Haiguang Xu

High-Resolution Observations of the Elliptical Galaxy NGC 4636 with the Reflection Grating Spectrometer on Board XMM-Newton

We present the first high spectral resolution X-ray observation of the giant elliptical galaxy NGC 4636, obtained with the reflection grating spectrometer on board the XMM-Newton Observatory. The resulting spectrum contains a wealth of emission lines from various charge states of oxygen, neon, magnesium, and iron. Examination of the cross-dispersion profiles of several of these lines provides clear, unambiguous evidence of resonance scattering by the highest oscillator strength lines, as well as a weak temperature gradient in the inner regions of the interstellar medium. We invoke a sophisticated new Monte Carlo technique that allows us to properly account for these effects in performing quantitative fits to the spectrum. Our spectral fits are not subject to many of the systematics that have plagued earlier investigations. The derived metal abundances are higher than have usually been inferred from prior, lower spectral resolution observations of this source, but are still incompatible with conventional chemical-enrichment models of elliptical galaxies. In addition, our data are incompatible with standard cooling-flow models for this system; our derived upper limit to the mass deposition rate is below the predicted value by a factor of 3-5.

ASCA Measurements of Silicon and Iron Abundances in the Intracluster Medium

We analyzed the ASCA X-ray data of 40 nearby Clusters of galaxies, whose intracluster-medium temperature distributes in the range of 0.9-10 keV. We measured the Si and Fe abundances of the intracluster medium, spatially averaging over each Cluster, but excluding the central ~ 0.15/I^Q1 Mpc region in order to avoid any possible abundance gradients and complex temperature structures. The Fe abundances of these Clusters are 0.2-0.3 solar, with only weak dependence on the temperature of the intracluster medium, hence on the Cluster richness. In contrast, the Si abundance is observed to increase from 0.3 to 0.6-0.7 solar from the poorer to richer Clusters. These results suggest that the Supernovae of both type-Ia and type-II significantly contribute to the metal enrichment of the intracluster medium, with the relative contribution of type-II Supernovae increasing towards richer Clusters. We suggest a possibility that a considerable fraction of type-II Supernova products escaped from poorer Systems.

X-Ray Probing of the Central Regions of Clusters of Galaxies

The results on ASCA X-ray study of the central regions of medium-richness clusters of galaxies are summarized, while emphasizing the differences between cD and non-cD clusters. The intra-cluster medium (ICM) is likely to consist of two (hot and cool) phases within ∼ 100kpc of a cD galaxy, where the ICM metallicity is also enhanced. In contrast, the ICM in non-cD clusters appears to be isothermal with a small metallicity gradient right of the center. The gravitational potential exhibits a hierarchical nesting around cD galaxies, while a total mass-density profile with a central cusp is indicated for a non-cD cluster, Abell 1060. The iron-mass-to-light ratio of the ICM decreases toward the center in both types of clusters, although it is radially constant in peripheral regions. The silicon-toiron abundance ratio in the ICM increases with the cluster richness, but remains close to the solar ratio around cD galaxies. These overall results are interpreted without appealing to the popular cooling-flow hypothesis. Instead, an emphasis is put on the halo-in-halo structure formed around cD galaxies.

Discovery of a Large-scale Abundance Gradient in the Cluster of Galaxies AWM 7 with ASCA

A large-scale gradient in the metal abundance has been detected with ASCA from an X-ray bright cluster of galaxies, AWM 7. The metal abundance shows a peak of 0.5 solar at the center and smoothly declines to 0.2 solar at a radius of 500 kpc. The gas temperature is found to be constant at 3.8 keV. The radial distribution of iron can be fitted with a β model with β ~ 0.8 assuming the same core radius (115 kpc) as that of the intracluster medium. The metal distribution in AWM 7 suggests that the gas injected from galaxies is not efficiently mixed in the cluster space and traces the distribution of galaxies.

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.

Three-Layered Atmospheric Structure in Accretion Disks Around Stellar-Mass Black Holes

Modeling of the x-ray spectra of the Galactic superluminal jet sources GRS 1915+105 and GRO J1655-40 reveals a three-layered atmospheric structure in the inner region of their accretion disks. Above the cold and optically thick disk with a temperature of 0.2 to 0.5 kiloelectron volts, there is a warm layer with a temperature of 1.0 to 1.5 kiloelectron volts and an optical depth around 10. Sometimes there is also a much hotter, optically thin corona above the warm layer, with a temperature of 100 kiloelectron volts or higher and an optical depth around unity. The structural similarity between the accretion disks and the solar atmosphere suggests that similar physical processes may be operating in these different systems.

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.

