Volume 648, April 2021
|Number of page(s)||17|
|Published online||08 April 2021|
A MeerKAT view of pre-processing in the Fornax A group
INAF – Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius, CA, Italy
2 University of Oulu, Space Physics and Astronomy Unit, Pentti Kaiteran katu 1, 90014 Oulu, Finland
3 Institute of Astronomy, Graduate School of Science, The University of Tokyo, 2-21-1 Osawa, Mitaka, Tokyo 181-0015, Japan
4 Kapteyn Astronomical Institute, University of Groningen, PO Box 800 9700 AV Groningen, The Netherlands
5 INAF – Astronomical Observatory of Capodimonte, Via Moiariello 16, Naples 80131, Italy
6 Netherlands Institute for Radio Astronomy (ASTRON), Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
7 Deptarment of Astronomy, Univ. of Cape Town, Private Bag X3, Rondebosch 7701, South Africa
8 Inter-University Institute for Data Intensive Astronomy, University of Cape Town, Cape Town, Western Cape 7700, South Africa
9 South African Radio Astronomy Observatory, 2 Fir Street, Black River Park, Observatory, Cape Town 7925, South Africa
10 Department of Physics and Electronics, Rhodes University, PO Box 94 Makhanda 6140, South Africa
11 Argelander-Institut für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany
12 Ruhr University Bochum, Faculty of Physics and Astronomy, Astronomical Institute, 44780 Bochum, Germany
13 Dipartimento di Fisica, Università di Cagliari, Cittadella Universitaria, 09042 Monserrato, Italy
14 Centre for Space Research, North-West University, Potchefstroom 2520, South Africa
15 Department of Physics, University of Pretoria, Private Bag X20, Hatfield 0028, South Africa
16 INAF – Istituto di Radioastronomia, Via Gobetti 101, 40129 Bologna, Italy
Accepted: 22 January 2021
We present MeerKAT neutral hydrogen (H I) observations of the Fornax A group, which is likely falling into the Fornax cluster for the first time. Our H I image is sensitive to 1.4 × 1019 atoms cm−2 over 44.1 km s−1, where we detect H I in 10 galaxies and a total of (1.12 ± 0.02) × 109 M⊙ of H I in the intra-group medium (IGM). We search for signs of pre-processing in the 12 group galaxies with confirmed optical redshifts that reside within the sensitivity limit of our H I image. There are 9 galaxies that show evidence of pre-processing and we classify each galaxy into their respective pre-processing category, according to their H I morphology and gas (atomic and molecular) scaling relations. Galaxies that have not yet experienced pre-processing have extended H I discs and a high H I content with a H2-to-H I ratio that is an order of magnitude lower than the median for their stellar mass. Galaxies that are currently being pre-processed display H I tails, truncated H I discs with typical gas fractions, and H2-to-H I ratios. Galaxies in the advanced stages of pre-processing are the most H I deficient. If there is any H I, they have lost their outer H I disc and efficiently converted their H I to H2, resulting in H2-to-H I ratios that are an order of magnitude higher than the median for their stellar mass. The central, massive galaxy in our group (NGC 1316) underwent a 10:1 merger ∼2 Gyr ago and ejected 6.6−11.2 × 108 M⊙ of H I, which we detect as clouds and streams in the IGM, some of which form coherent structures up to ∼220 kpc in length. We also detect giant (∼100 kpc) ionised hydrogen (Hα) filaments in the IGM, likely from cool gas being removed (and subsequently ionised) from an in-falling satellite. The Hα filaments are situated within the hot halo of NGC 1316 and there are localised regions that contain H I. We speculate that the Hα and multiphase gas is supported by magnetic pressure (possibly assisted by the NGC 1316 AGN), such that the hot gas can condense and form H I that survives in the hot halo for cosmological timescales.
Key words: galaxies: groups: general / galaxies: groups: individual: Fornax A / galaxies: evolution / galaxies: interactions / galaxies: ISM / radio lines: galaxies
© ESO 2021
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