Extremely strong DLAs at high redshift: gas cooling and H-2 formation

We present a spectroscopic investigation with the Very Large Telescope/X-shooter of seven candidate extremely strong damped Lyman-aabsorption systems [ESDLAs, N(H I) >= 5 x 10(21) cm(-2)] observed along quasar sightlines. We confirm the extremely high column densities, albeit slightly (0.1 dex) lower than the original ESDLA definition for four systems. We measured low-ionization metal abundances and dust extinction for all systems. For two systems, we also found strong associated H-2 absorption log N(H-2) (cm(-2)) = 18.16 +/- 0.03 and 19.28 +/- 0.06 at z = 3.26 and 2.25 towards J2205+1021 and J2359+1354, respectively, while for the remaining five we measured conservative upper limits on the H-2 column densities of typically log N(H-2) (cm(-2)) < 17.3. The increased H-2 detection rate (10-55 per cent at 68 per cent confidence level) at high H I column density compared with the overall damped Lyman-alpha population (similar to 5-10 per cent) confirms previous works. We find that these seven ESDLAs have similar observed properties as those previously studied towards quasars and gamma-ray burst afterglows, suggesting they probe inner regions of galaxies. We use the abundance of ionized carbon at the excited fine-structure level to calculate the cooling rates through the C II lambda 158 mu m emission, and compare them with the cooling rates from damped Lyman-alpha systems in the literature. We find that the cooling rate distribution of ESDLAs also presents the same bimodality as previously observed for the general (mostly lower H I column density) damped Lyman-alpha population.