The Metallicity of Low-Redshift [CLC]Ly[/CLC]α Forest Clouds

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Abstract
Using archival Hubble Space Telescope (HST) Faint Object Spectrograph spectra of 15 quasars, we measure an average metallicity for Lyα forest absorption clouds between redshifts of 0.3 and 0.8. A composite spectrum of all Lyα systems shows an average rest equivalent width of ~0.06 Å for C IV λ1548.20. This is a factor of 10 larger than the value found by Lu at redshifts between 1.7–3.0 using spectra with comparable resolution. Our results show that [C/H] ~ -1.6 ± 0.1 at z ~ 0.5 if C+3 is the dominant form of carbon and line clustering and extremely narrow or wide lines do not significantly affect our column density estimates. Allowing for these effects, we find [C/H] -1.9. If line clustering is comparable in the low- and high-redshift forest, we estimate [C/H] -1.3. Our derived [C/H] is an order of magnitude larger than values derived at z ~ 2.4. It is unlikely that this difference is solely the result of ionization or line-clustering differences. We conclude that the average metallicity has increased either because of a new population of Lyα absorbers at low redshift and/or because of star formation directly or indirectly enhancing the metals in the Lyα clouds. Using 97 Lyα lines with rest equivalent widths greater 0.32 Å, we confirm the results of previous surveys that show that the number density of absorbers at z < 0.8 is larger than predicted by a extrapolation from data at z > 1.6.