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 deg² out to the cluster virial radius. The survey has a spatial and velocity resolution of 67″ × 95″(∼6 × 9 kpc at the Fornax cluster distance of 20 Mpc) and 6.6 km s⁻¹ and a 3σ sensitivity of N_HI ∼ 2 × 10¹⁹ cm⁻² and M_HI ∼ 2 × 10⁷ M_⊙, 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 8 × 10⁶ to 1.5 × 10¹⁰ M_⊙. 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. 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_⋆ ≲ 10⁹ M_⊙, 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 ≳1−2 Gyr. 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.