The Metallicity of Low-Redshift Lyalpha Forest Clouds

Details The Metallicity of Low-Redshift Lyalpha Forest Clouds The Metallicity of Low-Redshift Lyalpha Forest Clouds

May 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998aj....115.1725b&link_type=abstract

The Astronomical Journal, Volume 115, Issue 5, pp. 1725-1736.

Astronomy and Astrophysics

Astronomy

25

Galaxies: Quasars: Absorption Lines

Scientific paper

Using archival Hubble Space Telescope (HST) Faint Object Spectrograph spectra of 15 quasars, we measure an average metallicity for Lyalpha forest absorption clouds between redshifts of 0.3 and 0.8. A composite spectrum of all Lyalpha systems shows an average rest equivalent width of ~0.06 Å for C iv lambda1548.20. 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 Lyalpha absorbers at low redshift and/or because of star formation directly or indirectly enhancing the metals in the Lyalpha clouds. Using 97 Lyalpha 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.

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