KAT-7 science verification: Cold gas, star formation, and substructure in the nearby Antlia Cluster

Abstract

The Antlia Cluster is a nearby, dynamically young structure, and its proximity provides a valuable opportunity for detailed study of galaxy and group accretion on to clusters. We present a deep HI mosaic completed as part of spectral line commissioning of the Karoo Array Telescope (KAT-7), and identify infrared counterparts from the Widefield Infrared Survey Explorer extended source catalogue to study neutral atomic gas content and star formation within the cluster. We detect 37 cluster members out to a radius of ~0.9 Mpc with MHI > 5 × 107M⊙. Of these, 35 are new HI detections, 27 do not have previous spectroscopic redshift measurements, and one is the Compton thick Seyfert II, NGC 3281, which we detect in HI absorption. The HI galaxies lie beyond the X-ray-emitting region 200 kpc from the cluster centre and have experienced ram pressure stripping out to at least 600 kpc. At larger radii, they are distributed asymmetrically suggesting accretion from surrounding filaments. Combining HI with optical redshifts, we perform a detailed dynamical analysis of the internal substructure, identify large infalling groups, and present the first compilation of the largescale distribution of HI and star-forming galaxies within the cluster. We find that elliptical galaxy NGC 3268 is at the centre of the oldest substructure and argue that NGC 3258 and its companion population are more recent arrivals. Through the presence of HI and ongoing star formation, we rank substructures with respect to their relative time since accretion on to Antlia.