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 (H₂) in low-redshift (0.05 ≲ z ≲ 0.7) Damped Lyα absorbers (DLAs) and sub-DLAs with N(H I) ≳ 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(H₂)=14.4 corresponding to a molecular fraction of log fH₂ = -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 fH₂ limit. The H₂ detection rate of 50_-12 ⁺²⁵ per cent 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 cm^-2. In spite of having N(H I) values typically lower by a factor of 10, low-z H₂ systems show molecular fractions (log fH₂ = -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 cm^-3. 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.