2003A&A...410..785R

A new molecular hydrogen cloud is found in the sub-damped Lyα absorber [logN(HI)=19.88±0.05] 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)=(8.7^+8.7_–4.0)x10¹⁶cm^–2 and a fractional molecular abundance f_H2=(2.3^+2.3_–1.1)x10^–3 derived from the H₂ lines arising from the J=0-5 rotational levels of the ground electronic vibrational state. The estimated rate of photodissociation at the cloud edge I₀≲1.8x10^–8s^–1 is much higher than the mean Galactic disk value, I_MW∼5.5x10^–11s^–1. This may indicate an enhanced star-formation activity in the z=1.15 system as compared with molecular clouds at z∼3 where I∼I_MW. We also find a tentative evidence that the formation rate coefficient of H₂ upon grain surfaces at z=1.15 is a factor of 10 larger than a canonical Milky Way value, R_MW≃3x10^–17cm³/s. The relative dust-to-gas ratio estimated from the [Cr/Zn] ratio is equal to ∼k=0.89±0.19 (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 L ∼0.25 pc may imply that the H₂ is confined within small and dense filaments embedded in a more rarefied gas giving rise to the z=1.15 sub-damped Lyα absorber.