A Survey of Analogs to Weak Mg ii Absorbers in the Present

Abstract

We present the results of a survey of the analogs of weak Mg II absorbers {[}rest-frame equivalent width W(r)(2796)< 0: 3 angstrom] at 0 < z < 0.3. Our sample consisted of 25 HST STIS echelle quasar spectra ( R 45; 000), which covered Si II lambda 1260 and C II lambda 1335 over this redshift range. Using those similar transitions as tracers of Mg II facilitates a much larger survey, covering a redshift path length of g( z) 5: 3 for an equivalent width limit of Mg ii corresponding to W(r)( 2796)> 0.02 angstrom, with 30% completeness for the weakest lines. Correcting for incompleteness, we find the number of weak Mg II absorber analogs with 0.02 angstrom < W(r)(2796)< 0.3 angstrom to be dN/ dz 1.00 +/- 0.20 for 0 < z < 0.3. This compares to a value of dN/ dz 1: 74 similar to 0: 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 Mg ii absorbers of the decreasing intensity of the extragalactic background radiation field from z similar to 1 to similar to 0. Assuming that all the objects that produce absorption at z similar to 1 are stable on a cosmological timescale and that no new objects are created, we would expect dN/ dz similar to 2 3 at z similar to 0. About 30% - 50% of this z similar to 0 population would be descendants of the parsec-scale structures that produce single-cloud, weak Mg ii absorbers at z similar to 1. The other 50% - 70% would be lower density, kiloparsec-scale structures that produce C iv absorption, but not detectable low-ionization absorption, at z similar to 1. We conclude that at least one, and perhaps some fraction of both, of these populations has evolved away since z similar to 1, in order to match the z similar to 0 value of 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 similar to 1.