High metallicity and molecular hydrogen in Damped Lyman-alpha systems

Astronomy and Astrophysics – Astrophysics

Scientific paper

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Scientific paper

A systematic search for molecular hydrogen (H_2) in damped Lyman-α systems (DLAs) at high redshift, zabs > 1.8, was recently carried out with UVES at the ESO Very Large Telescope (Ledoux et al. 2003). The output of this survey suggested that high metallicity is a good criterion for the presence of H_2 in DLAs. We systematically observed with UVES DLAs with the highest metallicities and constructed a new sample of 18 high redshift DLAs/sub-DLAs with N(HI) > 1019.5 cm-2 and metallicities -1.3≤{[X/H]}<-0.3, with X=Zn, S or Si. We found two new H_2-bearing DLAs toward the quasars Q 2343+125 and Q 2348-011. We show that the fraction of DLAs with large molecular fractions, log f>-4 (with f=2N(H_2)/(2N(H_2)+N(HI)) ), is as large as 50% for [X/H]>-0.7, when it is only ˜ 15% in the overall sample (-2.5<[X/H]<-0.3). This demonstrates that high metallicity is an important factor for the presence of molecular hydrogen in the diffuse neutral gas of high-redshift galaxies.

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