The Last Eight-billion Years of Intergalactic Si IV Evolution

Abstract

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 ≥ 3{σ_{{W_{r}}}}. The absorber line density of the G = 1 doublets was {{{d}}{{N}}_{Si IV}}/{{d}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) × 10^-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 → 0 with {{d}}{Ω_{{{Si^{+3}}}}}/{{d}}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.