Damped Lymanalpha systems

precious clues to the first cycles of nucleosynthesis, complementing those obtained from parallel stellar abundance studies in the Milky Way halo (e.g. Erni et al. 2006).

However, all this may be about to change. Thanks in part to the order-of-magnitude increase in the number of known QSOs - and therefore DLAs - brought about by the SDSS, there have been several reports in the last few months of discoveries of metal-rich DLAs, or sub-DLAs.1 These systems may make a significant contribution to the census of metals, particularly at redshifts z < 1 (Kulkarni et al. 2006). Particularly intriguing are the claims of highly supersolar abundances, by factors of 4-5, in two cases (Peroux et al. 2006; Prochaska et al. 2006). In principle, metallicity measurements from interstellar absorption lines are the most straightforward to perform - the underlying physics is simple, and practical complications such as saturation of the metal lines and blending of the Lya line with nearby unrelated absorption features would both work in the sense of decreasing the abundances deduced. Possibly, the gas is more highly ionised than can be judged from the available data. In any case, it is clear that these extreme cases are highly deserving of further study: at redshifts z < 1 imaging should presumably reveal the presence of massive, luminous galaxies close to the QSO sight-lines and, looking further ahead, observations of O i and N i absorption lines with a UV spec-trograph on the refurbished Hubble Space Telescope should clarify the importance of ionisation corrections.

In a related development, Wild et al. (2006) recently suggested that absorption systems with strong Caii A.A.3933, 3968 absorption lines (with an equivalent width greater than 0.5 A for the stronger member of the doublet) are likely to be high-column-density DLAs. The Ca ii doublet, which has been mapped extensively in the Galactic ISM since the discovery of its interstellar nature by Eddington (Adams 1941), has been relatively neglected at high redshifts, where it moves into a difficult part of the spectrum at z > 1. Now, with a database of QSO spectra as extensive as that available in the SDSS, it may provide an effective way to isolate DLAs specifically in that all-important redshift interval, z < 1, which accounts for more than half of the age of the Universe and yet is still relatively unexplored with 'conventional' DLAs, which require space observations to be identified in the first place. At these intermediate-to-low redshifts, it is possible with modern instrumentation to identify the galaxies responsible for the Ca ii absorption and measure their physical properties. In a pilot study, admittedly aimed at the more luminous galaxies within

1 The conventional definition of a damped Lya system is one in which the column density of neutral hydrogen is above the threshold N(H i) = 2 x 1020cm-2, but absorption systems with column densities one order of magnitude lowerthan this value also exhibit (obviously weaker) damping wings in the Lya line; they are usually referred to as sub-DLAs or 'super-Lyman-limit systems' - another example of the confusing power of labels in astronomy!

Figure 20.7. Colour composite (u, g, r, i, z) images of the eight SDSS QSOs with zabs < 0.5 Ca ii absorption systems studied by Zych et al. (2007, in preparation). In each panel the galaxy closest to the QSO sight-line is at the same redshift as the Ca ii lines in the QSO spectrum and is presumably the absorber. (The Ca ii-bearing galaxy is directly beneath the QSO image in the third panel from the left in the top row and the second panel in the bottom row).

Figure 20.7. Colour composite (u, g, r, i, z) images of the eight SDSS QSOs with zabs < 0.5 Ca ii absorption systems studied by Zych et al. (2007, in preparation). In each panel the galaxy closest to the QSO sight-line is at the same redshift as the Ca ii lines in the QSO spectrum and is presumably the absorber. (The Ca ii-bearing galaxy is directly beneath the QSO image in the third panel from the left in the top row and the second panel in the bottom row).

the SDSS sample of Ca ii absorbers at zabs < 0.5, Zych etal. (2007, in preparation) found that all eight galaxies targeted (see Figure 20.7) are metal-rich, with nebular abundances (O/H) > (O/H)0, and are sustaining vigorous rates of star formation in excess of a few Solar masses per year. These preliminary results are at odds with the conventional notion of metal-poor, quiescent, DLA galaxies and suggest that this picture may still be incomplete. They certainly stimulate efforts to clarify the relationship between Ca ii- and H i-selected DLAs.

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