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Issue A&A
Volume 410, Number 3, November II 2003
Page(s) 785 - 793
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20031313



A&A 410, 785-793 (2003)
DOI: 10.1051/0004-6361:20031313

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

D. Reimers1, R. Baade1, R. Quast1 and S. A. Levshakov2

1  Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
2  Department of Theoretical Astrophysics, Ioffe Physico-Technical Institute, 194021 St. Petersburg, Russia

(Received 30 June 2003 / Accepted 15 August 2003 )

Abstract
A new molecular hydrogen cloud is found in the sub-damped Ly $\alpha$ absorber [ $\log N$( $\ion{H}{i}$) = $19.88\pm0.05$] at the redshift $z_{\rm abs}$ = 1.15 toward the bright quasar HE 0515-4414 ( $z_{\rm em}$ = 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)  $= (8.7^{+8.7}_{-4.0})\times10^{16}$ cm -2 and a fractional molecular abundance $f_{{\rm H}_2} = (2.3^{+2.3}_{-1.1})\times10^{-3}$ 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 $I_0 \la 1.8\times10^{-8}$ s -1 is much higher than the mean Galactic disk value, $I_{\rm MW} \sim 5.5\times10^{-11}$ s -1. This may indicate an enhanced star-formation activity in the z = 1.15 system as compared with molecular clouds at $z \sim 3$ where $I \sim I_{\rm MW}$. 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, $R_{\rm MW} \approx 3\times10^{-17}$ cm 3 s -1. The relative dust-to-gas ratio estimated from the [Cr/Zn] ratio is equal to $\tilde{k} = 0.89\pm0.19$ (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 $L \sim 0.25$ 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.


Key words: cosmology: observations -- quasars: absorption lines -- quasars: individual: HE 0515-4414

Offprint request: S. A. Levshakov, lev@astro.ioffe.rssi.ru

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