The MeerKAT Fornax Survey

VI. The collapse of the galaxy H I Mass Function in Fornax

¹ Netherlands Institute for Radio Astronomy (ASTRON), Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
² INAF – Osservatorio Astronomico di Cagliari, Via della Scienza 5, I-09047 Selargius, (CA), Italy
³ INAF – Osservatorio Astronomico di Capodimonte, Salita Moiariello, 16, 80131 Napoli, (NA), Italy
⁴ Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa
⁵ Kapteyn Astronomical Institute, University of Groningen, PO Box 800 NL-9700 AV Groningen, The Netherlands
⁶ Ruhr University Bochum, Faculty of Physics and Astronomy, Astronomical Institute (AIRUB), 44780 Bochum, Germany

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

Received: 4 August 2025
Accepted: 18 November 2025

Abstract

We present the deepest H I mass function (H IMF) ever measured, outside the Local Group. The observations are part of the MeerKAT Fornax Survey and cover a 4 × 4 deg² field, corresponding to the approximate R_vir. The 3σ detection limit is log(M_HI/M_⊙) = 5.7 for a 50 km s⁻¹-wide point source. We detect H I in 35 galaxies and 44 clouds with no optical counterparts. Using deep optical images from the Fornax Deep Survey, we measure the 5σ optical flux limit of the H I clouds and show that they are a distinct population, separated by a four magnitude gap from the faintest H I-detected galaxies. Three quarters (33 out of 44) of the clouds are associated with the two galaxies with the most H I in the cluster – NGC 1365 and NGC 1427A, although the clouds contribute a negligible amount to the total M_HI budget. By performing a signal-to-noise analysis and computing the Rauzy statistic on the H I detections, we demonstrate that our catalogue is complete down to log(M_HI/M_⊙) = 6, and we are therefore readily able to probe the H IMF down to this level. We find an abrupt drop in the number density of H I-detected galaxies at log(M_HI/M_⊙) = 7, signifying a clear absence of galaxies between 6 < log(M_HI/M_⊙) ≤ 7. We use the modified maximum likelihood method to fit a Schechter function down to log(M_HI/M_⊙) ≥ 7, the range where the H IMF follows a power law. The measured low-mass slope is α = −1.31 ± 0.13, with a characteristic knee mass of log(M_*/M_⊙) = 10.52 ± 1.89. The low-mass slope matches the slope in the field, while the knee is defined by a single galaxy and is unconstrained. Below log(M_HI/M_⊙) = 7, there is a sharp departure from a Schechter function, and we report the first robust measurement of the collapse of a H IMF. For the H IMF below log(M_HI/M_⊙) = 7 to follow a power law, tens of galaxies are needed – a factor of ∼6 higher than what is observed. The collapse of the Fornax H IMF is likely due to the rapid removal of H I from low-mass galaxies.