Tidal interaction vs. ram pressure stripping effects as seen in X-rays^⋆

Hot gas in group and cluster galaxies

¹ Astronomisches Institut der Ruhr-Universität Bochum, Universitätsstrasse 150, 44780 Bochum, Germany
e-mail: mawez@astro.rub.de
² ESAC, XMM-Newton Science Operations Centre, PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
³ Obserwatorium Astronomiczne Uniwersytetu Jagiellońskiego, ul. Orla 171, 30-244 Kraków, Poland
⁴ Laboratoire d’Astrophysique de Bordeaux (OASU), Universite de Bordeaux, CNRS/INSU, BP 89, 33270 Floirac, France

Received: 7 July 2011
Accepted: 4 July 2012

Abstract

Context. Cluster and group spiral galaxies are very often affected by their environment. The hot intracluster/intragroup medium (ICM/IGM) and a high galaxy density can lead to perturbations of the galactic interstellar medium (ISM) due to ram pressure and/or tidal interaction effects. In radio polarimetry observations, both phenomena may manifest similar features. X-ray data can help to determine the real origin of the perturbation.

Aims. We analyse the distribution and physical properties of the hot gas in the Virgo cluster spiral galaxies NGC 4254 and NGC 4569, which indicate that the cluster environment has had a significant influence on their properties. By performing both spatial and spectral analyses of X-ray data, we try to distinguish between two major phenomena: tidal and ram pressure interactions. We compare our findings with the case of NGC 2276, in which a shock was reported, by analysing XMM-Newton X-ray data for this galaxy.

Methods. We use archival XMM-Newton observations of NGC 4254, NGC 4569, and NGC 2276. Maps of the soft diffuse emission in the energy band 0.2−1 keV are obtained. For the three galaxies, especially at the position of magnetic field enhancements we perform a spectral analysis to derive gas temperatures and thus to look for shock signatures. A shock is a signature of ram pressure resulting from supersonic velocities; weak tidal interactions are not expected to influence the temperature of the ionized gas.

Results. In NGC 4254, we do not observe any temperature increase at the position of the bright polarized radio ridge. This suggests that the feature is formed by tidal interactions, and not by ram pressure stripping. NGC 4569 shows a higher temperature at the position of the polarized features, which may be indicative of ram-pressure effects. For NGC 2276, we do not find clear indications of a shock. Although ram-pressure effects seem to be visible, the main driver of the observed distortions is most likely tidal interaction.

Conclusions. Determining gas temperatures via sensitive X-ray observations at the position of polarized radio ridges seems to be a good method for distinguishing between ram pressure and tidal interaction effects acting upon a galaxy.