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.
Highlights
The onset of star formation is intimately linked to the cooling and subsequent collapse of the neutral gas in galaxies
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
As the presence and strength of Lyα absorption does not depend on temperature, the Lyα line alone does not constrain the relative contribution of CNM and WNM in DLAs
Summary
The onset of star formation is intimately linked to the cooling and subsequent collapse of the neutral gas in galaxies. As the presence and strength of Lyα absorption does not depend on temperature, the Lyα line alone does not constrain the relative contribution of CNM and WNM in DLAs. Since the interplay between the warm and cold neutral gas is crucial for the regulation of star formation, understanding how DLAs trace the CNM and WNM is of great importance for galaxy evolution studies. A direct way of probing the cold neutral gas at high redshift is through the use of H i 21 cm absorption studies since its optical depth does depend on temperature (Gupta et al 2009, 2012; Curran et al 2010; Srianand et al 2012; Kanekar et al 2014; Dutta et al 2017). Based on the spin-temperature measurements from H i 21 cm absorption, Kanekar et al (2014) found that the
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