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

Abstract

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=1.15 toward the bright quasar HE0515-4414 (zem = 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}) 10^16 cm^-2, and a fractional molecular abundance f(H2)=(2.3^{+2.3}_{-1.1}) 10^-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 I_0<=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 H2 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 H2 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.