The MeerKAT Fornax Survey

We present the deepest H <ns0:sc>I</ns0:sc> mass function (H <ns0:sc>I</ns0:sc>MF) 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 Rvir. The 3σ detection limit is log(MHI/M⊙) = 5.7 for a 50 km s−1-wide point source. We detect H <ns0:sc>I</ns0:sc> 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 <ns0:sc>I</ns0:sc> clouds and show that they are a distinct population, separated by a four magnitude gap from the faintest H <ns0:sc>I</ns0:sc>-detected galaxies. Three quarters (33 out of 44) of the clouds are associated with the two galaxies with the most H <ns0:sc>I</ns0:sc> in the cluster – NGC 1365 and NGC 1427A, although the clouds contribute a negligible amount to the total MHI budget. By performing a signal-to-noise analysis and computing the Rauzy statistic on the H <ns0:sc>I</ns0:sc> detections, we demonstrate that our catalogue is complete down to log(MHI/M⊙) = 6, and we are therefore readily able to probe the H <ns0:sc>I</ns0:sc>MF down to this level. We find an abrupt drop in the number density of H <ns0:sc>I</ns0:sc>-detected galaxies at log(MHI/M⊙) = 7, signifying a clear absence of galaxies between 6 < log(MHI/M⊙) ≤ 7. We use the modified maximum likelihood method to fit a Schechter function down to log(MHI/M⊙) ≥ 7, the range where the H <ns0:sc>I</ns0:sc>MF 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(MHI/M⊙) = 7, there is a sharp departure from a Schechter function, and we report the first robust measurement of the collapse of a H <ns0:sc>I</ns0:sc>MF. For the H <ns0:sc>I</ns0:sc>MF below log(MHI/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 <ns0:sc>I</ns0:sc>MF is likely due to the rapid removal of H <ns0:sc>I</ns0:sc> from low-mass galaxies.