2011ApJ...729...87C

We identified 24 Si IV absorption systems with z ≲ 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 ≥ 3σ_Wr . The absorber line density of the G = 1 doublets was {{°} N_{Si iv}/{°}X}= 1.4^{+0.4}_{-0.3} 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^+0.5_–0.4)x10^–14 cm² and slope α_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:Ω_Si^+3= (3.7^{+2.8}_{-1.7}) ×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^+3.0_–1.9 higher than the 2 ≤ z ≤ 4.5 <Ω_Si^+3>. From a simple linear fit to Ω_Si^+3 over the age of the universe, we estimated a slow and steady increase from z = 5.5 {x2192} 0 with {°}Ω_Si^+3°t_age = (0.61±0.23) ×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.