THE LAST EIGHT-BILLION YEARS OF INTERGALACTIC Si IV EVOLUTION

We identified 24 Si IV absorption systems with z less than or similar to 1 from a blind survey of 49 low-redshift quasars with archival Hubble Space Telescope ultraviolet spectra. We relied solely on the characteristic wavelength separation of the doublet to automatically detect candidates. After visual inspection, we defined a sample of 20 definite (group G = 1) and 4 highly likely (G = 2) doublets with rest equivalent widths W-r for both lines detected at >= 3 sigma(Wr). The absorber line density of the G = 1 doublets was dN(Si IV)/dX = 1.4(-0.3)(+0.4) for log N(Si+3) > 12.9. The best-fit power law to the G = 1 frequency distribution of column densities f (N(Si+3)) had normalization k = (1.2(-0.4)(+0.5)) x 10(-14) cm(2) and slope alpha(N) = -1.6(-0.3)(+0.3). Using the power-law model of f (N(Si+3)), we measured the Si+3 mass density relative to the critical density: Omega(Si+3) = (3.7(-1.7)(+2.8)) x 10(-8) for 13 <= log N(Si+3) <= 15. From Monte Carlo sampling of the distributions, we estimated our value to be a factor of 4.8(-1.9)(+3.0) higher than the 2 <= z <= 4.5 . From a simple linear fit to Omega(Si+3) over the age of the universe, we estimated a slow and steady increase from z = 5.5 -> 0 with d Omega(Si+3)/dt(age) = (0.61 +/- 0.23) x 10(-8) Gyr(-1). We compared our ionic ratios N(Si+3)/N(C+3) to a 2 < z < 4.5 sample and concluded, from survival analysis, that the two populations are similar, with median < N(Si+3)/N(C+3)> = 0.16.