Dissecting cold gas in a high-redshift galaxy using a lensed background quasar

Abstract

We present a study of cold gas absorption from a damped Lyman-α absorber (DLA) at redshift zabs = 1.946 toward two lensed images of the quasar J144254.78+405535.5 at redshift zQSO = 2.590. The physical separation of the two lines of sight at the absorber redshift is dabs = 0.7 kpc 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 dabs. We measure the column densities of H I 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 ± 25 K for the two lines of sight. From the relative excitation of fine-structure levels of C I, we constrain the hydrogen volumetric densities to lie in the range of 40 − 110 cm−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 ≈ 0.1 pc. Using the transverse line-of-sight separation of 0.7 kpc together with the individual cloud size, we are able to place an upper limit to the volume filling factor of cold gas of fvol < 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 − 30 kpc), this is consistent with the relative incidence rate of C I absorbers and DLAs.