Detection of molecular hydrogen at z = 1.15 toward HE 0515-4414

A new molecular hydrogen cloud is found in the sub-damped Ly (alpha)absorber [ log(N)( 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 H 2 are identified in the UV spectrum of this quasar obtained with the Space Telescope Imaging Spectrograph (STIS) aboard the Hubble Space Telescope. The H 2-bearing cloud shows a total H 2 column density N(H 2)  = …  cm^(-2) and a fractional molecular abundance … derived from the H 2 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 <~ 1.8 * 10^(-8) s^(-1) is much higher than the mean Galactic disk value, I(MW) ~ 5.5* 10^(-11) s ^(-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 H 2 upon grain surfaces at z = 1.15 is a factor of 10 larger than a canonical Milky Way value, R(MW) ~~ 3* 10^(-17) cm^ 3 s^ -1. 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 H 2 is confined within small and dense filaments embedded in a more rarefied gas giving rise to the z = 1.15 sub-damped Ly (alpha)absorber.