# Extremely strong DLAs at high redshift: gas cooling and H2 formation

Abstract : We present a spectroscopic investigation with the Very Large Telescope/X-shooter of seven candidate extremely strong damped Lyman-α absorption systems [ESDLAs, N(H I) ≥ 5 × 1021 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$\rm _2$ absorption $\log N(\rm{H\rm _2})\,\text{(cm$^{-2}$)}=18.16\pm 0.03$ 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$\rm _2$ column densities of typically $\log N(\rm{H\rm _2})\,\text{(cm$^{-2}$)}\,\, 17.3$. The increased H2 detection rate (10-55 per cent at 68 per cent confidence level) at high H I column density compared with the overall damped Lyman-α population (~5-10 per cent) confirms previous works. We find that these seven ESDLAs have similar observed properties as those previously studied towards quasars and γ-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 λ158 μm emission, and compare them with the cooling rates from damped Lyman-α 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-α population.
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Journal articles

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Submitted on : Tuesday, August 9, 2022 - 12:20:42 PM
Last modification on : Wednesday, August 10, 2022 - 3:46:30 AM

### Citation

K. N. Telikova, S. A. Balashev, P. Noterdaeme, J. -K. Krogager, A. Ranjan. Extremely strong DLAs at high redshift: gas cooling and H2 formation. Monthly Notices of the Royal Astronomical Society, 2022, 510, pp.5974-5983. ⟨10.1093/mnras/stab3800⟩. ⟨insu-03748287⟩

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