The MeerKAT Fornax Survey

III. Ram-pressure stripping of the tidally interacting galaxy NGC 1427A in the Fornax cluster

¹ INAF - Osservatorio Astronomico di Cagliari, Via della Scienza 5, I-09047 Selargius (CA), Italy
² Netherlands Institute for Radio Astronomy (ASTRON), Oude Hoogeveensedijk 4 7991 PD Dwingeloo, the Netherlands
³ Kapteyn Astronomical Institute, University of Groningen, PO Box 800 NL-9700 AV Groningen, the Netherlands
⁴ Ruhr University Bochum, Faculty of Physics and Astronomy, Astronomical Institute (AIRUB), 44780 Bochum, Germany
⁵ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
⁶ Department of Physics and Electronics, Rhodes University, PO Box 94 Makhanda 6140, South Africa
⁷ Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa
⁸ Department of Astronomy, Columbia University, New York, NY 10027, USA
⁹ INAF - Astronomical Observatory of Capodimonte, Salita Moiariello 16, 80131 Naples, Italy
¹⁰ Centre for Space Research, North-West University, Potchefstroom 2520, South Africa
¹¹ School of Physics and Astronomy, Cardiff University, Queens Buildings, The Parade, Cardiff CF24 3AA, UK

Received: 25 March 2024
Accepted: 12 July 2024

Abstract

We present MeerKAT Fornax Survey H I observations of NGC 1427A, a blue irregular galaxy with a stellar mass of ∼2 × 10⁹ M_⊙ located near the centre of the Fornax galaxy cluster. Thanks to the excellent resolution (1–6 kpc spatially, 1.4 km s⁻¹ in velocity) and H I column density sensitivity (∼4 × 10¹⁹ to ∼10¹⁸ cm⁻² depending on resolution), our data deliver new insights on the long-debated interaction of this galaxy with the cluster environment. We confirm the presence of a broad, one-sided, starless H I tail stretching from the outer regions of the stellar body and pointing away from the cluster centre. We find the tail to have 50% more H I (4 × 10⁸ M_⊙) and to be 3 times longer (70 kpc) than in previous observations. In fact, we detect scattered H I clouds out to 300 kpc from the galaxy in the direction of the tail – possibly the most ancient remnant of the passage of NGC 1427A through the intracluster medium of Fornax. Both the velocity gradient along the H I tail and the peculiar kinematics of H I in the outer region of the stellar body are consistent with the effect of ram pressure given the line-of-sight motion of the galaxy within the cluster. However, several properties cannot be explained solely by ram pressure and suggest an ongoing tidal interaction. This includes: the close match between dense H I and stars within the disturbed stellar body; the abundant kinematically anomalous H I; and the inversion of the H I velocity gradient near the base of the H I tail. We rule out an interaction with the cluster tidal field, and conclude that NGC 1427A is the result of a high-speed galaxy encounter or of a merger started at least 300 Myr ago, where ram pressure shapes the distribution and kinematics of the H I in the perturbed outer stellar body and in the tidal tails.