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Appendix A: Notes on individual groups

A0160

This group/poor cluster of galaxies was classified as “intermediate type” because there is a distinct sub-peak to the east of the center of extended emission. However this sub-peak was excluded from our analysis of the ICM, as it appears not to be interacting with the surrounding material. There is a large degeneracy between temperature and normalization in the spectral fits, so the total error on the surface brightness was overestimated. We corrected for this unreasonably high uncertainty by choosing the smaller of the two errors.

This system was investigated by Drake et al. (2000), who found “wake”-like structures in the X-ray emission, which they interpreted as evidence for ram-pressure stripping of the cluster galaxies by the surrounding ICM.

Jetha et al. (2005) also inspected radio observations for this object, and found a spatial correlation between a wide angle tailed radio galaxy in the BCG and structures in the X-ray emitting gas. They concluded that the secondary peak (“source 1”) corresponds to a large elliptical at slightly higher redshift than the cluster, which they found not to be interacting with the cluster.

HCG 62

This group has been studied extensively, in radio, X-ray and optical bands. In the innermost regions, the emission shows traces of dynamical activity, although the overall shape is quite regular. We used only the newer two of the four available Chandra observations, because they are from a more recent different background epoch and together have  ~120   ks of exposure, which is more than sufficient for our analysis.

Using ASCA data, Fukazawa et al. (2001) and Nakazawa et al. (2007) reported an excess of hard emission in the spectra of this group. This hard excess could however not be confirmed by a more recent Suzaku observation (Tokoi et al. 2008).

Spavone et al. (2006) concentrated specifically on the brightest group member, NGC 4778, which shows distinct kinematical and morphological peculiarities, which they argued is an indication for a possible recent minor merger event. If this conclusion is correct, the group may still be dynamically disturbed, even though the emission appears to be spherical at large radii.

Two X-ray cavities in this group were studied by Morita et al. (2006), but found no clear explanation for their origin. These authors also found some evidence for a departure from hydrostatic equilibrium in this system. They used a triple β-model to describe the SBP.

Gu et al. (2007) identified an arc-shaped region spanning from south to northwest, with an average metal abundance about a factor of two higher than the surrounding regions. They concluded this may have been created by AGN activity, or by a recent merger, in accordance with Spavone et al.

Gitti et al. (2010) combined X-ray data from Chandra and XMM-Newton with GMRT radio data and detect low-frequency radio emission associated with the X-ray cavities.

IC 1262

This poor cluster appears quite regular and relaxed in the overall emission but shows some remarkable filamentary emission features close to the center which are clearly visible in the Chandra image.

Hudson et al. (2003) first reported a possible detection of a diffuse non-thermal component, measured with Beppo-SAX, which might also correspond to excess emission in radio. Hudson & Henriksen (2003) confirmed this observation with Chandra data, and concluded this cluster has recently merged with a smaller subclump, triggering the non-thermal emission by relativistic shocks.

On the other hand, Trinchieri et al. (2007) found the filamentary structures to be actually cooler than the surrounding regions, and rejected the shock front scenario proposed by Hudson & Henriksen. They suggested these structures may have been formed by ram pressure stripping of a nearby spiral galaxy or a past radio source that has since faded and is now only a relic.

IC 1633

This system is the hottest (T ~ 3   keV) and most massive (M₂₀₀ ~ 16  ×  ) object in the sample presented here.

IC 4296

This group is the faintest object in our sample, and in addition was observed with only half of the area of the CCD chips (“1/2 subarray mode”) to avoid pile-up, apparently because the observers were interested mostly in the giant radio-active elliptical galaxy at the center (Pellegrini et al. 2003). However, with this configuration large parts of the extended group emission are lost, so both observations essentially are useless for investigating the ICM. We nevertheless performed the analysis, but the object was an obvious outlier in all of the investigated scaling relations, with too high temperature and mass, clearly outside the normal scatter, so it was ultimately excluded from the fits, although it still appears in some of the tables.

MKW4

Using XMM-Newton observations, O’Sullivan et al. (2003) studied this poor HIFLUGCS cluster and found a central temperature gradient lacking strong cooling activity, as well as a sharp abundance peak at the cluster core, located at the position of the central cD galaxy NGC 4073. They argued that this system is close to establishing hydrostatic equilibrium and has developed a central temperature drop, but not yet a cooling flow.

Fukazawa et al. (2004) observed this group with Chandra and also found a central temperature drop and high central abundance. They attributed this high abundance to the weakness of the AGN in the cD galaxy, which allowed the ejected metals to remain near the cluster center.

MKW8

This poor cluster is the brightest object in our sample, as well as the second hottest. It is also included in the HIFLUGCS sample. The X-ray map is slightly asymmetric, and in the optical image there are two large galaxies visible at the center.

Hwang et al. (1999) investigated the properties of a small sample of groups and found MKW8 to be a dynamically complex system with a relatively low velocity dispersion.

NGC 326

The radio-active central galaxy of this system has an inversion symmetric (“Z”-shaped) morphology and has been studied in the radio regime by Murgia et al. (2001).

NGC 507

This rich group/poor cluster shows some interesting dynamical features, with quite irregular X-ray emission. The main X-ray peak is coincident with the NGC 507 galaxy, but the emission is extended in the north and east directions towards another large galaxy (NGC 508). In addition the cluster may be contaminated by or even interacting with the nearby group NGC 499, which is located only about 15   arcmin to the north of NGC 507, just outside the region observed with Chandra. This system is included in HIFLUGCS.

