The elusive H i→H₂ transition in high-z damped Lyman-α systems

¹ Institut d’Astrophysique de Paris, CNRS-UPMC, UMR 7095, 98bis bd Arago, 75014 Paris, France
e-mail: noterdaeme@iap.fr
² Inter-University Centre for Astronomy and Astrophysics, Post Bag 4, Ganeshkhind, 411 007 Pune, India

Received: 4 March 2015
Accepted: 17 May 2015

Abstract

We study the H₂ molecular content in high redshift damped Lyman-α systems (DLAs) as a function of the H i column density. We find a significant increase of the H₂ molecular content around log N(H i) (cm^-2) ~ 21.5−22, a regime unprobed until now in intervening DLAs, beyond which the majority of systems have log N(H₂) > 17. This is in contrast with lines of sight towards nearby stars, where such H₂ column densities are always detected as soon as log N(H i) > 20.7. This can qualitatively be explained by the lower average metallicity and possibly higher surrounding UV radiation in DLAs. However, unlike in the Milky Way, the overall molecular fractions remain modest, showing that even at a large N(H i) only a small fraction of overall H i is actually associated with the self-shielded H₂ gas. Damped Lyman-α systems with very high-N(H i) probably arise along quasar lines of sight passing closer to the centre of the host galaxy where the gas pressure is higher. We show that the colour changes induced on the background quasar by continuum (dust) and line absorption (H i Lyman and H₂ Lyman & Werner bands) in DLAs with log N(H i) ~ 22 and metallicity ~1/10 solar is significant, but not responsible for the long-discussed lack of such systems in optically selected samples. Instead, these systems are likely to be found towards intrinsically fainter quasars that dominate the quasar luminosity function. Colour biasing should in turn be severe at higher metallicities.