C IV and other Metal Absorption Line Systems in 18 z = 4 Quasars

Abstract

We present a modest survey of quasar metal line absorption systems at redshifts 2.3–4.5. Relatively high signal-to-noise ratio (~25 pixel^(-1)) spectra of 18 quasars at 2 Å FWHM resolution show many absorption systems with strong metal lines in the region redward of the Lyα emission lines. We conducted a systematic search and found 55 C IV doublets, 19 Si IV doublets, three Mg II doublets, and seven N V doublets. The present data alone hint that the number of C IV absorption doublets per unit redshift, N(z), decreases with increasing redshift for 2.3 < z < 4.5 but at only the 1–2 σ level, for either an Einstein–de Sitter model (Λ = 0) or a flat universe with Λ = 0.7. When we combine our sample with published data that extend to lower redshifts, we detect evolution at the 1–4 σ level, depending on the cosmological model and the strength of the C IV lines. There are fewer C IV systems per unit z with increasing z, and the systems with stronger C IV lines evolve much faster. At z ≃ 2.4, C IV with W > 0.3 Å are approximately 55% of all C IV systems with W > 0.15 Å, but by z ≃ 4 that percentage is less than 37%. Similar conclusions were reached by Sargent, Boksenberg, & Steidel and by Steidel, primarily at lower redshifts. However, we measure approximately twice the density of C IV systems at 2.3 < z < 3.8 with W > 0.15 Å that was reported by Steidel. The probability that our sample and previous samples come from the same distribution is only ~2%. But this could be a statistical accident because it is an a posteriori comparison. We believe that the systems that we report are real, and we have no other explanation for this difference. For Si IV absorption lines, there is a 1 σ hint of evolution with the same sense. In contrast, Lyα and Mg II systems are known to show evolution of the opposite sense with more absorbers at larger redshifts. The physical cause of this difference may be a mixture of ionization and chemical evolution effects.

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