2025ApJ...994..197G

A systematic search for H I 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 survey of three GOTOQs, where we achieved a remarkable 100% detection rate of H I 21 cm absorption. By combining our findings with the existing literature, we establish that GOTOQs constitute a distinct population in terms of H I 21 cm absorption, with significantly higher detection rates than those observed in damped Lyα-based or metal absorption-based searches. For the GOTOQs, we find a strong correlation between the line-of-sight reddening and the H I 21 cm optical depth, characterized by $\int \,\tau \,dv\,({\rm{km}}\,{{\rm{s}}}^{-1})=13.5{8}_{-2.35}^{+2.75}E(B-V)+0.6{8}_{-1.27}^{+1.06}$, consistent with the Milky Way sightlines. We also show that the H I 21 cm optical depth declines with the impact parameter, and find a tentative trend for the H I 21 cm detection rates to also decline 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 H I 21 cm absorbers.