A blind ATCA HI survey of the Fornax galaxy cluster: properties of the HI detections

We present the first interferometric blind HI survey of the Fornax galaxy cluster, which covers an area of 15 deg\(^2\) out to the cluster \(R_{vir}\). The survey has a resolution of 67''x95'' and 6.6 km\(s^{-1}\) with a 3\(\sigma\) sensitivity of N(HI)~2x10\(^{19}\) cm\(^{-2}\) and MHI 2x10\(^7\) M\(_\odot\). We detect 16 galaxies out of 200 spectroscopically confirmed Fornax cluster members. The detections cover ~3 orders of magnitude in HI mass, from 8x10\(^6\) to 1.5x10\(^{10}\) M\(_\odot\). 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, ~2 Gyr. 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 suggesting that they have been interacting within or on their way to Fornax. Our HI detections are HI-poorer and form stars at a lower rate than non-cluster galaxies in the same \(M_\star\) range. Low mass galaxies are more strongly affected throughout their infall towards the cluster. The MHI/\(M_\star\) ratio of Fornax galaxies is comparable to that in the Virgo cluster. At fixed \(M_\star\), our HI detections follow the non-cluster relation between MHI and the star formation rate, and we argue that this implies that so far they have lost their HI on a timescale \(\gtrsim\)1-2 Gyr. Deeper inside the cluster HI removal is likely to proceed faster, as confirmed by a population of HI-undetected but H\(_2\)-detected star-forming galaxies. Based on ALMA data, we find a large scatter in H\(_2\)-to-HI mass ratio, with several galaxies showing an unusually high ratio that is probably caused by faster HI removal. We identify an HI-rich subgroup of possible interacting galaxies dominated by NGC 1365, where pre-processing is likey to have taken place.