Detection of molecular hydrogen at z = 1.15 toward HE 0515–4414^*

¹ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
² Department of Theoretical Astrophysics, Ioffe Physico-Technical Institute, 194021 St. Petersburg, Russia

Corresponding author: S. A. Levshakov, lev@astro.ioffe.rssi.ru

Received: 30 June 2003
Accepted: 15 August 2003

Abstract

A new molecular hydrogen cloud is found in the sub-damped Lyα absorber [ () = ] at the redshift z_abs = 1.15 toward the bright quasar HE 0515–4414 (z_em = 1.71). More than 30 absorption features in the Lyman band system of H₂ are identified in the UV spectrum of this quasar obtained with the Space Telescope Imaging Spectrograph (STIS) aboard the Hubble Space Telescope. The H₂-bearing cloud shows a total H₂ column density N(H₂)  cm^-2 and a fractional molecular abundance derived from the H₂ lines arising from the rotational levels of the ground electronic vibrational state. The estimated rate of photodissociation at the cloud edge  s^-1 is much higher than the mean Galactic disk value,  s^-1. This may indicate an enhanced star-formation activity in the system as compared with molecular clouds at where . We also find a tentative evidence that the formation rate coefficient of H₂ upon grain surfaces at is a factor of 10 larger than a canonical Milky Way value,  cm³ s^-1. The relative dust-to-gas ratio estimated from the [Cr/Zn] ratio is equal to (in units of the mean Galactic disk value), which is in good agreement with a high molecular fraction in this system. The estimated line-of-sight size of  pc may imply that the H₂ is confined within small and dense filaments embedded in a more rarefied gas giving rise to the sub-damped Lyα absorber.