A MeerKAT view of pre-processing in the Fornax A group

27 Aug 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.