High-redshift DLA Research Articles

On the Nature and Redshift Evolution of DLA Galaxies

We extend our spiral galaxy models, which successfully describe nearby template spectra as well as the redshift evolution of CFRS and HDF spirals, to include – in a chemically consistent way – the redshift evolution of a series of individual elements. Comparison with observed DLA abundances shows that DLAs might well be the progenitors of present-day spiral types Sa through Sd. Our models bridge the gap between high redshift DLA and nearby spiral HII region abundances. The slow redshift evolution of DLA abundances is a natural consequence of the long SF timescales for discs, the scatter at any redshift reflects the range of SF timescales from early to late spiral types. We claim that, while at high redshift all spiral progenitor types seem to give rise to DLA absorption, towards low redshifts, the early-type spirals seem to drop out of DLA samples due to low gas and/or high metal and dust content. Model implications for the spectrophotometric properties of the DLA galaxy population are discussed in the context of campaigns for the optical identifications of DLA galaxies both at low and high redshift.

Chemical Evolution of Damped Lyα Galaxies: The [S/Zn] Abundance Ratio at Redshift ≥2

Relative elemental abundances, and in particular the α/Fe ratio, are important diagnostic tools of the chemical evolution of damped Lyα (DLA) systems. The S/Zn ratio is not affected by differential dust depletion and is an excellent estimator of the α/Fe ratio. We report six new determinations of sulphur abundance in DLA systems at zabs ≥ 2 with already-known zinc abundances. The combination with extant data from the literature provides a measure of the S/Zn abundance ratio for a total of 11 high-redshift DLA systems. The observed [S/Zn] ratios do not show the characteristic [α/Fe] enhancement observed in metal-poor stars of the Milky Way at a comparable level of metallicity ([Zn/H] ≈ -1). The behavior of DLA data is consistent with a general trend of a decreasing [S/Zn] ratio with increasing metallicity [Zn/H]. This would be the first evidence of the expected decrease of the α/Fe ratio in the course of the chemical evolution of DLA systems. However, in contrast to what is observed in our Galaxy, the α/iron peak ratio seems to attain solar values when the metallicity is still low ([Zn/H] ≤ -1) and to decrease below solar values at higher metallicities. The behavior of the α/Fe ratio challenges the frequently adopted hypothesis that high-redshift DLA systems are progenitors of spiral galaxies and instead favors an origin in galaxies characterized by low star formation rates, in agreement with the results from imaging studies of low-redshift DLA systems, where the candidate DLA galaxies show a variety of morphological types, including dwarf and low surface brightness galaxies and only a minority of spirals.

NICMOS Imaging of the Damped Lyα Absorber atz = 1.89 toward LBQS 1210+1731: Constraints on Size and Star Formation Rate

We report results of a high-resolution imaging search for the galaxy associated with the damped Lyα (DLA) absorber at z = 1.892 toward the z_(em) = 2.543 quasar LBQS 1210+1731, using the Hubble Space Telescope (HST) NICMOS. The images were obtained in the broad filter F160W and the narrow filter F190N with camera 2 on NICMOS and were aimed at detecting the absorber in the rest-frame optical continuum and in Hα line emission from the DLA absorber. After suitable point-spread function (PSF) subtractions, a feature is seen in both the broadband and narrowband images, at a projected separation of 0.25 from the quasar. This feature may be associated with the DLA absorber, although we cannot completely rule out that it could be a PSF artifact. If associated with the DLA, the object would be ≈ 2-3 h^(-)_(07) kpc in size with a flux of 9.8 ± 2.4 μJy in the F160W filter, implying a luminosity at λ_(central) = 5500 A in the rest frame of 1.5 × 10^(10) h^(-2)_(70) L_☉ at z = 1.89, for q_0 = 0.5. However, a comparison of the fluxes in the broad and narrow filters indicates that most of the flux in the narrowband filter is continuum emission, rather than redshifted Hα line emission. This suggests that if this object is the absorber, then either it has a low star formation rate (SFR), with a 3 σ upper limit of 4.0 h^(-2)_(70) M_☉ yr^(-1), or dust obscuration is important. It is possible that the Hα emission may be extinguished by dust, but this seems unlikely, given the typically low dust-to-gas ratios observed in DLAs. Alternatively, the object, if real, may be associated with the host galaxy of the quasar rather than with the damped Lyα absorber. H-band images obtained with the NICMOS camera 2 coronagraph show a much fainter structure ≈4-5 h^(-2)_(70) kpc in size and containing four knots of continuum emission, located 0.7 away from the quasar. This structure is not seen in images of comparison stars after similar PSF subtractions and is also likely to be associated with the absorbing galaxy or its companions, although we do not know its redshift. We have probed regions far closer to the quasar sight line than in most previous studies of high-redshift intervening DLAs. The two objects we report mark the closest detected high-redshift DLA candidates yet to any quasar sight line. If the features in our images are associated with the DLA, they suggest faint, compact, somewhat clumpy objects rather than large, well-formed protogalactic disks or spheroids. If the features are PSF artifacts, then the constraints on sizes and star formation rates of the DLA are even more severe. The size, luminosity, and SFR estimates mentioned above should therefore be conservatively considered as upper limits.