A survey of analogs to weak Mg II absorbers in the present

We present the results of a survey of the analogs of weak Mg II absorbers [rest-frame equivalent width W_r(2796) < 0.3 Å] at 0 < z < 0.3. Our sample consisted of 25 HST STIS echelle quasar spectra (R = 45,000), which covered Si II λ1260 and C II λ1335 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 Å, with 30% completeness for the weakest lines. Correcting for incompleteness, we find the number of weak Mg II absorber analogs with 0.02 Å < W_r,(2796) < 0.3 Å to be dN/dz = 1.00 ±0.20 for 0 < z < 0.3. This compares to a value of dN/dz = 1.74 ± 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 ∼ 1 to ∼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 ∼ 2-3 at z ∼ 0. About 30%-50% of this z ∼ 0 population would be descendants of the parsec-scale structures that produce single-cloud, weak Mg n absorbers at z ∼ 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 ∼ 1. We conclude that at least one, and perhaps some fraction of both, of these populations has evolved away since z ∼ 1, in order to match the z ∼ 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 ∼ 1.