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

We present the results of a systematic search for molecular hydrogen (H-2) in low-redshift (0.05 less than or similar to z less than or similar to 0.7) Damped Lya absorbers (DLAs) and sub-DLAs with N(H I) greater than or similar to 10(19.0) cm(-2), in the archival Hubble Space Telescope/CosmicOrigins 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(H-2)= 14.4 corresponding to a molecular fraction of log f(H2) = -4.9 at the median N(H I). H-2 is detected in 10 cases (3/5 DLAs and 7/22 sub-DLAs) down to this f(H2) limit. The H-2 detection rate of 50(-12)(+25) per cent seen in our sample, is a factor of greater than or similar to 2 higher than that of the high-z sample of Noterdaeme et al., for systems with N(H-2) > 10(14.4) cm(-2). In spite of having N(H I) values typically lower by a factor of 10, low-z H-2 systems show molecular fractions (log f(H2) = -1.93 +/- 0.63) that are comparable to the high-z sample. The rotational excitation temperatures (T-01 = 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 cm(-3). The impact parameters of the identified host-galaxy candidates are in the range 10 less than or similar to rho (kpc) less than or similar to 80. We, therefore, conjecture that the low-z H-2 bearing gas is not related to star-forming discs but stems from self-shielded, tidally stripped or ejected disc-material in the extended halo.