CO-dark molecular gas at high redshift: very large H${}_2$ content and high pressure in a low metallicity damped Lyman-alpha system

Abstract

We present a detailed analysis of a H${}_2$-rich, extremely strong intervening Damped Ly-$\alpha$ Absorption system (DLA) at $z_{\mathrm{abs}}=2.786$ towards the quasar J0843+0221, observed with the Ultraviolet and Visual Echelle Spectrograph on the Very Large Telescope. The total column density of molecular (resp. atomic) hydrogen is $\log N$(H${}_2$)=$21.21\pm 0.02$ (resp. $\log N$(HI)=$21.82\pm 0.11$), making it to be the first case in quasar absorption lines studies with H${}_2$ column density as high as what is seen in ${}^{13}$CO-selected clouds in the Milky-Way. We find that this system has one of the lowest metallicity detected among H${}_2$-bearing DLAs, with $[Zn/H]=-1.52_{-0.10}^{+0.08}$. This can be the reason for the marked differences compared to systems with similar H${}_2$ column densities in the local Universe: $(i)$ the kinetic temperature, $T\sim$120~K, derived from the $J=0,1$ H${}_2$ rotational levels is at least twice higher than expected; $(ii)$ there is little dust extinction with A${}_V<0.1$; $(iii)$ no CO molecules are detected, putting a constraint on the $X_{\mathrm{CO}}$ factor $X_{\mathrm{CO}}>2\times 10^{23}$ cm${}^{-2}$/(km/s\,K), in the very low metallicity gas. Low CO and high H${}_2$ contents indicate that this system represents "CO-dark/faint" gas. We investigate the physical conditions in the H${}_2$-bearing gas using the fine-structure levels of CI, CII, SiII and the rotational levels of HD and H${}_2$. We find the number density to be about $n\sim 260-380$cm${}^{-3}$, implying a high thermal pressure of $(3-5)\times 10^4$cm${}^{-3}$K. We further identify a trend of increasing pressure with increasing total hydrogen column density. This independently supports the suggestion that extremely strong DLAs (with $\log$N(H) $\sim 22$) probe high-z galaxies at low impact parameters.

21 pages, 21 figures. Accepted for publication in MNRAS

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