Xiang-Ping Wu

The LX-T and LX-σ Relationships for Galaxy Clusters Revisited

The relationships between the X-ray-determined bolometric luminosity, LX, the temperature, T, of the intracluster gas, and the optical measured velocity dispersion, σ, of the cluster galaxies are updated for galaxy clusters using the largest sample of 256 clusters drawn from literature. The newly established relationships, based on the doubly weighted orthogonal distance regression (ODR) method, are justified by both their self-consistency and co-consistency, which can then be used to test the theoretical models of cluster formation and evolution. The observationally determined LX-T and LX-σ relationships, LX ∝ T2.72±0.05 ∝ σ5.24±0.29, are marginally consistent with those predicted by the scenario in which both intracluster gas and galaxies are in isothermal and hydrostatic equilibrium with the underlying gravitational potential of clusters. A comparison between these observed and predicted LX-T relationships also suggests that the mean cluster baryon fraction, fb, remains approximately constant among different clusters, fb ≈ 0.17, which gives rise to a low mass density universe of Ωm ≈ 0.3.

The LX-T, LX-σ, and σ-T Relations for Groups and Clusters of Galaxies

While in the hierarchical model of structure formation, groups of galaxies are believed to be the scaled-down version of clusters of galaxies, a similarity breaking in the fundamental laws may occur on the group scale, reflecting a transition between galaxy-dominated and intracluster medium dominated properties. In this paper, we present an extensive study of the relations between the X-ray luminosity (Lx), the temperature (T) of hot diffuse gas and the velocity dispersion (sigma) of galaxies for groups and clusters of galaxies, based on the largest sample of 66 groups and 274 clusters drawn from literature. Our best fit Lx-T and Lx-sigma relations for groups read Lx propto T^(5.57+-1.79) propto sigma^(2.35+-0.21), which deviates remarkably from those for clusters: Lx propto T^(2.79+-0.08) propto sigma^(5.30+-0.21). The significance of these correlations have been justified by both the co-consistency test and the Kendalls tau statistics. We have thus confirmed the existence of similarity breaking in the Lx-T and Lx-sigma relations between groups and clusters as claimed in previous work, although the best fit sigma-T relations remain roughly the same in both systems: sigma propto T^0.64. Alternatively, the significant disagreement between the observationally fitted Lx-T and Lx-sigma relations for groups and those expected from a perfect hydrostatic equilibrium hypothesis indicates that the X-ray emission of individual galaxies and the non-gravitational heating must play a potentially important role in the dynamical evolution of groups. Therefore, reasonable caution should be exercised in the cosmological applications of the dynamical properties of groups.

Cosmic-ray protons and magnetic fields in clusters of galaxies and their cosmological consequences

The masses of clusters of galaxies estimated by gravitational lensing exceed in many cases the mass estimates based on hydrostatic equilibrium. This may suggest the existence of nonthermal pressure. We ask if radio galaxies can heat and support the cluster gas with injected cosmic-ray protons and magnetic field densities, which are permitted by Faraday rotation and gamma-ray observations of clusters of galaxies. We conclude that they are powerful enough to do this within a cluster radius of roughly 1 Mpc. If present, nonthermal pressures could lead to a revised estimate of the ratio of baryonic mass to total mass, and the apparent baryonic overdensity in clusters would disappear. In consequence, Ωcold, the clumping part of the cosmological density Ω0, would be larger than 0.4 h -->−1/250.

A comparison of different cluster mass estimates: consistency or discrepancy?

Rich and massive clusters of galaxies at intermediate redshift are capable of magnifying and distorting the images of background galaxies. A comparison of different mass estimators among these clusters can provide useful information about the distribution and composition of cluster matter and its dynamical evolution. Using the hitherto largest sample of lensing clusters drawn from the literature, we compare the gravitating masses of clusters derived from the strong/weak gravitational lensing phenomena, from the X-ray measurements based on the assumption of hydrostatic equilibrium, and from the conventional isothermal sphere model for the dark matter profile characterized by the velocity dispersion and core radius of galaxy distributions in clusters. While there is excellent agreement between the weak lensing, X-ray and isothermal sphere model-determined cluster masses, these methods are likely to underestimate the gravitating masses enclosed within the central cores of clusters by a factor of 2–4 as compared with the strong lensing results. Such a mass discrepancy has probably arisen from the inappropriate applications of the weak lensing technique and the hydrostatic equilibrium hypothesis to the central regions of clusters, as well as from assuming an unreasonably large core radius for both luminous and dark matter profiles. Nevertheless, it is pointed out that these cluster mass estimators may be safely applied on scales greater than the core sizes. Namely, the overall clusters of galaxies at intermediate redshift can still be regarded as the dynamically relaxed systems, in which the velocity dispersion of galaxies and the temperature of X-ray emitting gas are good indicators of the underlying gravitational potentials of clusters.