Paolillo et al. (2003) investigated this system and found two different components in the X-ray emission, one belonging to the halo core and another external, extended component. Accordingly, they used a bidimensional double β-model to fit the surface brightness distribution. Kraft et al. (2004) observed a sharp discontinuity in the surface brightness map and suggested this is caused by a radio lobe within the galaxy NGC 507, which is blowing higher abundance gas out into the group halo.

Recently, Sato et al. (2009) re-observed this group with Suzaku and created radial temperature and metallicity profiles, as well as several metal mass-to-light ratios and a 2-dimensional hardness map. They found it necessary to use a two-temperature model to fit the ICM spectra, however they saw no signs for a significant deviation from spherical symmetry.

NGC 1132

Mulchaey & Zabludoff (1999) have analyzed ASCA observations of the elliptical galaxy NGC 1132 and found an extended X-ray halo. They concluded this system is not an isolated galaxy but may be either the remnant of a poor group which has in the past merged into a single giant elliptical (fossil group) or has never managed to form a group due to a lack of other bright nearby galaxies (“failed” group).

NGC 1550

The X-ray emission of this system is slightly elliptical in the central regions. In the optical the group is dominated by the S0 galaxy NGC 1550.

Kawaharada et al. (2003) used ASCA data to estimate the mass of this group and, comparing this to the optical luminosity, found an unusually high mass-to-light ratio, which is closer to rich clusters than typical groups of similar mass. They proposed this system may be classified as a “dark group” with an especially low baryon content. A study with XMM-Newton data was also published by Kawaharada et al. (2009), where they investigated the metal distibution in the ICM and concluded that the group galaxies have in the past been merged into the cD galaxy, so NGC 1550 would qualify as a fossil group.

The temperature and entropy profiles of this group were investigated in detail by Sun et al. (2003), using Chandra observations. They found the temperature profile to be quite similar to those of other groups, indicating a universal profile. In the entropy profile they did not see evidence for the “entropy floor” predicted by simulations. Also, they studied the galactic composition of this system and concluded NGC 1550 is not a fossil group.

Sato et al. (2010) investigated the distribution of metals in this group using Suzaku data. They traced the abundances of several elements out to a radial region of  ~ 0.5   r₁₈₀, which would fit well the picture of a fossil group whose central dominant galaxy has enriched the ICM out to large radii.

NGC 4325

This group has been studied by Russell et al. (2007), who used both ROSAT and Chandra data. They found several indications for a disturbance in the group core and the surrounding ICM, e.g. X-ray cavities. They concluded that even though the group is radio-quiet, there is evidence for past AGN activity and, judging by the steep entropy profile, another outburst may be imminent.

NGC 5419 (Abell S753)

Subrahmanyan et al. (2003) investigated this poor cluster in the radio regime and presented several theories concerning the origin of the low surface brightness radio source found in this system. It may be a relic of activity in the dominant galaxy NGC 5419 or of another cluster galaxy, or may have been created as a lobe of a powerful radio galaxy, although there is no clear optical counterpart.

NGC 6269 (AWM5)

A small sub-peak is visible on the edge of the Chandra field of view which belongs to the group member NGC 6265, which appears to be moving through the ICM east of the cD galaxy NGC 6269.

Baldi et al. (2009) have performed a detailed study of this group and presented evidence for reheating of cool gas in the group core, probably due to past AGN activity. They also investigated the ram pressure stripped tail of NGC 6265 and estimated the mass of this galaxy.

NGC 6338

This system is very clearly in the process of merging, with a bright subclump just to the north of the main group center, which is localized at the position of the BCG. The subclump corresponds to a smaller galaxy apparently falling into the gravitational potential of the group.

NGC 6482

This group is both the coolest and least massive object in this sample. It was studied in detail by Khosroshahi et al. (2004), who found evidence this is a fossil group with short cooling times at the center but which nevertheless shows now sign of a central temperature drop.

RXC J1022

There is a secondary peak close to the group center, but no apparent change in the ICM surrounding it. This may either be an infalling group that has not yet effected on the ICM, or a chance alignment along the line of sight. There are currently no redshift measurements available to check the latter possibility. However in both cases it is advisable to remove this clump from the group emission, as it is not (or not yet) part of the main group’s gravitational potential, so we exclude this region from the analysis.

This is the most distant object in the sample presented here (z ~ 0.05).

RXC J2214 (3C 442A)

At the very center, this system shows signs of dynamical disturbance, where two bright galaxies (NGC 2736 and NGC 2737) are in the process of merging.

Worrall et al. (2007) and Hardcastle et al. (2007) have studied the interaction between the ICM and the radio lobes found in this system, which probably have been created by past activity of the radio galaxy 3C 442A. Both concluded that the ICM disturbed by the merger has a strong impact on the radio lobes, changing the dynamics and possibly even re-heating the radio plasma.

S0463

In this system there are two peaks of roughly the same size and luminosity, so it is not immediately clear which should be the emission peak. The clump in the south-western quadrant is more luminous, but the BCG lies at the position of the other one, so we assume this to be the peak of emission as it should correspond to the center of the group’s gravitational potential.

SS2S153 (NGC 3411, USGC152)

O’Sullivan et al. (2007) investigated this group in detail and reported a remarkable feature in the temperature profile. Although the overall X-ray emission is very regular and spherical, and the metallicity profile shows no unusual features, there appears to be a shell of cool gas surrounding the hotter group core. They concluded the most likely explanation is interplay between the cool core and a past episode of heating from the currently quiescent AGN.

Appendix B: Tables

Appendix C: Radial profiles

Left to right: temperature profile, metallicity profile, surface brightness profile.

Appendix D: Images

Left: optical images with X-ray contours. Right: exposure-corrected Chandra images, with point sources, smoothed with a 2 arcsec kernel.

© ESO, 2011