Tracing Cold Gas in Absorption Across Cosmic Time with the SKA
Elizabeth K. Mahony, Neeraj Gupta, Sergei A. Balashev, Yogesh Chandola, Francoise Combes, Rebecca Davies, Jens-Kristian Krogager, Wenkai Hu, Filippo M. Maccagni, Pasquier Noterdaeme, Mamta Pandey-Pommier, Elaine M. Sadler, Rasha M. Samir, Nick Seymour, Hyein Yoon
astro-ph.GA
Submitted: 2026-06-30
Comments: Published in Advancing Astrophysics with the SKAII (AASKAII), 2026 (arXiv:2606.20366). Report-no:AASKAII/Mahony01
License: http://creativecommons.org/licenses/by-nc-sa/4.0/
The gist: Observing the 21-cm HI line in absorption provides a powerful means of tracing the cold neutral gas in normal and active galaxies across cosmic time.
Terminology
Abstract
Observing the 21-cm HI line in absorption provides a powerful means of tracing the cold neutral gas in normal and active galaxies across cosmic time. The frequency coverage and sensitivity of SKAO will allow us to detect HI in absorption from z = 0 to beyond z = 6, enabling the characterisation of the properties of cold gas in and around galaxies at all epochs. This chapter summarises recent advances in absorption-line studies, lessons learned from precursor surveys, and updates the science case presented in Kanekar and Briggs (2004) and Morganti et al. (2015), focusing on the capabilities enabled by the SKA design baseline, Array Assembly 4 (AA4). We expand on these earlier works by presenting new opportunities to simultaneously search for OH 18-cm absorption, an efficient tracer of diffuse molecular gas that complements the atomic gas traced by HI absorption, as well as the need for sub-arcsecond scale spectroscopic imaging and multi-wavelength data from large surveys. These advances will allow SKAO absorption surveys to address key questions surrounding the fuelling and feedback cycles of AGN and the evolution of the cold neutral gas across cosmic time.
Sources
- Discovery of a 21 cm absorption system at z=2.327 with CHIME
- H I Properties of Field Galaxies at $\boldsymbol{z\approx 0.2}$-0.6: Insights into Declining Cosmic Star Formation
- Empirical Calibration of Na I D and Other Absorption Lines as Tracers of High-Redshift Neutral Outflows
- Associated HI absorption and AGN feedback
- The LOFAR Two-metre Sky Survey: VII. Third Data Release
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