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

Abstract

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 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)*E¹⁶ cm^-2 and a fractional molecular abundance f_{H_2} = (2.3^+2.3_-1.1)*E^-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 <~ 1.8*E^-8 s^-1 is much higher than the mean Galactic disk value, I_MW ~ 5.5*E^-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*E^-17 cm³ s^-1. The relative dust-to-gas ratio estimated from the [Cr/Zn] ratio is equal to tilde {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 Lyalpha absorber.

Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by Aura, Inc., under NASA contract NAS 5-2655; and on observations collected at the VLT/Kueyen telescope ESO, Paranal, Chile, programme ID 066.A-0212.