2015MNRAS.448.2840M

We present the results of a systematic search for molecular hydrogen (H2) in low-redshift (0.05 ≲ z ≲ 0.7) Damped Lyα absorbers (DLAs) and sub-DLAs with N(H I) \gtrsim 10^19.0/ cm2, in the archival Hubble Space Telescope/Cosmic Origins Spectrograph spectra. Our core sample is comprised of 27 systems with a median logN(H I) = 19.6. On the average, our survey is sensitive down to logN(H2) = 14.4 corresponding to a molecular fraction of logf_H_2 = -4.9 at the median N(H I). H2 is detected in 10 cases (3/5 DLAs and 7/22 sub-DLAs) down to this f_H_2 limit. The H2 detection rate of 50^+25_-12 percent seen in our sample, is a factor of ≳ 2 higher than that of the high-z sample of Noterdaeme et al., for systems with N(H2) > 10^14.4/ cm2. In spite of having N(H I) values typically lower by a factor of 10, low-z H2 systems show molecular fractions (logf_H_2=-1.93±0.63) that are comparable to the high-z sample. The rotational excitation temperatures (T01 = 133±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 interstellar medium field for a particle density in the range 10^–100/ cm3. The impact parameters of the identified host-galaxy candidates are in the range 10 ≲ ρ (kpc) ≲ 80. We, therefore, conjecture that the low-z H2 bearing gas is not related to star-forming discs but stems from self-shielded, tidally stripped or ejected disc-material in the extended halo.