 DLA towards J073149+285449, a galaxy with impact parameter b= 1.54 arcsec = 11.89 h-172 kpc and an implied star formation rate (SFR) of ~5 h-272 M. yr-1 is identified as the most reliable candidate. In the case of the z~ 2.92 DLA towards J211444-005533, no likely host is found down to a 3σ SFR limit of 1.4 h-272 M. yr-1. Studying the H i column density as a function of the impact parameter, including six DLAs with known hosts from the literature, we find evidence that the observed H i distribution is more extended than what is generally predicted from numerical simulation. © 2010 The Authors. Journal compilation © 2010 RAS.