An HST/COS survey of molecular hydrogen in DLAs & sub-DLAs at z < 1: Molecular fraction and excitation temperature

Abstract

We present the results of a systematic search for molecular hydrogen (H2) in low redshift ($0.05\lesssim z\lesssim 0.7$) DLAs and sub-DLAs with $N(HI)\gtrsim 10^{19.0}$ cm${}^{-2}$, in the archival HST/COS spectra. Our core sample is comprised of 27 systems with a median $\log N(HI)=19.6$. On the average, our survey is sensitive down to $\log N(H2)=14.4$ corresponding to a molecular fraction of $\log f_{H2}=-4.9$ at the median $N(HI)$. H2 is detected in 10 cases (3/5 DLAs and 7/22 sub-DLAs) down to this $f_{H2}$ limit. The H2 detection rate of $50_{-12}^{+25}$ percent seen in our sample, is a factor of $\gtrsim 2$ higher than that of the high-$z$ sample of Noterdaeme et al. (2008), for systems with $N(H2)>10^{14.4}$ cm${}^{-2}$. In spite of having $N(HI)$ values typically lower by a factor of 10, low-$z$ H2 systems show molecular fractions ($\log f_{H2}=-1.93\pm 0.63$) that are comparable to the high-$z$ sample. The rotational excitation temperatures ($T_{01}=133\pm 55$ K), as measured in our low-$z$ sample, are typically consistent with high-$z$ measurements. Simple photoionization models favour a radiation field much weaker than the mean Galactic ISM field for a particle density in the range 10 - 100 cm${}^{-3}$. The impact parameters of the identified host-galaxy candidates are in the range $10\lesssim \rho$ (kpc) $\lesssim 80$. We, therefore, conjecture that the low-$z$ H2 bearing gas is not related to star-forming disks but stems from self-shielded, tidally stripped or ejected disk-material in the extended halo.

Published in the MNRAS (Definitive version at: http://mnras.oxfordjournals.org/content/448/3/2840.full?keytype=ref&ijkey=wgV8z9RldXtPf3r)

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