Properties of QSO Metal Line Absorption Systems at High Redshifts: Nature and Evolution of the Absorbers and the Ionizing Radiation Background

Abstract

In Keck HIRES spectra of 9 QSOs we identify a sample of 908 CIV absorber components in 188 systems outside the Lyman forest in the redshift range 1.6 < z < 4.4, with related lines of SiIV, CII, SiII and NV. The properties of the CIV absorbers are almost constant with z. We find a mild increase in Omega(CIV) with decreasing z with a mean = (3.8+/-0.7)*10^(-8) (spatially flat LCDM cosmology and h = 0.71). Using Omega(b) from the CMB and ionization fractions from our data we obtain [C/H]_(z = 4.0) >/= -3.11(+0.14/-0.19) and [C/H]_(z = 2.1) >/= -2.64(+0.15/-0.22), suggesting a rise by about a factor 3. Relating Omega(H) more directly to regions containing the absorbers our values become >~ -2.2 and >~ -2.0, respectively. CIV components exhibit strong clustering at Delta(v) < 300 km/s but there is no clustering on any scale between systems. We argue that for our sample the CIV clustering is entirely due to the peculiar velocities of gas present in the outer extensions of galaxies. We find no change with z in the median column density ratio SiIV/CIV, contrary to previous observations; other ionic ratios vary continuously with redshift. We show that these are only partial indicators of ionization state and remedy this by use of specific pairs of ratios. We demonstrate that the majority of absorbers are photoionized and find that at z < 2.65 QSOs dominate the ionization whereas at z > 3.4 an additional, dominant contribution from galaxies with specific spectral characteristics and high radiative escape fraction in the range 1-4 Ryd is required. These results also indicate that [Si/C] = 0.0-0.4 fits the data well. We conclude that the heavy element absorbers at z > 3.4 are located close to galaxies and irradiated dominantly by them, consistent with our independent conclusion from clustering properties.