A&A 545, A140 (2012) Astronomy DOI: 10.1051/0004-6361/201219775 & c ESO 2012 Astrophysics Hot gas in groups: NGC 5328 and the intriguing case of NGC 4756 with XMM-Newton, G. Trinchieri1,A.Marino2, P. Mazzei3, R. Rampazzo3, and A. Wolter1 1 INAF – Osservatorio Astronomico di Brera, via Brera 28, 20121 Milano, Italy e-mail: [ginevra.trinchieri;anna.wolter]@brera.inaf.it 2 Dipartimento di Fisica e Astronomia G. Galilei, Università di Padova, vicolo dell’Osservatorio 3, 35122 Padova, Italy e-mail: [email protected] 3 INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy e-mail: [paola.mazzei;roberto.rampazzo]@oapd.inaf.it Received 8 June 2012 / Accepted 26 July 2012 ABSTRACT Context. Environment appears to have a strong influence on the fundamental properties of galaxies, modifying both their morpholo- gies 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 galax- ies. 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 clus- ter 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 41 −1 luminosity of Lx ∼ 10 erg s 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 41 −1 connection. The X-ray emission from NGC 5328 is clearly peaked on the galaxy, has Lx ∼ 10 erg s , 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. Key words. galaxies: elliptical and lenticular, cD – galaxies: groups: general – galaxies: groups: individual: NGC 5328 – galaxies: groups: individual: NGC 4756 – X-rays: galaxies – X-rays: ISM 1. Introduction z ∼ 1(Kovacˆ et al. 2010; Peng et al. 2010). Cosmological hy- drodynamical simulations by e.g. Kobayashi et al. (2005)and ∼ Observationally, we see that a large fraction ( 50–60%) of the Feldmann et al. (2010, 2011) that study the evolution of groups galaxies in the local Universe is in groups. These could either suggest that the star-forming massive galaxies at the center of be isolated systems, or parts of filaments or chains, that are group-type potentials at z 1 can become the massive, gas- near to or infalling into clusters (see e.g. Eke et al. 2004; Tago poor early-type systems observed at the center of groups today. et al. 2008). Groups, for which the galaxy velocity dispersion is comparable to the internal velocities of the galaxies, provide We can therefore expect that the galaxies we observe in a controlled environment in which interactions (mergers) act to clusters today are likely to have experienced pre-processing modify galaxy properties, for example by removing the gas that in groups at some point in their history, before the group fuels star formation. Recent surveys have indeed verified that the itself fell into the cluster formed within the same infalling transformation from star-forming systems into “passive” ones halo. In their study of close pairs in the Sloan Digital Sky (Bai et al. 2010, and references therein) is a result of the envi- Survey (SDSS), Perez et al. (2009) found that galaxies are effi- ronmental action over the past eight billion years, since redshift ciently pre-processed by close encounters and mergers while in intermediate-density environments. With the same mechanism Based on XMM-Newton observations (Obs. ID 0551600101 and of interaction between pairs, mergers have probably depleted the 0401480201 P.I. G. Trinchieri). reservoir of galaxies in the halo while building the dominant el- Appendix A is available in electronic form at liptical of the group. In addition to mergers, simulations suggest http://www.aanda.org that ram-pressure stripping, once thought to operate only in rich Article published by EDP Sciences A140, page 1 of 19 A&A 545, A140 (2012) environments, is active in the form of “strangulation” in groups 2. Characterization of the environments as well. This depletes gas-rich galaxies of their hot interstellar of NGC 4756 and NGC 5328 medium (ISM) content, which is their largest gas reservoir, leav- ing their molecular gas content nearly intact. On a timescale of NGC 4756 is the brightest unperturbed elliptical galaxy in a about 1 Gyr, this process leads to the quenching of star formation loose group. The structure of the group is filamentary and com- in gas-rich disk galaxies, transforming them into S0s (Kawata plex, extending for about half a degree. The central part of the group contains a significant fraction of early-type galaxies. At et al. 2008). Jeltema et al. (2008) interpret the evidence that the LK − LX relation for early-type galaxies in groups is system- about 7 southwest (SW) of NGC 4756, a compact, Hickson- atically below that of field objects as indirect evidence of hot type, clump of galaxies with signatures of recent interaction gas stripped by viscous- or ram-pressure. This is supported by has been identified (Grützbauch et al. 2005a). The NGC 5328 more direct evidence of the action of a stripping event seen in group, dominated by an old (12.4 ± 3.7 Gyr, Annibali et al. 2007) the X-ray image of the S0 galaxy NGC 6265, located ∼250 kpc elliptical galaxy, has the characteristics of an evolved group west (W) of the NGC 6269 group (see also Baldi et al. 2009;or (Grützbauch et al. 2005b) and is at the same distance as the Kim et al. 2008, for NGC 7619). Abell 3574 cluster of galaxies, as we explain below. However, a different scenario is proposed by Berrier et al. Both NGC 4756 and NGC 5328 and the groups associated (2009). Their simulations actually suggest that only a small frac- with them were extensively studied by Grützbauch et al. (2005a) tion (30%) of galaxies today in clusters may have been accreted and Grützbauch et al. (2005b). However, prior to the observa- from groups, while the majority have been accreted directly from tions presented here, inadequate X-ray data were available for the field and then reprocessed in the cluster environment. both galaxies. An unpublished snapshot Chandra observation To better understand the processes related to galaxy evolu- shows emission at the position of NGC 5328, which we later use tion in different environments, a careful analysis of the dynamics to determine the presence of a point source at the center of this and the characteristics of galaxies in groups in the local Universe galaxy (see Sect. 3.1.2). NGC 4756 was observed by Einstein, provides the observational data necessary to interpret the differ- the ROSAT HRI and ASCA. A strong emission peaked on the ent scenarios proposed. galaxy was detected in all observations. An extension to the Tully (2010) shows that evolved groups, which are charac- north (N) and emission from MCG-2-33-38, a Seyfert galaxy terized by a high fraction of elliptical galaxies, have at least one in the SW sub-group, were also observed. In spite of the lim- dominant E galaxy and relatively few intermediate-luminosity ited quality of the available data, the comparison of adaptively galaxies than pristine spiral-rich groups (see e.g. Grützbauch smoothed images from ASCA in two energy bands showed an et al. 2009). The evolution of massive ellipticals at the centers intriguing feature that prompted us to ask for higher quality of groups, and the evolution of their hot gas content are begin- X-ray data. The soft (1–2 keV) and hard (2–8 keV) energy-band ning to be clarified through self-consistent models (Bettoni et al. images from ASCA suggest the presence of two separate compo- 2012). nents, with different spatial distributions: a soft peak, consistent In this context, we adopt a multiwavelength approach to with the hot intergalactic medium associated with NGC 4756, studying groups (and their members) characterized by different and a hard-band centroid to the N, which is roughly at the posi- observational properties (e.g., galaxy density and composition, tion of a concentration of galaxies in Abell 1631 (see Grützbauch dynamical properties) to understand whether these are repre- et al.