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

Labanya Kumar Guha, Raghunathan Srianand, Rajeshwari Dutta

First listed 2025-04-01 | Last updated 2026-01-05

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-$α$ (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 $\int τ\, dv\, (\rm{km\,s^{-1}}) = 13.58^{+2.75}_{-2.35} E(B-V) + 0.68^{+1.06}_{-1.27}$, 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.

Short digest

Pilot uGMRT 21-cm searches toward three SDSS-identified Galaxies On Top Of Quasars (J0758+1136, J1159+5820, J1451+0857) yield a 100% detection rate of HI absorption at extremely small impact parameters. Combining with literature, the authors argue GOTOQs form a distinct absorber population with much higher detection rates than DLA- or metal-selected samples. They find a Milky Way–like reddening relation, ∫τ dv (km s⁻¹) = 13.58(+2.75/−2.35) E(B−V) + 0.68(+1.06/−1.27), and show both detection rate and optical depth drop rapidly with impact parameter. This demonstrates that fiber-selected GOTOQs efficiently trace cold HI close to star-forming galaxies and set the stage for a large, intervening 21-cm absorber sample with upcoming wide-field spectroscopy.

Key figures to inspect

• Figure 1: Use the SDSS/eBOSS emission-line fits ([O II] doublet, others) to confirm the foreground galaxy redshifts captured in the quasar fibers and compare DESI-LIS images; note that only J1451+0857 shows the galaxy directly, enabling a measured impact parameter, while the others provide fiber-size upper limits.
• Figure 2: Inspect the multi-Gaussian 21-cm absorption profiles for J0758+1136, J1159+5820, and J1451+0857; read component velocities relative to the galaxy emission redshift and sum components to reproduce the reported integrated optical depths.
• Figure 3: Examine the trend of integrated 21-cm optical depth versus impact parameter; locate the new uGMRT GOTOQs (boxed squares) against earlier QGP compilations to see the sharp decline in τ with increasing D and where detections cluster.
• Figure 4: Compare covering fraction versus impact parameter for GOTOQs/QGPs to the red points from Gupta et al. (2021); note the assumed ∫τ dv sensitivity (~0.3 km s⁻¹) and how f_c rises at the smallest D, highlighting why GOTOQs are effective low-D probes.