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Kleiner, D.; Serra, P.; Maccagni, F. M.; Venhola, A.; Morokuma-Matsui, K.; Peletier, R.; Iodice, E.; Raj, M. A.; de Blok, W. J. G.; Comrie, A.; Józsa, G. I. G.; Kamphuis, P.; Loni, A.; Loubser, S. I.; Molnár, D. Cs; Passmoor, S. S.; Ramatsoku, M.; Sivitilli, A.; Smirnov, O.; Thorat, K.; Vitello, F.
Astronomy & astrophysics, 04/2021, Volume: 648Journal Article
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 × 10 19 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) × 10 9 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 H 2 -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 H 2 -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 H 2 , resulting in H 2 -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 × 10 8 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 (H α ) filaments in the IGM, likely from cool gas being removed (and subsequently ionised) from an in-falling satellite. The H α filaments are situated within the hot halo of NGC 1316 and there are localised regions that contain H I . We speculate that the H α 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.
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