A blind ATCA HI survey of the Fornax galaxy cluster

Properties of the HI detections

¹ INAF-Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius, CA, Italy
e-mail: alessandro.loni@inaf.it
² Dipartimento di Fisica, Universitá di Cagliari, Cittadella Universitaria, 09042 Monserrato, Italy
³ International Centre for Radio Astronomy Research (ICRAR), The University of Western Australia, 35 Stirling Hwy, Crawley, WA 6009, Australia
⁴ ARC Centre of Excellence for All-Sky Astrophysics in 3 Dimensions (ASTRO3D), Australia
⁵ CSIRO Astronomy and Space Science, Australia Telescope National Facility, PO Box 76 Epping, NSW 1710, Australia
⁶ Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa
⁷ School of Physics and Astronomy, Cardiff University, Queens Buildings The Parade, Cardiff CF24 3AA, UK
⁸ INAF-Astronomical observatory of Capodimonte, via Moiariello 16, Naples 80131, Italy
⁹ Kapteyn Astronomical Institute, University of Groningen, PO Box 800 9700 AV Groningen, The Netherlands
¹⁰ Australian Research Council, Centre of Excellence for All-sky Astrophysics (CAASTRO), Australia
¹¹ Department of Physics and Electronics, Rhodes University, PO Box 94, Makhanda 6140, South Africa

Received: 30 October 2020
Accepted: 26 January 2021

Abstract

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.