2010MNRAS.408..362F

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 HI 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. Mgii 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 = 1.54arcsec = 11.89h^–1₇₂kpc and an implied star formation rate (SFR) of ∼5h^–2₇₂M_☉/yr 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 1.4h^–2₇₂M_☉/yr. Studying the HI column density as a function of the impact parameter, including six DLAs with known hosts from the literature, we find evidence that the observed HI distribution is more extended than what is generally predicted from numerical simulation.