Abstract
We present the results of a systematic search for molecular hydrogen (H2) in
low redshift (0.05 < z < 0.7) damped and sub-damped Lya absorption systems
(DLAs/sub-DLAs) with N(HI) > 10^19.0 cm^-2, in the archival Hubble Space
Telescope (HST)/Cosmic Origins Spectrograph (COS) spectra. Our core sample is
comprised of 27 systems with a median log N(HI) = 19.6. This is a factor of ~
10 lower than that of the high-z (z > 2) sample of Noterdaeme et al. H2
absorption is detected in 10 cases (3/5 in DLAs and 7/22 in sub-DLAs). On the
average, our survey is sensitive down to log N(H2) = 14.4 corresponding to a
molecular mass fraction of log f_H2 = -4.9 at the median N(HI). The H2
incidence rate of 50^+25_-12 percent, is a factor of > 2 higher than that
of the high-z sample, for systems with N(H2) > 10^14.4 cm^-2. The enhanced
incidence rate could result from the increase of the cosmic mean metallicities
of DLAs and sub-DLAs with cosmic time. In spite of having systematically lower
N(HI) values, low-z H2 systems show molecular mass fractions (log f_H2 = -1.93
$\pm$ 0.63) that are comparable to the high-z measurements. 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 demonstrate that the radiation field prevailing in the
low-z H2 systems cannot have an appreciable stellar contribution for densities
of 10-100 cm^-3. The impact parameters of the nearest possible host-galaxy
candidates are typically large (e.g. 10 < $\rho$ (kpc) < 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.
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