H I absorption in MHONGOOSE

Spin temperatures and cold neutral medium in nearby disc galaxies

¹ Netherlands Institute for Radio Astronomy (ASTRON), Oude Hoogeveensedijk 4, 7991 PD, Dwingeloo, The Netherlands
² Department of Astronomy, University of Cape Town, Private Bag X3 Rondebosch 7701, South Africa
³ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV, Groningen, The Netherlands
⁴ INAF – Osservatorio Astronomico di Cagliari, Via della Scienza 5, I-09047 Selargius, (CA), Italy
⁵ Wits Centre for Astrophysics, School of Physics, University of the Witwatersrand, 1 Jan Smuts Avenue, 2000 Johannesburg, South Africa
⁶ Université de Strasbourg, CNRS, Observatoire astronomique de Strasbourg, UMR 7550, 67000 Strasbourg, France
⁷ INAF – Arcetri Astrophysical Observatory, Largo Enrico Fermi 5, 50125 Firenze, Italy
⁸ Dipartimento di Fisica e Astronomia, Università degli Studi di Firenze, Via G. Sansone 1, 50019 Sesto Fiorentino, Firenze, Italy
⁹ Max Planck Institute for Radio Astronomy, Auf dem Hügel 69, 53121 Bonn, Germany
¹⁰ National Centre for Radio Astrophysics, Tata Institute of Fundamental Research, Pune University Campus, Post Bag 3, Ganeshkhind, Pune 411007, India
¹¹ Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, University of Manchester, Oxford Road, Manchester M13 9PL, UK

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 2 April 2026
Accepted: 10 May 2026

Abstract

Combined H I emission–absorption studies constrain the spin temperature and phase structure of the neutral atomic hydrogen interstellar medium (ISM). However, these studies have largely been limited to the Milky Way and the Local Group. We extend this technique to galaxies at distances of 7–22 Mpc using deep data from the MeerKAT H I Observations of Nearby Galactic Objects – Observing Southern Emitters (MHONGOOSE) survey and quantify the detection fraction and cold neutral medium (CNM) properties at these distances. We searched for H I absorption towards 56 background continuum sources in 21 out of the 30 MHONGOOSE galaxies (with nine galaxies lacking suitable background sources) and detected absorption associated with the H I discs of the galaxies in three cases: one sightline in NGC 289 and two in NGC 7424. This corresponds to detection rates of 5% (3/56) for the full sample and 10% (3/31) for a clean sub-sample of sightlines when considering only unresolved background sources behind 14 low-inclination galaxies. Detections occur only where both the continuum flux and the foreground H I column density are high, with optical-depth sensitivity as the primary limiting factor. For the detected sightlines, we modelled the absorption and emission spectra to derive spin temperatures and CNM fractions using the standard combined emission–absorption method. The CNM spin temperatures and line widths are comparable to Local Group measurements, but the inferred CNM fractions are systematically lower. We argue that this difference is primarily a resolution effect. At the distances of our galaxies, the emission spectra average over several hundred parsecs, diluting structured CNM relative to the smoother warm neutral medium. This demonstrates that emission–absorption analyses can be extended beyond the Local Group, provided that care is taken in constructing representative emission spectra.