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J. C. Charlton, A. Narayanan, R. Lynch (Penn State), J. R. Masiero (U. Hawaii)
We present the results of a survey of the analogs of weak MgII absorbers (rest-frame equivalent width W(2796) < 0.3Å) at 0 < z < 0.3. Our sample consisted of 25 HST/STIS echelle quasar spectra (R=45,000) which covered SiII 1260 and CII 1335 over this redshift range. Using those similar transitions as tracers of MgII facilitates a much larger survey, covering a redshift pathlength of g(z)=5.3 for an equivalent width limit of MgII corresponding to Wr(2796) >0.02Å, with 30% completeness for the weakest lines. Correcting for incompleteness, we find the number of weak MgII absorber analogs with 0.02 < W(2796) < 0.3Åto be dN/dz = 0.96 ±0.19 for 0 < z < 0.3. This compares to a value of dN/dz =1.74±.10 found by Churchill et al. for the higher redshift range, 0.4 < z < 1.4, and is consistent with cosmological evolution of the population. We consider the expected effect on observability of weak MgII absorbers of the decreasing intensity of the extragalactic background radiation field from z ~1 to z ~0. Assuming that all the objects that produce absorption at z ~1 are stable on a cosmological timescale, and that no new objects are created, we would expect dN/dz ~5 at z ~0. About 20% of this z ~0 population would be decendants of the parsec-scale structures that produce single-cloud, weak MgII absorbers at z ~1. The other 80% would be lower density, kiloparsec-scale structures that produce CIV absorption, but not detectable low ionization absorption, at z ~1. We conclude that at least the latter, and perhaps both of these populations have evolved away since z ~1, in order to match the z ~0 dN/dz measured in our survey. This would follow naturally for a population of transient structures whose generation is related to star-forming processes, whose rate has decreased since z ~1.
The author(s) of this abstract have provided an email address for comments about the abstract: charlton@astro.psu.edu
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Bulletin of the American Astronomical Society, 36 5
© 2004. The American Astronomical Society.