2021A&A...648A..32K

We present MeerKAT neutral hydrogen (HI) observations of the Fornax A group, which is likely falling into the Fornax cluster for the first time. Our HI image is sensitive to 1.4x10¹⁹atoms/cm² over 44.1km/s, where we detect HI in 10 galaxies and a total of (1.12±0.02)x10⁹M_☉ of HI 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 HI 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 HI morphology and gas (atomic and molecular) scaling relations. Galaxies that have not yet experienced pre-processing have extended HI discs and a high HI content with a H₂-to-HI ratio that is an order of magnitude lower than the median for their stellar mass. Galaxies that are currently being pre-processed display HI tails, truncated HI discs with typical gas fractions, and H₂-to-HI ratios. Galaxies in the advanced stages of pre-processing are the most HI deficient.If there is any HI, they have lost their outer HI disc and efficiently converted their HI to H₂, resulting in H₂-to-HI 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 ∼2Gyr ago and ejected 6.6-11.2x10⁸M_☉ of HI, which we detect as clouds and streams in theIGM, some of which form coherent structures up to ∼220kpc in length. We also detect giant (∼100kpc) ionised hydrogen (Hα) filaments in theIGM, 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 HI. 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 HI that survives in the hot halo for cosmological timescales.