Ram pressure stripping of the multiphase ISM in the Virgo cluster spiral galaxy NGC 4438

¹ CDS, Observatoire astronomique de Strasbourg, 11 rue de l'université, 67000 Strasbourg, France e-mail: bvollmer@astro.u-strasbg.fr
² Astronomical Observatory, Jagiellonian University, ul. Orla 171, 30-244 Kraków, Poland
³ Jansky fellow at NRAO, 1003 Lopezville Rd, Socorro, NM 87801, USA
⁴ Observatoire de Paris, Section Meudon, GEPI, CNRS-UMR 8111, and Université Paris 7, 5 place Janssen, 92195 Meudon, France
⁵ Laboratoire d'Astrophysique de Bordeaux (OASU), Université de Bordeaux, UMR 5804, CNRS/INSU, BP 89, 33270 Floirac, France
⁶ Laboratoire d'Astrophysique de Marseille, 13376 Marseille, France
⁷ Max-Planck-Insitut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Received: 13 October 2008
Accepted: 8 January 2009

Abstract

Ram pressure stripping of the multiphase ISM is studied in the perturbed Virgo cluster spiral galaxy NGC 4438. This galaxy underwent a tidal interaction ∼100 Myr ago and is now strongly affected by ram pressure stripping. Deep VLA radio continuum observations at 6 and 20 cm are presented. We detect prominent extraplanar emission to the west of the galactic center, which extends twice as far as the other tracers of extraplanar material. The spectral index of the extraplanar emission does not steepen with increasing distance from the galaxy. This implies in situ re-acceleration of relativistic electrons. The comparison with multiwavelength observations shows that the magnetic field and the warm ionized interstellar medium traced by Hα emission are closely linked. The kinematics of the northern extraplanar Hα emission, which is ascribed to star formation, follow those of the extraplanar CO emission. In the western and southern extraplanar regions, the Hα measured velocities are greater than those of the CO lines. We suggest that the ionized gas of this region is excited by ram pressure. The spatial and velocity offsets are consistent with a scenario where the diffuse ionized gas is more efficiently pushed by ram pressure stripping than the neutral gas. We suggest that the recently found radio-deficient regions compared to 24 μm emission are due to this difference in stripping efficiency.