ner_tags: O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Formula I-Formula I-Formula I-Formula O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Formula I-Formula I-Formula O O O O B-Formula I-Formula I-Formula I-Formula O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Tag O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Wavelength O O O O O O B-Wavelength O O O O O O O O O O O O O B-Formula I-Formula I-Formula I-Formula O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O B-Formula I-Formula I-Formula I-Formula I-Formula O O O O O O O O O O O O O O O O O O O O O O O B-CelestialObject O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation I-Citation I-Citation O B-Citation I-Citation I-Citation I-Citation I-Citation O B-Citation O O O O O O B-Citation I-Citation I-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-CelestialObject I-CelestialObject I-CelestialObject I-CelestialObject O B-Formula I-Formula I-Formula I-Formula I-Formula I-Formula I-Formula I-Formula I-Formula O O O O O O O O B-CelestialObject I-CelestialObject I-CelestialObject I-CelestialObject O B-Formula I-Formula I-Formula I-Formula I-Formula I-Formula I-Formula
tokens: independently confirms that the H 2 -bearing cloud is likely pressure confined. We note that while a high dust content reduces the H 2 formation time-scale, the number density can have major importance since t ∝ n −2 . It has two direct consequences: first, in low-metallicity environments, H 2 may trace only the densest regions in the patchy medium of DLAs. High number densities will obviously severely reduce the covering fraction of the H 2 -bearing gas. Secondly, the formation time-scale of H 2 can be shortened by supersonic turbulences that can create pockets of compressed gas, where H 2 is quickly formed, and consequently redistributed to the adjacent medium. In Section 4.2 , we showed that the H 2 -bearing gas in the DLA at z = 2.7865 has a kinetic temperature T ∼ 122 K, which is significantly higher than what is expected at such H 2 column density. We supposed that such high temperature was likely attributed to the low metallicity in the medium. The cooling time can be easily estimated from the cooling rate, l c , derived from C ii* , as ∼2 k B T / l c , where k B is the Boltzmann constant (the factor of 2 was chosen since the molecular fraction is not well constrained and the correct coefficients are 3/2 and 5/2 for monatomic and diatomic gas, respectively). This indicates a cooling time of about 0.1 Myr, which is at least an order shorter than the time-scales mentioned above. Hence, the cloud is likely in thermal equilibrium and the high kinetic temperature is indeed due to the low metallicity and not to the formation of H 2 in a shock-compressed medium. 5.3 CO content, <italic>X</italic><sub>CO</sub> and CO-dark gas In spite of extremely high H 2 column density, we did not detect CO. This is not surprising given the facts that we measured a low associated extinction and possibly an enhanced UV flux: CO is easily photodestructed by UV photons in the diffuse medium and therefore at least moderate level of extinction A V ≳ 1 is required for this molecule to survive. Models indicate that the abundance of CO scales with the metallicity, but there are only scarce measurements at low metallicities (e.g. Shi et al. 2015 ). Our upper limit on N (CO) is therefore very valuable since it provides a stringent limit on X CO at Z ∼ 0.03 Z ⊙ for the first time. Fig. 17 shows the dependence of the CO column density on that of H 2 measured in both the MW and high-redshift ISM. Values for high- z DLAs are from Srianand et al. ( 2008a ) and Noterdaeme et al. ( 2010 , 2017 ) for quasar absorption systems and Prochaska et al. ( 2009 ) for the GRB-DLA. 6 We also derived upper limits on the total CO column density in known strong H 2 absorption systems, using the covered A-X bands in the archival spectra and assuming that the population of CO rotational levels is dominated by the CMB excitation. We found N (CO) 12.55 in the DLA towards Q 1232 + 082 with log N (H 2 ) = 19.57 ± 0.10 and N (CO) 12.81 in the DLA towards Q 0812 + 3208 with log N (H 2 ) = 19.93