X-ray Bright Groups Research Articles

The group evolution multiwavelength study: the near-infrared luminosity function of nearby galaxy groups

We present J- and K-band luminosity functions (LFs) for the group evolution multiwavelength study (GEMS) sample of 60 nearby groups of galaxies, with photometry from the Two-Micron All Sky Survey (2MASS). We find that, as seen in B- and R-band photometry of a subsample of these groups in our earlier work, the LFs of the X-ray dim groups (L x < 10 41.7 erg s -1 ) show a depletion of galaxies of intermediate luminosity around M K = -23, within a radius of 0.3 R 500 from the centres of these groups. This feature is seen neither in the X-ray brighter groups nor in either kind of group when the LFs are determined all the way out to R 500 . We conclude that an enhanced level of star formation is not responsible for this feature. From the faint end of the LFs, we find support for the underabundance of low surface brightness dwarfs in the 2MASS. We find that for all kinds of groups, the modelling of the LF, with universal forms for the LFs of galaxies of different morphological types, fails when simultaneously required to fit the B- and K-band LFs. This means that the dip-like features seen in LFs are not merely due to the varying proportions of galaxies of different morphological types among the X-ray dim and bright groups. We argue that this supports our hypothesis that this feature is due to the enhanced merging of intermediate-mass galaxies in the dynamically sluggish environment of X-ray dim groups.

The Formation of Fossil Galaxy Groups in the Hierarchical Universe

We use a set of twelve high-resolution N-body/hydrodynamical simulations in the $\Lambda$CDM cosmology to investigate the origin and formation rate of fossil groups (FGs), which are X-ray bright galaxy groups dominated by a large elliptical galaxy, with the second brightest galaxy being at least two magnitudes fainter. The simulations invoke star formation, chemical evolution with non-instantaneous recycling, metal dependent radiative cooling, strong star burst driven galactic super winds, effects of a meta-galactic UV field and full stellar population synthesis. We find an interesting correlation between the magnitude gap between the first and second brightest galaxy and the formation time of the group. It is found that FGs have assembled half of their final dark matter mass already at $z\ga1$, and subsequently typically grow by minor merging only, wheras non-FGs on average form later. The early assembly of FGs leaves sufficient time for galaxies of $L \sim L_*$ to merge into the central one by dynamical friction, resulting in the large magnitude gap at $z=0$. A fraction of 33$\pm$16% of the groups simulated are found to be fossil, whereas the observational estimate is $\sim$10-20%. The FGs are found to be X-ray over-luminous relative to non-FGs of the same optical luminosity, in qualitative agreement with observations. Finally, from a dynamical friction analysis is found that only because infall of $L \sim L_*$ galaxies happens along filaments with small impact parameters do FGs exist at all.

X-ray bright groups and their galaxies

Combining X-ray data from the ROSAT PSPC and optical data drawn from the literature, we examine in detail the relationship between the X-ray and optical properties of X-ray bright galaxy groups. We find a relationship between optical luminosity and X-ray temperature consistent with that expected from self-similar scaling of galaxy systems, L_B \propto T^{1.6 +/- 0.2}. The self-similar form and continuity of the L_B : T relation from clusters to groups and the limited scatter seen in this relation, implies that the star formation efficiency is rather similar in all these systems. We find that the bright extended X-ray components associated with many central galaxies in groups appear to be more closely related to the group than the galaxy itself, and we suggest that these are group cooling flows rather than galaxy halos. In addition we find that the optical light in these groups appears to be more centrally concentrated than the light in clusters. We also use the optical and X-ray data to investigate whether early or late type galaxies are primarily responsible for preheating in groups. Using three different methods, we conclude that spiral galaxies appear to play a comparable role to early types in the preheating of the intragroup medium. This tends to favour models in which the preheating arises primarily from galaxy winds rather than AGN, and implies that spirals have played a significant role in the metal enrichment of the intragroup medium.

The morphology—density relation in X-ray-bright galaxy groups

We have examined the morphological make-up of X-ray bright groups. The brighter galaxies in these groups exhibit clear morphology-density and morphology-radius relations. The group morphology-density relation is offset from the cluster relation in the sense that at a given surface density, X-ray bright groups have a lower spiral fraction. After correcting for projection effects the morphology-3D density relation is still shifted towards fewer spirals for a given 3D density, in comparison with clusters. A simple model which corrects the group data for the effects of projection and for the expected higher merging rate in groups, brings the morphology-density relation into good agreement with that of clusters, suggesting that the relation may be driven by two-body interactions. The fraction of S0 galaxies in these X-ray bright groups is at least as high as that observed in nearby clusters. Given the low velocity dispersion of groups, this indicates that ram pressure stripping is not the dominant mechanism for S0 formation.

The Velocity Dispersion In The X-Ray Scaling Laws

We used a sample of 20 poor groups of galaxies to study the low mass tail of the relationships among the X-ray temperature T, the X-ray luminosity L x and the optical velocity dispersion σ. We obtained redshifts for the fainter members of these groups. We find that X-ray bright groups have more members and higher velocity dispersions on average. Using the fainter group members and Monte Carlo tests, we define the number of group members required to calculate a robust velocity dispersion. There is a tendency to underestimate the group velocity dispersion for samples of fewer than ∼ 10 members that introduces systematic errors in the slope of the relationships among L x , T, and σ. A comparison with the data of Ponman and Mahdavi shows that our improved velocity dispersions provide better constraints on the relationship between the kinematics of the hot gas and galaxies in these common environments.

Mass and Metallicity of Five X‐Ray–bright Galaxy Groups

We present ASCA X-ray observations of a sample of five groups selected from a cross-correlation of the ROSAT All-Sky Survey with the White et al. optical catalog of groups. These X-ray bright groups significantly increase the number of known systems with temperatures between 2 and 3 keV. They have element abundances of roughly 0.3 times solar that are typical of clusters, but their favored ratio of Si/Fe abundance is lower than the cluster value. Combining the ASCA results with ROSAT imaging data, we calculate total masses of a few to several times 10^13 M_sun, gas mass fractions of ~10%, and baryonic mass fractions of at least 15-20% within a radius of 0.5 Mpc. Upper limits for the ratios of gas to galaxy mass and of the iron mass to galaxy luminosity overlap with the range observed in rich clusters and extend to lower values, but not to such low values as seen in much poorer groups. These results support the idea that groups, unlike clusters, are subject to the loss of their primordial and processed gas, and show that this transition occurs at the mass scale of the 2-3 keV groups. A discussion of ASCA calibration issues and a comparison of ROSAT and ASCA temperatures are included in an Appendix.