While in the hierarchical model of structure formation, groups of galaxies are believed to be a scaled-down version of clusters of galaxies, a similarity break 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 (L_X), the temperature (T) of hot diffuse gas, and the velocity dispersion (σ) of galaxies for groups and clusters of galaxies, based on the largest sample of 66 groups and 274 clusters drawn from the literature. Our best-fit L_X-T and L_X-σ relations for groups are given by L_X ∝ T^{5.57 ± 1.79} ∝ σ^{2.35 ± 0.21}, which deviates remarkably from those for clusters: L_X ∝ T^{2.79 ± 0.08} ∝^{5.30 ± 0.21}. The significance of these correlations has been justified by both the co-consistency test and the Kendall τ statistics. We have thus confirmed the existence of a similarity break in the L_X-T and L_X-σ relations between groups and clusters as claimed in previous work, although the best-fit σ-T relations remain roughly the same in the two systems: σ ∝ T^0.64. Alternatively, the significant disagreement between the observationally fitted L_X-T and L_X-σ relations for groups and those expected from a perfect hydrostatic equilibrium hypothesis indicates that the X-ray emission of individual galaxies and the nongravitational 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.

Yan Jie Xue;向平 武

Astrophysical Journal

2000-7-20

The cusped Navarro, Frenk, & White (NFW) universal density profile suggested by typical cold dark matter (CDM) models has been challenged in recent years by the discovery of soft cores with finite central density for a broad range of masses from dwarf galaxies to clusters of galaxies. It is thus desirable that a new, analytic model should instead become available for virialized dark halos. One promising candidate is probably the empirical density profile proposed by Burkert, which resembles an isothermal profile with a constant core in the inner region and matches the NFW profile at large radii. Meanwhile, such a revised dark halo (RDH) profile has turned out to be a great success on galactic scales. This stimulates us to apply the RDH profile to more massive systems such as clusters of galaxies. In this paper we have made an attempt to derive the radial density profile of intracluster gas from the RDH profile under the isothermal and hydrostatic equilibrium hypotheses, and compare it with those revealed by X-ray observations and inferred from the NFW profile. We show that the RDH-predicted gas density can be well represented by the conventional β model with a typical β parameter of β ∼ 0.7-0.9. Alternatively, fitting the theoretically predicted X-ray surface brightness profile to an ensemble of 45 X-ray clusters observed by ROSAT, we find that the RDH and NFW profiles become almost indistinguishable from each other, and their characteristic density and scale length parameters are strongly correlated. Yet, unlike the NFW model, the RDH profile can allow us to work out straightforwardly the central dark matter density from X-ray measurements of the surface brightness and temperature of clusters. It appears that the resulting central densities of the 45 clusters have an average value of 〈p₀〉 ≈ 0.01 M_⊙ pc^-3, in agreement with the result estimated on galactic scales, which reinforces the claim for the presence of soft halo cores over the entire mass range.

向平 武;Yan Jie Xue

Astrophysical Journal

2000-10-20

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 (R_e). 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 L_b = 4.4_-1.4^+2.0 × 10 ³⁸ ergs s^-1. We identified 210 globular cluster (GC) candidates in a HST WFPC2 optical image of the galaxy's 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.

Yueheng Xu;Haiguang Xu;Zhongli Zhang;Arunav Kundu;Yu Wang;向平 武

Astrophysical Journal

2005-10-1

The energy source has remained to be the great mystery in understanding of the gamma-ray bursts (GRBs), if the events are placed at cosmological distances as indicated by a number of recent observations. The currently popular models include (1) the merger of two neutron stars or a neutron star and a black hole binary and (2) the hypernova scenario, i.e., the collapse of a massive member in a close binary. Since a neutron star will inevitably collapse into a black hole if its mass exceeds the limit M_max ≈ 3 M_⊙, releasing a total binding gravitational energy of ∼10⁵⁴ ergs, we explore semiempirically the possibility of attributing the energy source of GRBs to the accretion-induced collapse of a neutron star (AICNS) in a massive X-ray binary system consisting of a neutron star and a type O/B companion. This happens because a significant mass flow of ∼10^-3-10^-4 M_⊙ yr^-1 may be transferred onto the neutron star through the Roche-lobe overflow and primarily during the spiralin phase when it plunges into the envelope of the companion, which may eventually lead to the AICNS before the neutron star merges with the core of the companion. In this scenario, a "dirty" fireball with a moderate amount of beaming is naturally expected because of the nonuniformity of the stellar matter surrounding the explosion inside the companion, and a small fraction (∼0.1%) of the energy is sufficient to create the observed GRBs. In addition, the bulk of the ejecting matter of the companion star with a relatively slow expansion rate may act as the afterglow. Assuming a nonevolutionary model for galaxies, we estimate that the birthrate of the AICNS events is about two per day within a volume to redshift z = 1 for an Ω₀ = 1 universe, consistent with the reported GRB rate. It appears that the AICNS scenario, as a result of stellar evolution, may provide a natural explanation for the origin of GRBs and therefore deserves to be further investigated in the theoretical study of GRBs.

