H <scp>I</scp> 21 cm Absorption Associated with Foreground Galaxies on Top of Quasars

Abstract 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 Ly α -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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mo>∫</mml:mo> </mml:mrow> <mml:mspace width="0.25em"/> <mml:mi>τ</mml:mi> <mml:mspace width="0.25em"/> <mml:mi>d</mml:mi> <mml:mi>v</mml:mi> <mml:mspace width="0.25em"/> <mml:mo stretchy="false">(</mml:mo> <mml:mi mathvariant="normal">km</mml:mi> <mml:mspace width="0.25em"/> <mml:msup> <mml:mrow> <mml:mi mathvariant="normal">s</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> <mml:mo stretchy="false">)</mml:mo> <mml:mo>=</mml:mo> <mml:mn>13.5</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>8</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>2.35</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>2.75</mml:mn> </mml:mrow> </mml:msubsup> <mml:mi>E</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mi>B</mml:mi> <mml:mo>−</mml:mo> <mml:mi>V</mml:mi> <mml:mo stretchy="false">)</mml:mo> <mml:mo>+</mml:mo> <mml:mn>0.6</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>8</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1.27</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>1.06</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> , 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.