The mass-temperature relation of 22 nearby clusters

We present a new investigation of the mass-temperature (Mtot-TX) relation of 22 nearby clusters based on the analysis of their ROSAT X-ray surface brightness profiles (SX ) and their ASCA emission-weighted temperatures. Two methods of the cluster mass estimations are employed and their results are compared: (1) the conventional β-model for gas distribution along with the isothermal and hydrostatic equilibrium assumptions, and (2) the NFW profile for dark matter distribution whose characteristic density and length are determined by the observed SX(r). These two models yield essentially the same goodness of fits for SX(r) and the similar Mtot-TX relations, with the latter demonstrating a significant departure from the simple gravitational scaling of Mtot ∝ T. It is also shown that the best-fit Mtot-TX relations could be reconciled with the theoretical expectation if the low-temperature clusters (TX < 3.5 keV) are excluded from the list, which lends support to the scenario that the intracluster medium is preheated in the early phase of cluster formation. Together with the entropy-temperature distribution, the existence of a similarity break at TX = 3-4 keV in the dynamical scaling relations for galaxy clusters has been confirmed.

A Statistical Comparison of Cluster Mass Estimates from Optical/X-Ray Observations and Gravitational Lensing

We present a statistical comparison of three different estimates of cluster mass, namely, the dynamical masses obtained from the velocity dispersion of optical galaxies, the X-ray masses measured from the temperature of X-ray-emitting gas under the assumption of isothermal hydrostatic equilibrium, and the gravitational lensing masses derived from the strong/weak distortions of background galaxy images. Using a sample of 29 lensing clusters available in literature, we show that the dynamical masses are in agreement with the gravitational lensing masses while the X-ray method has systematically underestimated cluster masses by a factor of 2-3 as compared with the others. These results imply that galaxies indeed trace the gravitational potential of their clusters and that there is no bias between the velocities of the dark matter particles and the galaxies in clusters. The X-ray cluster mass discrepancy probably arises from the simplification in the models for the X-ray gas distribution and dynamical evolution.

A combined analysis of cluster mass estimates from strong lensing, X-ray measurement and the universal density profile

(Abridged)We present a combined analysis of mass estimates in the central cores of galaxy clusters from the strong lensing, the X-ray measurements and the universal density profile (NFW). Special attention is paid to the questions (1)whether the previously claimed mass discrepancy between the strong lensing and X-ray measurements is associated with the presence of cooling/non-cooling flows, (2)whether the cusped NFW density model can provide a consistent cluster mass with the strong lensing result and (3)whether a non-zero cosmological constant can be of any help to reducing the strong lensing - X-ray mass ratios. We analyse a sample of 26 arc-like images among 21 clusters. The X-ray and NFW cluster masses are obtained by assuming that the intracluster gas is isothermal and in hydrostatic equilibrium with the underlying gravitational potential of the clusters. A statistical comparison of these three mass estimates reveals that the mass discrepancies for all the events are well within a factor of 2. We confirm the result of Allen that the larger mass discrepancy is only detected in the intermediate cooling, especially non-cooling flow clusters, thus attributing the mass discrepancy to the local dynamical activities in the central regions. We show that the NFW profile yields a consistent cluster mass with the conventional X-ray measurement, and any difference between these two models must occur at even smaller radii (e.g. within the arc-like images) or at large radii. Finally, a non-zero cosmological constant is able to moderately reduce the mass ratios of m_lens to m_xray.

