The MeerKAT Fornax Survey

II. The rapid removal of H I from dwarf galaxies in the Fornax cluster

¹ Netherlands Institute for Radio Astronomy (ASTRON), Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
e-mail: kleiner@astron.nl
² INAF – Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius, CA, Italy
³ Kapteyn Astronomical Institute, University of Groningen, PO Box 800 9700 AV Groningen, The Netherlands
⁴ Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa
⁵ Max-Plank-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁶ Department of Physics and Electronics, Rhodes University, PO Box 94 Makhanda 6140, South Africa
⁷ Ruhr University Bochum, Faculty of Physics and Astronomy, Astronomical Institute, 44780 Bochum, Germany
⁸ Armagh Observatory and Planetarium, College Hill, Armagh BT61 9DG, UK
⁹ Centre for Space Research, North-West University, Potchefstroom 2520, South Africa

Received: 20 March 2023
Accepted: 18 May 2023

Abstract

We present MeerKAT Fornax Survey atomic hydrogen (H I) observations of the dwarf galaxies located in the central ∼2.5 × 4 deg² of the Fornax galaxy cluster (R_vir ∼2°). The H I images presented in this work have a 3σ column density sensitivity between 2.7 and 50 × 10¹⁸ cm⁻² over 25 km s⁻¹ for spatial resolution between 4 and 1 kpc. We are able to detect an impressive M_HI = 5 × 10⁵ M_⊙ 3σ point source with a line width of 50 km s⁻¹ at a distance of 20 Mpc. We detected H I in 17 out of the 304 dwarfs in our field, with 14 out of the 36 late-type dwarfs (LTDs) and three out of the 268 early-type dwarfs (ETDs). The H I-detected LTDs have likely just joined the cluster and are on their first infall as they are located at large clustocentric radii, with comparable M_HI and mean stellar surface brightness at fixed luminosity as blue, star-forming LTDs in the field. By contrast, the H I-detected ETDs have likely been in the cluster longer than the LTDs and acquired their H I through a recent merger or accretion from nearby H I. Eight of the H I-detected LTDs host irregular or asymmetric H I emission and disturbed or lopsided stellar emission. There are two clear cases of ram pressure shaping the H I, with the LTDs displaying compressed H I on the side closest to the cluster centre and a one-sided, starless tail pointing away from the cluster centre. The H I-detected dwarfs avoid the most massive potentials (i.e. cluster centre and massive galaxies), consistent with massive galaxies playing an active role in the removal of H I. We created a simple toy model to quantify the timescale of H I stripping in the cluster by reproducing the observed M_r′–M_HI relation. We find that a M_HI = 10⁸ M_⊙ dwarf is stripped in ∼240 Myr. The model is consistent with our observations, where low-mass LTDs are directly stripped of their H I from a single encounter and more massive LTDs can harbour a disturbed H I morphology due to longer times or multiple encounters being required to fully strip their H I. This is the first time dwarf galaxies with M_HI ≲ 1 × 10⁶ M_⊙ have been detected and resolved beyond the local group and in a galaxy cluster.