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-$\alpha$ absorber (DLA) at redshift $z_{\mathrm{abs}}=1.946$ towards two lensed images of the quasar J144254.78+405535.5 at redshift $z_{\mathrm{QSO}}=2.590$. The physical separation of the two lines of sight at the absorber redshift is $d_{\mathrm{abs}}=0.7$~kpc based on our lens model. We observe absorption lines from neutral carbon and H${}_2$ along both lines of sight indicating that cold gas is present on scales larger than $d_{\mathrm{abs}}$. We measure column densities of HI to be $\log N(\mathrm{H}\mathrm{ı})=20.27\pm 0.02$ and $20.34\pm 0.05$ and of H${}_2$ to be $\log N({\mathrm{H}}_2)=19.7\pm 0.1$ and $19.9\pm 0.2$. The metallicity inferred from sulphur is consistent with Solar metallicity for both sightlines: $[S/H{]}_A=0.0\pm 0.1$ and $[S/H{]}_B=-0.1\pm 0.1$. Based on the excitation of low rotational levels of H${}_2$, we constrain the temperature of the cold gas phase to be $T=109\pm 20$ and $T=89\pm 25$ K for the two lines of sight. From the relative excitation of fine-structure levels of CI, we constrain the hydrogen volumetric densities 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 $l\approx 0.1$ pc. Using the transverse line-of-sight separation of 0.7 kpc together with the individual cloud size, we are able to put an upper limit to the volume filling factor of cold gas of $f_{\mathrm{vol}}<0.2$ %. 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 CI absorbers and DLAs.

arXiv comment12 pages + 5 pages of appendix. Accepted for publication in A&A

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