The low HI-mass galaxy population in the Fornax galaxy cluster

Neutral hydrogen (HI) is the galaxy component which extends at the outskirts of galaxies. Because of that, in dense environments such as galaxy clusters, HI can be actively removed from galaxies due to gravitational and hydrodynamical interactions with the local environments. Since HI is also the primary reservoir of fuel for star formation, a galaxy whose HI was removed, will lose its capability to form new stars. We have explored the HI content of the Fornax galaxy cluster using interferometric ATCA HI data with a resolution of 67′′×95′′ and a sensitivity of MHI=2x10^7M⊙. To understand whether environmental interactions have already removed a significant amount of HI, we compared the HI content of Fornax galaxies and that of non-cluster galaxies, finding that Fornax galaxies are HI deficient. We also constrained the gas removal time scales in Fornax by linking the cold gas content of Fornax galaxies, their SFR and the distribution of HI detections and HI non-detections both on the sky and in the projected phase space diagram. Indeed, the lack of HI in the virialized part of the cluster indicates that HI is removed within a cluster crossing time ∼2Gyr, while our HI detections reside outside the virialised region of the cluster and their HI content is consistent with the SFR of non-cluster galaxies with similar HI deficiency. Thus, on their way to the cluster they have lost/used HI in a time scale longer than the typical H2 depletion time (∼2Gyr). Another powerful tool to study the HI content of a portion of the Universe is the HI mass function (HIMF), which measures the density of objects as a function of the HI mass. Because of gas removal in cluster members, the shape of the HIMF of dense environments is expected to be different from that of the local universe, which is well described by a steep Schechter function, and thus it can provide further information on how galaxy evolution proceeds in different environments. One of the goals of our research was that of measuring the HIMF of Fornax. However, the low number of ATCA HI detections did not allow us to measure the low-mass end of it. We overcame this problem by applying a new Bayesian method which measures the shape of the HIMF below the nominal detection limit. Our preliminary results show that the distribution of MHI in Fornax might be better described by a Schechter function truncated below MHI∼10^7M⊙. The last project we are working on is part of the MeerKAT Fornax Survey, which provides us with data with a sensitivity a factor of 10 better than ATCA, and concerns the only truncated HI disc detected in Fornax with MeerKAT: NGC 1436. An appealing aspect of this galaxy is that observations suggest that it is currently moving from a spiral to a lenticular morphology. Although there is evidence that the majority of lenticulars in clusters are the end products of the evolutionary path of spirals, it is very rare to observe the aforementioned transformation in action. Thus we addressed the question of whether the Fornax environment played a role in this morphological transformation and the answer is: yes it did! To tackle this question, we used 10’’ MeerKAT data to inspect the HI emission of the galaxy and then we compared the HI distribution and kinematics with that of H2 using data from ALMA. This established that the cold gas disc is well settled, and therefore NGC 1436 is currently in a state of slow evolution. Finally, we fit MUSE spectra extracted from the inner, gas-rich, star-forming disc and from the outer, gas-poor, quiescent disc in order to measure and compare their star formation histories. Preliminary results show that this galaxy fell for the first time into the cluster about 5Gyr ago when environmental interactions caused a temporary enhancement of the SFR in the inner disc and a sudden drop in the SFR in the outer disc. Before this epoch, and thus before interacting with Fornax, it was evolving as a normal field spiral galaxy.