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Astron. Astrophys. 341, 709-724 (1999)

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5. Implications for DLA systems

5.1. DLAs at low redshift: abundances versus gas content

In Fig. 9 we present a reduced version of Fig. 7a to show that the upper envelope to the observed DLA abundances (heavy line) clearly increases from [FORMULA] towards [FORMULA] but declines or remains constant from [FORMULA] to [FORMULA]. This behavior is seen for all eight elements investigated in Fig. 7. For [FORMULA] through [FORMULA] abundance data for all these elements completely cover the range between our models for Sa through Sd galaxies. For [FORMULA] the situation changes. The lower the redshift of DLA absorbers the closer do their observed abundances fall to our early type spiral models. It looks as if at [FORMULA] Sa disks would no longer have large enough cross sections at the high HI column densities required for damped [FORMULA] absorption to appear in DLA samples. Virtually all of the [FORMULA] abundance data fall close to our models for Sc or Sd galaxies. In our simple 1-zone galaxy models we do not have any information about gas densities but only about the total gas content.

[FIGURE] Fig. 9. This reduced version of Fig. 7a shows the upper envelope (heavy line) to the observed Fe abundances in DLAs and our chemically consistent evolution models for Sa and Sd galaxies.

Interestingly, this global gas content for our Sa model drops from more than 50% (of the total mass) at [FORMULA] to [FORMULA]% at [FORMULA] while that of our Sd model still is more than 60% at [FORMULA]. So, neglecting any density structure of the HI disk which is not included in our modelling the comparison between observed abundance data and the enrichment evolution of our models suggests that as the global gas content falls below 50% galaxies drop out of the DLA absorber sample. From their efforts to optically identify DLA systems Steidel et al. (1995) suspect that there is "a selection effect against luminous spiral galaxies (like our Galaxy) for moderate redshift DLA systems" (see also Steidel et al. 1997). It would be interesting to check our prediction for low-z DLA galaxy types with HST.

5.2. Implications for optical identifications

The comparison of our spiral models with observed DLA abundances suggests that the bulk of the DLAs are the high redshift progenitors of present-day spiral galaxies from Sa through Sd. This has implications for the possibilities of optical detections.

Spiral galaxies at [FORMULA] have luminosities in the range from [FORMULA] for Sd galaxies to [FORMULA] for Sa galaxies, when using the mean values for these types in the Virgo cluster as given by Sandage et al. (1985). In any case, typical spirals are significantly fainter than [FORMULA]. Moreover we argued in Sect. 5.1 that the brighter early type spirals seem to drop out of the DLA sample towards low redshift due to their gas content becoming too scarce. This leads us to expect that low redshift DLA samples should be dominated by the particularly faint gas-rich late-type spiral (or even irregular or LSB) galaxies . Hence we do expect early type spirals to be among the DLA absorbers only for redshift larger than [FORMULA].

With evolutionary and cosmological corrections calculated from our chemically consistent spectrophotometric evolutionary synthesis code (Möller et al. 1998) we find that the typical luminosity of Sa galaxies increases up to [FORMULA] in the range [FORMULA]. Taking into account bolometric distance moduli for [FORMULA] and 3 and using cosmological parameters [FORMULA] and [FORMULA] this yields apparent magnitudes of about [FORMULA] which are close to the detection limit. Later galaxy types are fainter, e.g. an average Sd galaxy at [FORMULA] has [FORMULA] and [FORMULA], at [FORMULA] it has [FORMULA] and [FORMULA] and at [FORMULA] an average Sd galaxy has [FORMULA] and [FORMULA]. In terms of [FORMULA] -magnitudes we expect the average early type spiral Sa to have [FORMULA] [FORMULA] at [FORMULA] and [FORMULA] [FORMULA] at [FORMULA]. This explains why deep surveys did not detect DLAs at [FORMULA] down to [FORMULA] [FORMULA] (cf. Steidel et al. 1998). In the K-band our models predict [FORMULA] for Sa and [FORMULA] for Sd galaxies at [FORMULA]. This also makes us understand that on deep K images of ten QSOs with DLA systems in the redshift range [FORMULA] investigated by Aragon-Salamanca et al. (1996) only two candidates with [FORMULA] (i.e. from the bright end of the spiral galaxy luminosity function) have been detected. Since our chemical evolution models suggest that the brighter early type galaxies should drop out of the DLA sample due to too scarce gas content towards low redshift we expect optical identifications of low redshift DLA systems not to be easier: an average late-type spiral at [FORMULA] is expected to have [FORMULA], [FORMULA] [FORMULA] and [FORMULA]. The galaxies identified by Steidel et al. (1994and 1995) in the fields of 3C 286 (Q1328+307), PKS 1229-021 and PKS 0454+039 as candidates for DLA absorbers at [FORMULA], [FORMULA] and 0.7568 and [FORMULA], respectively, indeed have typical luminosities between [FORMULA] and -19.5 of late type spiral galaxies. We conclude that many DLA candidates are out of reach for present day imaging capabilities and hence for optical identification.

5.3. Possible implications for abundance ratios

If our result that DLA absorbers might well be associated with the progenitors of normal present-day spiral galaxies of all types from Sa through Sd were confirmed by high resolution imaging observations, a direct by-product of our models could be the ISM abundance ratios of various elements and their evolution with redshift. To the extent that observed Milky Way disk star abundance ratios directly reflect the ISM abundance ratios at birth of those stars we would then expect a disk-like rather than a halo-like abundance pattern for DLA systems (cf. the controversy on halo- vs. disk-like DLA abundance patterns between Lu et al. (1996) and Kulkarni et al. (1997)). Due to uncertainties in the stellar input yields, a rather small number of observed DLA abundance ratios available at present, and a deficient knowledge of how to correct for the effect of selective dust condensation, however, we estimate it premature to base any conclusion on such a comparison at present.

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© European Southern Observatory (ESO) 1999

Online publication: December 16, 1998