HI 21-cm Absorption Associated with Foreground Galaxies on Top of Quasars

Abstract: A systematic search for HI 21-cm absorption in Quasar-Galaxy Pairs (QGPs) provides a powerful means to map the distribution of cold gas around high-redshift star-forming galaxies. Fiber spectroscopy of high-redshift quasars enables the serendipitous detection of foreground star-forming galaxies at extremely small impact parameters, forming a unique subset of QGPs known as Galaxies On Top Of Quasars (GOTOQs). In this study, we present results from a pilot upgraded Giant Metrewave Radio Telescope (uGMRT) survey of three GOTOQs, where we achieved a remarkable 100\% detection rate of HI 21-cm absorption. By combining our findings with existing literature, we establish that GOTOQs constitute a distinct population in terms of HI 21-cm absorption, with significantly higher detection rates than those observed in Damped Lyman-$\alpha$ (DLA)-based or metal absorption-based searches. For the GOTOQs, we find a strong correlation between the line-of-sight reddening and the HI 21-cm optical depth, characterized by $\int \tau\, dv\, (\rm{km\,s^{-1}}) = 13.58^{+2.75}_{-2.35} E(B-V) + 0.68^{+1.06}_{-1.27}$, consistent with the Milky Way sightlines. We also show that the HI 21-cm detection rates and optical depth declines rapidly with the impact parameter. With upcoming wide-field spectroscopic surveys expected to substantially expand the catalog of known GOTOQs, the success of this pilot survey lays the foundation for constructing a statistically significant sample of intervening HI 21-cm absorbers.