On the origin of metallicity in Lyα forest systems

Masashi Chiba, Biman B. Nath

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9 Citations (Scopus)


We investigate the hypothesis that Lyα absorption lines arise in two populations of halos-minihalos of small circular velocity (Vc ≲ 55 km s-1), in which star formation and metal production are inhibited by photoionization of the UV background radiation, and large galactic halos (55 ≲ Vc ≲ 250 km s-1) that contain stars and metals. Based on the model of Lyα-absorbing gas confined in both populations of halos, we attempt to explain the recent observations of (1) associations of visible galaxies with Lyα lines at low redshifts z ≲ 1 and (2) metal lines associated with a nonnegligible fraction of low H I column density Lyα lines at z ∼ 3. For galactic halos, we find that photoionized gas clouds confined in the pressure of ambient hot gas can produce Lyα absorption for H I column densities as low as 1014 cm-2 and that the impact parameter of a sight line for such absorptions matches well with the observed radius of the gaseous envelope of a typical luminous galaxy. Using the Press-Schechter prescription for the mass function of halos, we also show that the fraction of Lyα lines with associated metal lines can be understood in terms of the fraction of Lyα absorbers associated with galactic halos. In particular, the reported fraction of ∼ 0.5-0.75 at z ∼ 3 is reproduced when the boundary value of Vc to separate minihalos or galactic halos is 40-60 km s-1, which is consistent with the theoretical prediction of galaxy formation under photoionization. The average metallicity of both Lyα forest and damped Lyα systems at z ∼ 3 is explained in terms of the model of halo-formation history combined with the age-metallicity relationship of Galactic halo stars. Possible methods to test this hypothesis and alternative scenarios are also discussed.

Original languageEnglish
Pages (from-to)638-647
Number of pages10
JournalAstrophysical Journal
Issue number2 PART I
Publication statusPublished - 1997
Externally publishedYes


  • Galaxies: abundances
  • Galaxies: halos
  • Quasars: absorption lines

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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