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

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4. Discussion

The redshift of the DLA absorber towards Q0151+048A is larger than the redshift of the quasar, so the system resembles the [FORMULA] damped absorber at [FORMULA] towards the quasar PKS0528-250, for which we have previously reported the detection of Ly [FORMULA] emission (x1). The detection of emission in the trough of the Q0151+048A absorber therefore makes the resemblance closer. The line could be Ly [FORMULA] emission from the absorber or from a companion. We defer a detailed discussion of the nature of the emitter to a subsequent paper, where we will report on narrow-band imaging observations of the line (Fynbo, Moller, & Warren, in preparation). However it is interesting to note that Pettini et al. (1995) have discovered a similar, slightly offset, emission line in the trough of a third [FORMULA] damped absorber, towards the quasar 2059-360. It appears, therefore, that Ly [FORMULA] emission may be more common in or near DLA absorbers near quasars than in or near intervening DLA absorbers. Therefore in this section we firstly consider whether these [FORMULA] DLA absorbers are representatives of a different population to the intervening DLA absorbers. Two possibilities are considered; that the clouds belong to the class of intrinsic absorbers, probably ejected by the quasar, or that we are seeing the disks of the host galaxies. Both possibilities are rejected, so it is probable that the [FORMULA] DLA absorbers are similar to intervening DLA absorbers. This leads us to consider briefly the likely reason for enhanced Ly [FORMULA] emission near quasars.

In the following we limit ourselves to a discussion of the Q0151+048A and PKS0528-250 systems, as the relevant information for the quasar 2059-360 has yet to be published.

4.1. The nature of [FORMULA] DLA systems

4.1.1. Intrinsic [FORMULA] systems

If the [FORMULA] DLA absorbers are different from the intervening systems, one possibility is that they belong to the class of intrinsic absorbers, which includes the broad absorption lines (BALs), and the narrow intrinsic [FORMULA] systems optically thin in the continuum (Savaglio et al., 1994; Moller et al., 1994; Hamann 1997), which are possibly related to BAL systems. Both types of intrinsic absorber typically display complex, but generally smooth absorption profiles (e.g. Barlow & Sargent, 1997), whereas both the damped systems under discussion are well fit by single-component Voigt profiles. Intrinsic [FORMULA] systems also typically have very high metal abundances, solar or several tens times solar (Petitjean et al., 1994; Moller et al., 1994; Hamann 1997). The metallicity of the DLA absorber in PKS0528-250, on the other hand, was measured by Meyer et al. (1989) to be only 12% solar, and by Lu et al. (1996) to be 17% solar. These values are representative of other DLA absorbers. The metallicity of the Q0151+048A DLA absorber has yet to be measured.

The intrinsic [FORMULA] systems are also typically characterised by high ionization parameter. If one were to increase the column density of such a system to the point where it became optically thick to Lyman continuum photons, low ionization absorption lines would become visible. However, the part of the cloud facing the quasar would remain highly ionized, and one would have a system with mixed ionization (strong CIV and NV as well as SiII and CII). However neither of the DLA systems under discussion show strong NV absorption. Therefore, on the basis of absorption profile, metallicity, and ionisation parameter these two absorbers appear to be representative of other DLA absorbers, rather than the intrinsic [FORMULA] systems.

4.1.2. Quasar host galaxies

Another possible explanation might be that we are seeing neutral gas in the quasar host galaxy disk. However for Q0151+048A the quasar systemic redshift [FORMULA] (x3) and the absorber redshift [FORMULA] are significantly different. The same is probably true of PKS0528-250. Here the absorber redshift is [FORMULA], which differs from the quasar emission redshift [FORMULA], measured by us from the CIV line, by [FORMULA]. As discussed in x3 the systemic redshift of the quasar will be higher than the value measured from the CIV line. However, if we follow Tytler & Fan (1992) the correction is only [FORMULA], whereas Espey's (1997) work would suggest a correction of no more than [FORMULA]. Therefore these two DLA absorbers do not appear to be the signatures of disks of the quasar host galaxies.

4.2. Ly [FORMULA] emission and the effect of the quasar

If, as strongly suggested by the above discussion, the [FORMULA] DLA systems are the same as intervening DLA systems, the enhanced Ly [FORMULA] emission in or near the [FORMULA] absorbers implies that they occupy different environments to the intervening systems. The most obvious explanation that comes to mind is that the emission lines in the troughs of the Q0151+048A and 2059-360 absorbers, if Ly [FORMULA] , are due to photoionisation by the quasar. However, one can imagine several other possible explanations for the enhanced emission. For example gravitational interaction between the quasar and absorber might induce star formation. In any case the Ly [FORMULA] emission from the PKS0528-250 DLA appears to be due to star formation, as we have detected continuum emission from the absorber, as well as from two Ly [FORMULA] emitting companions (Paper III). This might suggest, instead, that the explanation for enhanced Ly [FORMULA] emission near quasars is that quasar activity (whatever the cause) is more common in regions where young galaxies are actively forming stars.

Another factor which could play a rôle is the so called proximity effect (e.g. Bajtlik et al., 1988). Powerful quasars are able to ionize the neutral hydrogen in the Lyman forest out to large distances from the quasar. The effect of this would be to reduce any line blanketing of Ly [FORMULA] emission from galaxies in the vicinity of the quasar. Although the average line blanketing in the Ly [FORMULA] forest of the continuum of a quasar is only modest at this redshift, [FORMULA], the average line blanketing of the Ly [FORMULA] emission line of a galaxy might be greater as it would be enhanced by the cloud-galaxy correlation function.

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Online publication: January 8, 1998