2018A&A...619A.142K

We present a study of cold gas absorption from a damped Lyman-α absorber (DLA) at redshift z_abs=1.946 toward two lensed images of the quasar J144254.78+405535.5 at redshift z_QSO=2.590. The physical separation of the two lines of sight at the absorber redshift is d_abs=0.7kpc according to our lens model. We observe absorption lines from neutral carbon and H₂ along both lines of sight, indicating that cold gas is present on scales larger than d_abs. We measure the column densities of HI to be log N(HI)=20.27±0.02 and 20.34±0.05 and those of H₂ to be log N(H₂)=19.7±0.1 and 19.9±0.2. The metallicity inferred from sulphur is consistent with solar metallicity for both sightlines: [S/H]_A=0.0±0.1 and [S/H]_B=-0.1±0.1. Based on the excitation of low rotational levels of H₂, we constrain the temperature of the cold gas phase to be T=109±20 and T=89±25K for the two lines of sight. From the relative excitation of fine-structure levels of CI, we constrain the hydrogen volumetric densities to lie in the range of 40-110cm^–3. Based on the ratio of observed column density and volumetric density, we infer the average individual "cloud" size along the line of sight to be l~=0.1pc. Using the transverse line-of-sight separation of 0.7kpc together with the individual cloud size, we are able to place an upper limit to the volume filling factor of cold gas of f_vol<0.1%. Nonetheless, the projected covering fraction of cold gas must be large (close to unity) over scales of a few kpc in order to explain the presence of cold gas in both lines of sight. Compared to the typical extent of DLAs (∼10-30kpc), this is consistent with the relative incidence rate of CI absorbers and DLAs.