Atomic hydrogen reservoirs in quiescent galaxies at <i>z</i> = 0.4

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 &gt; 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 n 4000, 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 n 4000, 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.