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 Wr for both lines detected at ≥3σWr. The absorber line density of the G = 1doublets was dNSiiv/dX=1.4+0.4−0.3for 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)×10−14cm2 and slope α N=−16+0.3−0.3. Using the power-law model off(N(Si+3)), we measured the Si+3 mass density relative to the critical density: Si+3=(3.7+2.8−1.7)×10−8for 13logN(Si+3)15. From Monte Carlo sampling of the distributions, we estimated our value to be a factor of 4.8+3.0−1.9higher than the 2z4.5Si+3. From a simple linear fit to Si+3 over the age of the universe, we estimated a slow and steady increase from z=5.5→0 with dSi+3/d Ω age =(0.61±0.23)×10−8Gyr−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 medianN(Si+3)/N(C+3) =0.16.