2021A&A...648A..31L

We present the first interferometric blind HI survey of the Fornax galaxy cluster, which covers an area of 15deg² out to the cluster virial radius. The survey has a spatial and velocity resolution of 67''x95''(∼6x9kpc at the Fornax cluster distance of 20Mpc) and 6.6km/s and a 3σ sensitivity of N_HI∼2x10¹⁹cm^–2 and M_HI∼2x10⁷M_☉, respectively. We detect 16 galaxies out of roughly 200 spectroscopically confirmed Fornax cluster members. The detections cover about three orders of magnitude in HI mass, from 8x10⁶ to 1.5x10¹⁰M_☉. They avoid the central, virialised region of the cluster both on the sky and in projected phase-space, showing that they are recent arrivals and that, in Fornax, HI is lost within a crossing time, ∼2Gyr. Half of these galaxies exhibit a disturbed HI morphology, including several cases of asymmetries, tails, offsets between HI and optical centres, and a case of a truncated HI disc. This suggests that these recent arrivals have been interacting with other galaxies, the large-scale potential or the intergalactic medium, within or on their way to Fornax. As a whole, our Fornax HI detections are HI-poorer and form stars at a lower rate than non-cluster galaxies in the same M_* range. This is particularly evident at M_*≤10⁹M_☉, indicating that low mass galaxies are more strongly affected throughout their infall towards the cluster. The M_HI/M_* ratio of Fornax galaxies is comparable to that in the Virgo cluster. At fixed M_*, our HI detections follow the non-cluster relation between M_HI and the star formation rate, and we argue that this implies that thus far they have lost their HI on a timescale ≥1-2Gyr. Deeper inside the cluster HI removal is likely to proceed faster, as confirmed by a population of HI-undetected but H₂-detected star-forming galaxies. Overall, based on ALMA data, we find a large scatter in H₂-to-HI mass ratio, with several galaxies showing an unusually high ratio that is probably caused by faster HI removal. Finally, we identify an HI-rich subgroup of possible interacting galaxies dominated by NGC 1365, where pre-processing is likely to have taken place.