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 s
ystems 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 $50^{+25}_{-12}$ 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 ($log f_{H2}=-1.93pm0.63$) that are comparable to the high-$z$ sample. The rotational excitation temperatures ($T_{01} = 133pm55$ 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.
We report the detection of H2 in a zabs= 0.0963 Damped Lyman-{alpha} (DLA) system towards zem = 0.4716 QSO J1619+3342. This DLA has log N(H I) = 20.55 (0.10), 18.13 < log N(H2) < 18.40, [S/H] = -0.62 (0.13), [Fe/S] = -1.00 (0.17) and the molecular fr
action -2.11 < log f(H2) < -1.85. The inferred gas kinetic temperature using the rotational level population is in the range 95 - 132 K. We do not detect C I or C II* absorption from this system. Using R- and V-band deep images we identify a sub-L* galaxy at an impact parameter of 14 kpc from the line of sight, having consistent photometric redshift, as a possible host for the absorber. We use the photoionization code CLOUDY to get the physical conditions in the H2 component using the observational constrains from H2, C I, C II* and Mg I. All the observations can be consistently explained if one or more of the following is true: (i) Carbon is underabundant by more than 0.6 dex as seen in halo stars with Z ~ 0.1 Z_sun, (ii) H I associated with H2 component is less than 50% of the H I measured along the line of sight and (iii) the H2 formation rate on the dust grains is at least a factor two higher than what is typically used in analytic calculations for Milky Way interstellar medium. Even when these are satisfied, the gas kinetic temperature in the models are much lower than what is inferred from the ortho-to-para ratio of the molecular hydrogen. Alternatively the high kinetic temperature could be a consequence of contribution to the gas heating from non-radiative heating processes seen in hydrodynamical simulations.
[Abridged] We present a detailed study of the largest sample of intervening O VI systems in the redshift range 1.9 < z < 3.1 detected in high resolution (R ~ 45,000) spectra of 18 bright QSOs observed with VLT/UVES. Based on Voigt profile and apparen
t optical depth analysis we find that (i) the Doppler parameters of the O VI absorption are usually broader than those of C IV (ii) the column density distribution of O VI is steeper than that of C IV (iii) line spread (delta v) of the O VI and C IV are strongly correlated (at 5.3sigma level) with delta v(O VI) being systematically larger than delta v(C IV) and (iv) delta v(O VI) and delta v(C IV) are also correlated (at > 5sigma level) with their respective column densities and with N(H I) (3 and 4.5 sigma respectively). These findings favor the idea that C IV and O VI absorption originate from different phases of a correlated structure and systems with large velocity spread are probably associated with overdense regions. The velocity offset between optical depth weighted redshifts of C IV and O VI absorption is found to be in the range 0 < |Delta v (O VI - CIV)| < 48 km/s with a median value of 8 km/s. We compare the properties of O VI systems in our sample with that of low redshift (z < 0.5) samples from the literature and find that (i) the O VI components at low-z are systematically wider than at high-z with an enhanced non-thermal contribution to their b-parameter, (ii) the slope of the column density distribution functions for high and low-z are consistent, (iii) range in gas temperature estimated from a subsample of well aligned absorbers are similar at both high and low-z, and (iv) Omega_{O VI} = (1.0 pm 0.2) times10^{-7} for N(O VI) > 10^{13.7} cm^{-2}, estimated in our high-z sample, is very similar to low-z estimations.
