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R. Lynch, J. Charlton (Penn State), T.-S. Kim, R. Carswell (Cambridge U.)
We present a survey of weak MgII absorbers, W(2796)<0.3, in ~30 quasar spectra from the UVES high resolution spectrograph on the VLT, with a cumulative redshift path of ~30 at z ~2. Weak MgII absorbers are regions of high metallicity that occur apart from luminous galaxies, yet cover the same fraction of the sky. These structures could be related to invisible dwarf galaxies in the dark matter halos predicted by cold dark matter or to intergalactic star clusters. Similar surveys have been conducted at z ~0 and z ~1 by Narayanan et al. and Churchill et al., respectively. They found dN/dz=0.96 for z ~0 and dN/dz = 1.74 for z ~1, consistent with the expected cosmological evolution over that redshift interval. The number of weak MgII absorbers found at z ~ 0 is five times less than what is expected based on the evolution of the extragalactic background radiation. This suggests that these weak MgII structures are not stable, and that the rate of the processes that generate a transient population must be diminishing. At z ~2, given the redshift path, we would expect a dN/dz ~2.5 based on the cosmological evolution of the z ~1 population of weak MgII absorbers. For a stable population we would expect fewer weak MgII absorbers at high redshift based on an increase in the extragalactic background radiation, but if the absorbers are transient then we must consider the processes by which they are formed, e.g. superwinds or any processes related to star formation should be more important at z=2 than at z=1. We will present preliminary results of our study, which will shed light on the evolution of the population and provide clues to its origins.
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Bulletin of the American Astronomical Society, 36 5
© 2004. The American Astronomical Society.