Astron. Astrophys. 330, 19-24 (1998)

1. Introduction

Two optical strategies have been employed to search for high-redshift normal galaxies. Using narrow-band imaging to detect Ly , relatively few have been discovered (e.g. Lowenthal et al. 1991, Moller & Warren 1993 (hereafter Paper I), Francis et al., 1996). However, recently Steidel and collaborators have had considerable success with broad-band imaging, identifying candidates by the expected Lyman break in their spectra (Steidel et al., 1996). The same strategy applied to the Hubble Deep Field data has provided a measurement of the global star formation rate in galaxies in the redshift interval (Madau et al 1997).

The searches for starlight from high-redshift galaxies are complemented by the analysis of the damped Ly (DLA) absorption lines in the spectra of high-redshift quasars. The DLA studies have yielded measurements of the mass density of neutral hydrogen in the universe (e.g. Wolfe, 1987; Lanzetta et al., 1991), and the abundance of heavy elements in the gas (e.g. Pettini et al., 1994; Lu et al., 1996), and how these quantities have changed with redshift. The relation between the DLA absorbers and the Lyman-break galaxies is not yet well established, but is important, as it will connect the measured global rate of star formation with the evolution of the global density of neutral gas, and its chemical enrichment. This will allow a more detailed comparison with theories of how galaxies are assembled. For this reason considerable effort has been devoted to the detection of DLA absorbers in emission, in order to measure the star formation rate for the absorber, and the sizes of the cloud of neutral gas, and of the region of star formation.

Unfortunately to date only two DLA absorbers have been successfully, and unambiguously, identified. These are i.) the system at , of column density , seen in the spectrum of the quasar PKS0528-250 (Paper I), and ii.) the system at , of column density , seen in the spectrum of the quasar Q2233+131 (Djorgovski et al 1996). In the latter case the column density is below the value usually recognised as defining a DLA system, but we will treat it as a DLA absorber here. The PKS0528-250 DLA absorber has been the subject of extensive imaging and spectroscopic observations by ourselves. These have yielded clues to the connection between DLA absorbers and the Lyman break galaxies, described below. The absorber is nevertheless unusual as the redshift is similar to the redshift of the quasar. There are only a few such DLA absorbers known.¹

The detection of Ly emission from the PKS0528-250 DLA absorber, as well as from two companions with similar redshifts, was reported in Paper I. The DLA absorber (as well as the companions) has subsequently been detected in the continuum, with the Hubble Space Telescope (HST), confirming that the Ly emission is due to star formation rather than photoionisation by the quasar (Moller & Warren, 1996). The measured half light radius of the continuum emission, , and the apparent magnitude, , are within the range measured for Lyman-break galaxies, which led us to suggest that the two are essentially the same population (Moller & Warren, 1997, hereafter Paper III). The two companions in this field are similarly small in size. These HST observations support our earlier suggestion (Warren & Moller, 1996, hereafter Paper II), based on dynamical evidence, that these three objects are sub-units of a galaxy in the process of assembly.

Pettini et al (1995) have detected a similar slightly-offset emission line in the trough of a second damped absorber at , towards the quasar 2059-360. Ly emission may therefore be more common in or near DLA absorbers near quasars than in or near intervening DLA absorbers. Because of the unusual nature of the PKS0528-250 absorber, and to investigate the possibility that Ly emission in DLA absorbers is in some way enhanced in systems, over intervening systems, we have obtained spectra of a third DLA absorber, the system at seen in the spectrum of the quasar Q0151+048A. This absorber was first studied by Williams & Weymann (1976). The quasar (=UM144=PHL1222, 1950.0 coordinates RA 1 51 17.43, Dec 4 48 15.1) is radio quiet, optically is relatively bright (, ), non variable, and has a faint () companion quasar Q0151+048B lying 3.3" to the NE, and at a similar redshift, discovered by Meylan et al. (1990).

The spectroscopic observations are described in x2. In x3 we present the spectrum. By fitting a Voigt profile to the damped absorption line we find evidence for an emission line near the base of the trough, just blueward of the absorption line centre. In x4 we provide a discussion of the question of whether the DLA absorbers are intrinsically different to the intervening DLA absorbers, and in x5 we list our conclusions.

© European Southern Observatory (ESO) 1998

Online publication: January 8, 1998
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