THE LAST EIGHT-BILLION YEARS OF INTERGALACTIC Si IV EVOLUTION

Abstract

We identified 24 SiIV absorption systems with z <~ 1 from a blind survey of\n49 low-redshift quasars with archival Hubble Space Telescope ultraviolet\nspectra. We relied solely on the characteristic wavelength separation of the\ndoublet to automatically detect candidates. After visual inspection, we defined\na sample of 20 definite (group G = 1) and 4 "highly-likely" (G = 2) doublets\nwith rest equivalent widths W_r for both lines detected at > 3 sigma. The\nabsorber line density of the G = 1 doublets was dN_SiIV/dX = 1.4+0.4/-0.3 for\nlog N(Si+3) > 12.9. The best-fit power law to the G = 1 frequency distribution\nof column densities f(N(Si+3)) had normalization k = (1.2+0.5/-0.4) x 10^-14\ncm2 and slope alpha = -1.6+0.3/-0.3. Using the power-law model of f(N(Si+3)),\nwe measured the Si+3 mass density relative to the critical density: Omega(Si+3)\n= (3.7+2.8/-1.7) x 10^-8 for 13 < log N(Si+3) < 15. From Monte Carlo sampling\nof the distributions, we estimated our value to be a factor of 4.8+3.0/-1.9\nhigher than the 2 < z < 4.5 <Omega(Si+3)>. From a simple linear fit to\nOmega(Si+3) over the age of the Universe, we estimated a slow and steady\nincrease from z = 5.5 --> 0 with dOmega/dt_age = (0.61+/-0.23) x 10^-8 Gyr^-1.\nWe compared our ionic ratios N(Si+3)/N(C+3) to a 2 < z < 4.5 sample and\nconcluded, from survival analysis, that the two populations are similar, with\nmedian <N(Si+3)/N(C+3)> = 0.16.\n