%0 Generic %1 balashev2017codark %A Balashev, S. A. %A Noterdaeme, P. %A Rahmani, H. %A Klimenko, V. V. %A Ledoux, C. %A Petitjean, P. %A Srianand, R. %A Ivanchik, A. V. %A Varshalovich, D. A. %D 2017 %K DLA H2 metallicity %T CO-dark molecular gas at high redshift: very large H$_2$ content and high pressure in a low metallicity damped Lyman-alpha system %U http://arxiv.org/abs/1705.10283 %X We present a detailed analysis of a H$_2$-rich, extremely strong intervening Damped Ly-$\alpha$ 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 $N$(H$_2$)=$21.21\pm0.02$ (resp. $łog N$(H$\,$I)=$21.82\pm0.11$), 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 $Zn/H=-1.52^+0.08_-0.10$. 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\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 $X_CO$ factor $X_CO> 210^23 $ 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 $n \sim 260-380\,$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 $łog\,$N(H) $22$) probe high-z galaxies at low impact parameters.

@misc{balashev2017codark, abstract = {We present a detailed analysis of a H$_2$-rich, extremely strong intervening Damped Ly-$\alpha$ Absorption system (DLA) at $z_{\rm abs}=2.786$ 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$_2$)=$21.21\pm0.02$ (resp. $\log N$(H$\,$I)=$21.82\pm0.11$), 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 $\rm [Zn/H]=-1.52^{+0.08}_{-0.10}$. 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\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 $X_{\rm CO}$ factor $X_{\rm 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 C$\,$I, C$\,$II, Si$\,$II 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.}, added-at = {2017-05-30T09:48:53.000+0200}, author = {Balashev, S. A. and Noterdaeme, P. and Rahmani, H. and Klimenko, V. V. and Ledoux, C. and Petitjean, P. and Srianand, R. and Ivanchik, A. V. and Varshalovich, D. A.}, biburl = {https://www.bibsonomy.org/bibtex/285c1c90518aa8a993367743fb0b2055a/miki}, description = {[1705.10283] CO-dark molecular gas at high redshift: very large H$_2$ content and high pressure in a low metallicity damped Lyman-alpha system}, interhash = {a573945fcc837646eec1ad37e883ee62}, intrahash = {85c1c90518aa8a993367743fb0b2055a}, keywords = {DLA H2 metallicity}, note = {cite arxiv:1705.10283Comment: 21 pages, 21 figures. Accepted for publication in MNRAS}, timestamp = {2017-05-30T09:48:53.000+0200}, title = {CO-dark molecular gas at high redshift: very large H$_2$ content and high pressure in a low metallicity damped Lyman-alpha system}, url = {http://arxiv.org/abs/1705.10283}, year = 2017 }