Atomic hydrogen reservoirs in quiescent galaxies at z = 0.4

¹ Department of Physics and Astronomy, Università degli Studi di Padova, Vicolo dell’Osservatorio 3, I-35122 Padova, Italy
² INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, I-35122 Padova, Italy
³ National Center for Nuclear Research, Pasteura 7, 02-093 Warsaw, Poland
⁴ SISSA, Via Bonomea 265, 34136 Trieste, Italy
⁵ Département d’Astronomie, Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland
⁶ Institut für Astrophysik, Universität Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland
⁷ Department of Physics and Astronomy, University of the Western Cape, Robert Sobukwe Rd, 7535 Bellville, Cape Town, South Africa
⁸ Inter-university Institute for Data Intensive Astronomy, Department of Astronomy, University of Cape Town, 7701 Rondebosch, Cape Town, South Africa
⁹ Inter-university Institute for Data Intensive Astronomy, Department of Physics and Astronomy, University of the Western Cape, 7535 Bellville, Cape Town, South Africa
¹⁰ INAF – Istituto di Radioastronomia, Via Gobetti 101, 40129 Bologna, Italy
¹¹ Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281 S9, 9000 Gent, Belgium
¹² Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh EH9 3HJ, United Kingdom
¹³ INAF – Osservatorio Astronomico di Cagliari, Via della Scienza 5, I-09047 Selargius, CA, Italy
¹⁴ Astrophysics, Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK
¹⁵ Centre for Radio Astronomy Techniques and Technologies, Department of Physics and Electronics, Rhodes University, PO Box 94 Makhanda 6140, South Africa
¹⁶ South African Radio Astronomy Observatory, 2 Fir Street, Black River Park, Observatory, Cape Town 7925, South Africa

^⋆ Corresponding author: alessandro.bianchetti@phd.unipd.it

Received: 11 July 2025
Accepted: 10 September 2025

Abstract

Context. Based on local Universe observations, quiescent galaxies (QGs) host lower or negligible HI compared to star-forming galaxies (SFGs), but no constraints have been derived to date at higher redshift (z > 0.1). Understanding whether QGs can retain significant HI reservoirs at higher z is crucial for refining quenching and gas accretion models and for constraining overall star formation efficiency at different epochs.

Aims. We aim to probe HI in candidate QGs at intermediate redshifts (⟨z⟩≈0.36) and to understand whether a class of QGs exists that retains consistent HI reservoir, as well as which parameters (dust content, stellar mass, D_n4000, morphology, and environment) effectively capture HI-rich QGs.

Methods. We performed 21-cm spectral line stacking on MIGHTEE-HI data at ⟨z⟩ = 0.36, targeting two different samples of QGs, defined by means of a color-selection criterion and a spectroscopic criterion based on D_n4000, respectively. We also performed stacking on subsamples of the spectroscopically selected quiescent sample to investigate the correlation between the HI content and other galaxy properties.

Results. We find that QGs with an IR counterpart (i.e., dusty galaxies) host a substantial HI content, on average only 40% lower than that of SFGs. In contrast, color-selected QGs still retain HI, but at levels lower than those of SFGs by a factor of ∼3. Among dusty objects, we find that morphology has a mild impact on the atomic gas content, with spirals hosting approximately 15 − 30% more HI than spheroids. Environmental effects are also present: galaxies in low-density regions are richer in HI than those in high-density regions, by approximately 30% for spirals and 60% for spheroids. We suggest that, in general, HI content is influenced by several factors, including slow quenching mechanisms and interstellar medium (ISM) enrichment processes. Also, QGs – and especially dusty systems – seem to yield HI more consistently than in the local Universe.