THE LAST EIGHT-BILLION YEARS OF INTERGALACTIC Si iv EVOLUTION

We surveyed the Hubble Space Telescope UV spectra of 49 low-redshift quasars for z =3sigma{sub W{sub r}}. We conducted Monte Carlo completeness tests to measure the unblocked redshift (DELTAz) and co-moving path length (DELTAX) over which we were sensitive to C IV doublets of a range of equivalent widths and column densities. The absorber line density of (G = 1+2) doublets is dN{sub C{sub IV}}/dX= 4.1{sup +0.7}{sub -0.6} for log N(C{sup +3}) >= 13.2, and dN{sub C{sub IV}}/dX has not evolved significantly since z = 5. The best-fit power law to the G = 1 frequency distribution of column densities f(N(C{sup +3}))ident tok(N(C{sup +3})/N{sub 0}){sup {alpha}{sub N}} has coefficient k = 0.67{sup +0.18}{sub -0.16} x 10{sup -14} cm{sup 2} and exponent {alpha}{sub N} = -1.50{sup +0.17}{sub -0.19}, where N{sub 0} = 10{sup 14} cm{sup -2}. Using the power-lawmore » model of f(N(C{sup +3})), we measured the C{sup +3} mass density relative to the critical density: OMEGA{sub C{sup +3}}= (6.20{sup +1.82}{sub -1.52})x10{sup -8} for 13 0, with dOMEGA{sub C{sup +3}}/dt{sub age}= (0.42+-0.2)x10{sup -8} Gyr{sup -1}.« less

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