Hot gas in groups: NGC 5328 and the intriguing case of NGC 4756 with XMM-Newton

Context. Environment appears to have a strong influence on the fundamental properties of galaxies, modifying both their morphologies and their star formation histories. Similarly, galaxies play a role in determining the properties of the hot intergalactic medium in groups, heating and enriching it through a variety of mechanisms. NGC 5238 and NGC 4756 are the brightest unperturbed elliptical galaxies in their respective loose groups, but the analysis of their environment suggests that they may be at different evolutionary stages.
Aims: We aim to characterize the properties of the hot gas in both the halos of the brightest galaxy members and in the environment. In NGC 4756, we are also interested in the properties of a substructure identified to the southwest and the region connecting the two structures, to search for a physical connection between the two. However, we have to take into account that the group is projected against the bright, X-ray emitting cluster A1361, which heavily contaminates and confuses the emission from the foreground structure.
Methods: We present XMM-Newton observations of the groups and a careful analysis to separate different components. We examine the X-ray morphology, hot gas distribution, and spectral characteristics of both NGC 4756 and NGC 5328 and their companion galaxies. To better characterize the environment, we also present a re-evaluation of the dynamical properties of the systems. Smoothed particle hydrodynamical simulations are used to interpret the results.
Results: We find that the X-ray source associated with NGC 4756 indeed sits on top of extended emission from the background cluster A1361, but can be distinguished relatively well from it as a significant excess out to r ~ 150″ (~40 kpc). NGC 4756 has an X-ray luminosity of L_x ~ 10⁴¹ erg s^-1 due to hot gas, with an average temperature of kT ~ 0.7 keV. We measure a faint diffuse emission in the region of the subclump to the SW, but more interestingly, we detect gas between the two structures, indicating a possible physical connection. The X-ray emission from NGC 5328 is clearly peaked on the galaxy, has L_x ~ 10⁴¹ erg s^-1, and extends to r ~ 110 kpc. Simulations provide an excellent reproduction of the spectral energy distribution and the global properties of both galaxies, which are caught at two different epochs of the same evolutionary process, with NGC 5328 ~2.5 Gyr younger than NGC 4756.

Based on XMM-Newton observations (Obs. ID 0551600101 and 0401480201 P.I. G. Trinchieri).Appendix A is available in electronic form at http://www.aanda.org