CO-dark molecular gas at high redshift: very large H2 content and high pressure in a low-metallicity damped Lyman alpha system

Abstract

We present a detailed analysis of an H 2 -rich, extremely strong intervening damped Ly-α absorption system (DLA) at z abs = 2.786 towards the quasar J 0843+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 ± 0.02 (resp. log N(H i)=21.82 ± 0.11), making it to be the first case in quasar absorption line 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 +0.08 −0.10 . 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 ∼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 CO factor X CO > 2 × 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 C i, C ii, Si ii and the rotational levels of HD and H 2 . We find the number density to be about n ∼ 260 − 380 cm −3 , implying a high thermal pressure of 3 − 5 × 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) ∼ 22) probe high-z galaxies at low impact parameters.

X-shooter observations of strong H2-bearing DLAs at high redshift

“…These values are slightly higher than what is measured towards nearby stars (T ∼ 80 K, see, e.g., Savage et al 1977), but lower than previously seen in the overall population of high-z DLAs (T ∼ 150 K; Srianand et al 2005). However, T 01 temperatures in H 2 -bearing DLAs are known to decrease with increasing H 2 column density (Muzahid et al 2015;Balashev et al 2017), a trend also seen in the local ISM. The systems in our sample have typically higher H 2 column densities than those from Srianand et al (2005), but lower than towards nearby stars, that can explain the difference.…”

“…The strong-H i sample, by construction, probes the high end of H i column densities. Although the incidence rate of H 2 in that sample is quite high, ∼ 50 per cent, the actual H 2 column densities correspond to a very wide range 4 , including the two highest H 2 column densities found in ESDLAs, towards J 0843+0221 and J 1513+0352 (Balashev et al 2017;Ranjan et al 2018).…”

“…From a galactic-scale point-of-view, we also expect the corresponding lines-of-sight to probe gas at small galacto-centric distances, as confirmed by the detection of Lyman-α emission in composite ESDLA spectra ) and in individual systems (Noterdaeme et al 2012a;Ranjan et al 2018). This picture is corroborated by the possible increase of thermal pressure with increasing H i column density (Balashev et al 2017), indicating that such lines-of-sight probe more central regions of galaxies. An enhanced H 2 detection rate in ESDLAs has indeed been confirmed through followup of ESDLAs Ranjan et al 2019) as well as in stacked SDSS spectra .…”

Quasar 2175 Å dust absorbers – II. Correlation analysis and relationship with other absorption line systems

“…Neutral carbon has been considered an excellent tracer of molecular hydrogen (H 2 ) because the ionization potential of C i (11.26 eV) is very close to the dissociation energy of H 2 (e.g., Srianand et al 2005). When H 2 is detected, C i is always present in the same system (e.g., Ge & Bechtold 1999;Ge, Bechtold, & Kulkarni 2001;Srianand et al 2008;Guimarães et al 2012;Noterdaeme et al 2015;Balashev et al 2017.) Neutral Chlorine (Cl i), although rarely detected, is an alternative excellent tracer of H 2 (Jura 1974;Moomey, Federman, & Sheffer 2012;Noterdaeme et al 2010;Ma et al 2015;Balashev et al 2015).…”

“…Targeting strong C i absorbers leads to more successful detections (Noterdaeme et al 2009(Noterdaeme et al , 2010(Noterdaeme et al , 2011Balashev et al 2017) after the first detection at high-redshift by Srianand et al (2008). However, this condition is not sufficient to obtain CO in detectable amounts because CO requires more shielding from UV photons than H 2 and C i given a dissociation energy of 11.09 eV.…”