EDP Sciences
Free Access
Volume 410, Number 3, November II 2003
Page(s) 785 - 793
Section Extragalactic astronomy
DOI https://doi.org/10.1051/0004-6361:20031313
Published online 17 November 2003

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 )

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

SIMBAD Objects

© ESO 2003

Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.

Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.

Initial download of the metrics may take a while.