Measuring the [C II]-to-H I connection in high-redshift absorption-selected galaxies

Abstract: HI neutral gas is the basic building block of galaxies. It dictates the star formation rate and hence the metal enrichment rates within high redshift galaxies. HI neutral gas cannot be directly observed beyond z ≈ 0.38 due to the weakness of the 21 cm transition, therefore alternate methods need to be developed for studying high redshift galaxies. The emission luminosity of [C II]-157.741 μm transition as a potential tracer for neutral HI gas mass is explored. The [C II] line emission from galaxies in the range z ≈ (1.6, 3.7) is studied using absorption-selected QSO whose sightline passes through DLAs. 83 QSO-DLA spectroscopic data was obtained from X-shooter and HIRES spectrograph and some were fitted with Voigt profile using VoigtFit. The results were used to find a relationship between the luminosity L[C II] and the gas mass MHI. Using the [C II]-to-H I relation, log β[C II], it is found that log β[C II] is decreasing as a function of impact parameter. Further analysis on the metallicity dependence of log β[C II] shows a decreasing trend as metallicity increases. The metallicity-log β[C II] relation was compared to Gamma Ray Burst data from Heintz et al. (2021). The linear regression of the log β[C II] as function of metallicity for QSO is shown to be decreasing linearly with a similar slope but a factor of 10 higher than GRB relation for all N(H I). It was then concluded that the usage of QSO sightlines to probe HI using [C II] emission as a tracer is not feasible.