Chandra X-Ray Observatory Observation of A1689: New Determination of Mass Distribution and Comparison to Lensing Measurements

We present a new estimate of the projected X-ray mass of A1689 observed with Chandra X-Ray Observatory, in an attempt to clarify the issue of whether or not there exists a discrepancy between X-ray and gravitational lensing mass estimates claimed in previous investigations based on Einstein, ROSAT, and ASCA observations. Particular attention is paid to examining whether there is an offset between the X-ray centroid and the central dominant cD galaxy, which may be an indicator of the presence of local dynamical activities of intracluster gas in the central core and therefore explain the discrepancy between X-ray and lensing mass estimates, if any. The unprecedentedly high spatial resolution achieved by Chandra allows us to precisely localize the X-ray centroid of A1689, which appears to coincide perfectly with the central cD galaxy. This fact, along with the symmetry and regularity of the X-ray surface brightness and temperature distributions, suggests that A1689 is a fully relaxed cluster. We thus employ the hydrostatic equilibrium hypothesis to determine the projected mass profile of A1689 and compare it with the results obtained by different lensing techniques available in the literature. Our analysis confirms the existence of a discrepancy of a factor of ~2 between X-ray and lensing mass estimates in the central region of r ≈ 0.2 Mpc, although the two methods yield essentially consistent result on large radii. If the perfect coincidence between the X-ray center and the cD galaxy of A1689 detected by Chandra observation is not a projection effect, the central mass discrepancy between X-ray and lensing measurements may pose a challenge to our conventional understanding of the dynamical evolution of the intracluster gas in the central regions of clusters.

Reexamination of the Galaxy Formation-regulated Gas Evolution Model in Groups and Clusters

As an alternative explanation of the entropy excess and the steepening of the X-ray luminosity-temperature relation in groups and clusters, the galaxy formation-regulated gas evolution (GG) model proposed recently by Bryan makes an attempt to incorporate the formation of galaxies into the evolution of gas without additional heating by nongravitational processes. This seems to provide a unified scheme for our understanding of the structures and evolution of both galaxies and gas in groups and clusters. In this paper, we present an extensive comparison of the X-ray properties of groups and clusters predicted by the GG model and those revealed by current X-ray observations, using various large data sources in the literature and also taking the observational selection effects into account. These include an independent check of the fundamental working hypothesis of the GG model (i.e., galaxy formation was less efficient in rich clusters than in groups), a new test of the radial gas distributions revealed by both the gas mass fraction and the X-ray surface brightness profiles, and a reexamination of the X-ray luminosity-temperature and entropy-temperature relations. In particular, it shows that the overall X-ray surface brightness profiles predicted by the GG model are very similar in shape and insensitive to the X-ray temperature and that the shallower X-ray surface brightness profiles seen at low-temperature systems may arise from the current observational selection effect. This can be used as the simplest approach to distinguishing between the GG model and the preheating scenario. The latter yields an intrinsically shallower gas distribution in groups than in rich clusters.

CHANDRA STUDY OF X-RAY POINT SOURCES IN THE EARLY-TYPE GALAXY NGC 4552 (M89)

We present a Chandra ACIS study of the early-type galaxy NGC 4552. We detect 47 X-ray point sources, most of which are likely low-mass X-ray binaries (LMXBs), within four effective radii (Re). The brightest X-ray source coincides with the optical, UV, and radio center of the galaxy and shows variability on >1 hr timescales, indicating the possible existence of a low-luminosity active galactic nucleus (AGN). The 46 off-center sources and the unresolved point sources contribute about 29% and 20% to the total luminosity of the galaxy, respectively. We find that after correcting for the incompleteness at the low-luminosity end, the observed cumulative X-ray luminosity function (XLF) of the off-center sources is best fitted by a broken power-law model with a break at Lb = 4.4 × 1038 ergs s-1. We identified 210 globular cluster (GC) candidates in a HST WFPC2 optical image of the galaxys central region. Of the 25 off-center LMXBs that fall within the WFPC2 field of view, 10 sources are coincident with a GC. Thus, the fraction of the GCs hosting bright LMXBs and the fraction of the LMXBs associated with GCs are 4.8% and 40%, respectively. In the V and I bands, the GCs hosting bright LMXBs are typically 1-2 mag brighter than the GCs with no detected LMXBs. There are about 1.9 ± 0.4 times as many LMXBs in the red, metal-rich GCs as there are in the blue, metal-poor ones. We find no obvious difference between the luminosity distributions of LMXBs in GCs and in the field, but the cumulative spectrum of the LMXBs in GCs tends to be softer than that of the LMXBs in field. We detected three X-ray sources that have isotropic luminosities larger than 1039 ergs s-1. Only one of these is located in the joint Chandra-HST field and is found to be associated with a GC. By studying its ACIS spectra we infer that the this may be a candidate black hole system with a mass of 15-135 M☉. One of the other sources with a luminosity brighter than 1039 ergs s-1 reveals temporal variations in brightness on timescales greater than 1 hr.