The Building the Bridge survey for z =3 Lyα emitting galaxies I. Method and first results

Abstract

We present the first results of an observational programme at the ESO Very Large Telescope aimed at detecting a large sample of high-redshift galaxies fainter than the current spectroscopic limit of $R=25.5$ for Lyman-Break galaxies. In this paper, we describe the results of deep narrow and broad-band imaging and subsequent follow-up multi-object spectroscopy of faint high-redshift galaxies in the fields of the QSOs BRI 1346-0322 and Q 2138-4427. These QSOs have intervening high neutral hydrogen column density absorbers, at redshifts $z=2.85$ and $z=3.15$ respectively, for which redshifted Ly α emission falls within less than a few A from the central wavelengths of existing VLT (~60 A-wide) narrow-band filters. We selected 37 and 27 candidate emission-line galaxies in the two fields respectively. Most (~85%) of the candidates have R -band magnitudes fainter than $R=25.5$. The first spectroscopic follow-up of a sub-sample of the candidates resulted in 41 confirmed candidates and 4 foreground emission line galaxies (three [$\ion{O}{ii}$] emitters and one $\ion{C}{iv}$ emitter). The confirmation rate for Ly α emitters is 82% and 68% in the field of BRI 1346-0322 and Q 2138-4427 respectively. In addition, we serendipitously detect a number of other emission-line sources on some of the slitlets not used for candidates. Of these, 9 are also most likely Ly α emitters with redshifts ranging from 1.98 to 3.47. The redshift distribution of confirmed candidates in the field of BRI 1346-0322 is consistent with being drawn from a uniform distribution weighted by the filter response curve, whereas the galaxies in the field of Q 2138-4427 have redshifts clustering very close to the redshift of the damped Ly α absorber. This latter fact indicates the existence of a large “pancake”-like structure confirming the earlier suggestions of Francis & Hewitt ([CITE]).