The metallicity of low-redshift Lyα forest clouds

Using archival Hubble Space Telescope (HST) Faint Object Spectrograph spectra of 15 quasars, we measure an average metallicity for Ly alpha forest absorption clouds between redshifts of 0.3 and 0.8. A composite spectrum of all Ly alpha systems shows an average rest equivalent width of similar to 0.06 Angstrom for C IV lambda 1548.20. 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] similar to-1.6 +/- 0.1 at z similar to 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] greater than or similar to -1.9. If line clustering is comparable in the low-and high-redshift forest, we estimate [C/H] greater than or similar to -1.3. Our derived [C/H] is an order of magnitude larger than values derived at z similar to 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 Ly alpha. absorbers at low redshift and/or because of star formation directly or indirectly enhancing the metals in the Ly alpha clouds. Using 97 Ly alpha lines with rest equivalent widths greater 0.32 Angstrom, 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.