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

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6. Conclusions

We have compiled from the literature all available data on element abundances in DLA systems measured over the last decade, referred them all to one set of oscillator strength and solar reference values and subdivided them into reliable and less reliable data.

We extended our chemical galaxy evolution model to calculate abundances for altogether 16 different elements in spiral galaxies in a chemically consistent way, i.e. accounting for the steadily increasing initial metallicities of successive generations of stars by using input data bases (stellar yields, evolutionary tracks, lifetimes and remnant masses) for 5 different metallicities from Z = 0 [FORMULA] Z = [FORMULA]. Dynamical effects, however, are not included in our simple 1-zone models.

A detailed comparison of model results with observations for eight different element abundances yields the following main results:

  • The conformity between observed element abundances in DLA systems and those calculated from our models for spiral galaxies spanning the whole redshift range [FORMULA] indicates that DLA galaxies may well evolve into the full range of present-day spiral galaxies.

  • Without any adjustments and only using SFHs that proved successful for the spectrophotometric description of spiral galaxies from zero to high redshift our models successfully bridge the gap between abundances observed in high redshift DLA absorbers and the H II regions of present-day normal spiral galaxies Sa through Sd.

  • The slow redshift evolution of DLA abundances (compared to halo CIV systems) is a consequence of the relatively long timescales for star formation in disk galaxies.

  • The large scatter observed in element abundances in DLAs of similar redshift is naturally explained by the range of star formation rates at any redshift between early and late type spiral galaxies.

  • The few observations exceeding the abundances calculated for our Sa model can be explained either by a temporarily enhanced SFR or a small starburst in early type spirals if only SNII products are enhanced or by a shorter characteristic timescale for star formation, very early star formation enhancement or a local overabundance along the line of sight if both SNI and SNII products are enhanced.

  • Comparison of our chemically consistent models with models using only solar metallicity input physics shows that differences in the redshift evolution are small for some elements but large for others. For the elements Zn, Ni and Al with large differences the chemically consistent models provide a significantly better agreement with observed DLA abundances.

  • Using a Salpeter IMF instead of Scalo yields larger element abundances throughout and many data points fall below the curve of the Sd model. This could be compensated by using a lower SFR and means that more DLAs could be LSB galaxies.

Our comparison of element abundances observed in DLA systems with those resulting from our chemically consistent galaxy evolution models has important implications for the nature of low redshift DLAs and the possibility of optical identification of DLAs over the whole redshift range.

  • The upper envelope to observed DLA abundances increases from [FORMULA] to [FORMULA] and decreases to smaller redshifts whereas our Sa model abundances increase steadily. This leads to the conclusion that Sa galaxies at low redshift may not have gas at sufficient HI column densities over large enough cross sections to cause damped [FORMULA] absorption, whereas Sd galaxies appear as DLA systems down to [FORMULA]. This can explain why much less DLA systems have been found at low redshift in the HST key project QSO Absorption Lines (Bahcall et al. 1993) than expected from their high redshift frequency.

  • The comparison between our models and observations suggests that DLA systems could be the progenitors of Sa to Sd type galaxies with intrinsically faint late type spirals dominating at low redshift. Estimates from our chemically consistent spectrophotometric evolution models - including evolutionary and cosmological corrections - predict comparable luminosities for the brighter early type spiral DLA galaxies at [FORMULA] and for the intrinsically fainter late type spirals expected to dominate DLA samples at [FORMULA]: [FORMULA], [FORMULA] [FORMULA] and [FORMULA].

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© European Southern Observatory (ESO) 1999

Online publication: December 16, 1998