A MeerKAT view of pre-processing in the Fornax A group
D. Kleiner, P. Serra, F. M. Maccagni, A. Venhola, K. Morokuma-Matsui, R. Peletier, E. Iodice, M. A. Raj, W. J. G. de Blok, A. Comrie, G. I. G. Józsa, P. Kamphuis, A. Loni, S. I. Loubser, D. Cs. Molnár, S. S. Passmoor, M. Ramatsoku, A. Sivitilli, O. Smirnov, K. Thorat, F. Vitello
Abstract
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.