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

A new molecular hydrogen cloud is found in the sub-damped Lyalpha absorber [log N(H I) = 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) = (8.7(-4.0)(+8.7)) x 10(16) cm(-2) and a fractional molecular abundance f(H2) = (2.3(-1.1)(+2.3) x 10(-3) 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 I-0 less than or similar to 1.8 x 10(-8) s(-1) is much higher than the mean Galactic disk value, I-MW similar to 5.5 x 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 similar to 3 where I similar to 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 approximate to 3 x 10(-17) cm(3) s(-1). The relative dust-to-gas ratio estimated from the [Cr/Zn] ratio is equal to (k) over tilde = 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 similar to 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 Lyalpha absorber.