THE NEXT GENERATION VIRGO CLUSTER SURVEY. X. PROPERTIES OF ULTRA-COMPACT DWARFS IN THE M87, M49, AND M60 REGIONS

We use imaging from the Next Generation Virgo cluster Survey (NGVS) to present a comparative study of ultra-compact dwarf (UCD) galaxies associated with three prominent Virgo sub-clusters: those centered on the massive red-sequence galaxies M87, M49, and M60. We show how UCDs can be selected with high completeness using a combination of half-light radius and location in color–color diagrams (u^*iK_s or u^*gz). Although the central galaxies in each of these sub-clusters have nearly identical luminosities and stellar masses, we find large differences in the sizes of their UCD populations, with M87 containing ~3.5 and 7.8 times more UCDs than M49 and M60, respectively. The relative abundance of UCDs in the three regions scales in proportion to sub-cluster mass, as traced by X-ray gas mass, total gravitating mass, number of globular clusters (GCs), and number of nearby galaxies. We find that the UCDs are predominantly blue in color, with ~85% of the UCDs having colors similar to blue GCs and stellar nuclei of dwarf galaxies. We present evidence that UCDs surrounding M87 and M49 may follow a morphological sequence ordered by the prominence of their outer, low surface brightness envelope, ultimately merging with the sequence of nucleated low-mass galaxies, and that envelope prominence correlates with distance from either galaxy. Our analysis provides evidence that tidal stripping of nucleated galaxies is an important process in the formation of UCDs.

Probing the Mass Distributions in NGC 1407 and Its Associated Group with the X-Ray Imaging Spectroscopic and Optical Photometric and Line-Strength Indices Data

We have studied the mass distributions in the bright E0 galaxy NGC 1407 and its associated group by analyzing Chandra and ROSAT data. To probe the stellar mass distribution, we calculated the stellar mass-to-light ratios by comparing the observed line-strength indices and multicolor photometric data with different stellar synthesis models. We find that the gas is single phase, with a temperature of similar or equal to 0.7 keV within 1R(e). Outside 1Re, the gas temperature increases quickly outward to > 1 keV, indicating its group origin. We reveal that the X-ray surface brightness profile shows a central excess in the innermost region, and on both the total mass and dark matter profiles there is a flattened feature at about less than or similar to 1R(e), where the gas temperature increases rapidly. This may be a mark of the boundary between the galaxy and group halos. The total mass and dark matter distributions within 0.85R(e) are cuspy and can be approximated by power-law profiles with indices of similar or equal to 2, which are marginally consistent with the generalized NFW profiles with zeta = 2. The mass in outer regions can be well fitted by a single NFW profile, and the derived concentration parameter c (18.6 +/- 1.5) is larger than the 68% upper limit for a halo at z = 0 with the given M-vir. We find that the NGC 1407 group has a baryon-dominated core, while the mass in the > 1R(e) region is dominated by dark matter. At the virial radius r(200) 572 +/- 118 kpc, the inferred mass andmass-to-light ratio are M-200 2.20 +/- 0.42; 10(13) M circle dot and M-vir/L-B 311 +/- 60 M circle dot/L-B, circle dot, respectively, showing that the NGC 1407 group is an extremely dark system, similar to many clusters of galaxies.

A CHANDRA STUDY OF TEMPERATURE SUBSTRUCTURES IN INTERMEDIATE-REDSHIFT GALAXY CLUSTERS

By analyzing the gas temperature maps created from the Chandra archive data, we reveal the prevailing existence of temperature substructures on ~100 h –1 70 kpc scales in the central regions of nine intermediate-redshift (z ≈ 0.1) galaxy clusters, which resemble those found in the Virgo and Coma Clusters. Each substructure contains a clump of hot plasma whose temperature is about 2-3 keV higher than the environment, corresponding to an excess thermal energy of ~1058-1060 erg per clump. If there were no significant nongravitational heating sources, these substructures would have perished in 108-109 yr due to thermal conduction and turbulent flows, whose velocity is found to range from about 200 to 400 km s–1, we conclude that the substructures cannot be created and sustained by inhomogeneous radiative cooling. We also eliminate the possibilities that the temperature substructures are caused by supernova explosions, or by the nonthermal X-ray emission due to the inverse-Comptonization of the cosmic microwave background photons. By calculating the rising time of active galactic nucleus (AGN)-induced buoyant bubbles, we speculate that the intermittent AGN outbursts (≥1060 erg per burst) may have played a crucial role in the formation of the high-temperature substructures. Our results are supported by the recent study of McNamara and Nulsen, posing a tight observational constraint on future theoretical and numerical studies.