Bo Qin;向平 武;Ming Chung Chu;Li Zhi Fang;Jing Yao Hu

Astrophysical Journal

1998

We address the question of whether the angular power spectrum of the thermal Sunyaev-Zel'dovich (SZ) sky is further distorted by weak gravitational lensing of foreground large-scale structures. Using an analytic approach to both gaseous and dark halo models, we show that the contamination of weak lensing in the measurement of SZ power is negligibly small, and relative corrections |δC_l|/C_l are less than 3 per cent up to l = 10 ⁵. This arises from both the weaker gravitational potentials of low-redshift matter inhomogeneities that can act as lenses for SZ sources (clusters), and the shallower shape of the intrinsic SZ power spectrum at large l, in contrast to the cosmic microwave background which can be significantly affected by weak lensing because of the distant location and significant damping of its intrinsic power spectrum at small angular scales.

向平 武

Monthly Notices of the Royal Astronomical Society

2004-4-11

While X-ray measurements have so far revealed an increase in the volume-averaged baryon fractions f_b(r) of galaxy clusters with cluster radii r, f_b(r) should asymptotically reach a universal value f_b(∞) = f_b, provided that clusters are representative of the Universe. In the framework of hydrostatic equilibrium for intracluster gas, we have derived the necessary conditions for f_b(∞) = f_b. The X-ray surface brightness profile described by the β model and the temperature profile approximated by the polytropic model should satisfy γ ≈ 2(1 - 1/3β) and γ ≈ 1 + 1/3β for β < 1 and β > 1, respectively, which sets a stringent limit to the polytropic index: γ < 4/3. In particular, a mildly increasing temperature with radius is required if the observationally fitted β parameter is in the range 1/3 < β < 2/3. It is likely that a reliable determination of the universal baryon fraction can be achieved in the small β clusters because the disagreement between the exact and asymptotic baryon fractions for clusters with β > 2/3 breaks down at rather large radii (≳30r_c) where hydrostatic equilibrium has probably become inapplicable. We further explore how to obtain the asymptotic value f_b(∞) of the baryon fraction from the X-ray measurement made primarily over the finite central region of a cluster. We demonstrate our method using a sample of 19 strong lensing clusters, which enables us to place a useful constraint on f_b(∞): 0.094 ± 0.035 ≤ f_b(∞) ≤ 0.41 ± 0.18, corresponding to a cosmological density parameter 0.122 ± 0.069 ≤ Ω_M ≤ 0.53 ± 0.28 for H₀ = 50 km s^-1 Mpc^-1. An optimal estimate of f_b(∞) based on three cooling flow clusters with β < 1/2 in our lensing cluster sample yields 〈f_b(∞)〉 = 0.142 ± 0.007 or Ω_M = 0.35 ± 0.09.

向平 武;Yan Jie Xue

Monthly Notices of the Royal Astronomical Society

2000-2-1

It is widely believed that the global baryon content and the mass-to-light ratio of groups and clusters of galaxies are fair representatives of the matter mix of the universe and therefore can be used to reliably determine the cosmic mass density parameter Ω_M. However, this fundamental assumption is challenged by growing evidence from optical and X-ray observations that the average gas mass fraction and mass-to-light ratio increase mildly with scale from poor groups to rich clusters. Although a number of time-consuming hydrodynamical simulations combined with semianalytic approaches have been carried out that permit a sophisticated treatment of some complicated processes in the formation and evolution of cosmic structures, the essential physics behind the phenomenon still remains a subject of intense debate. In this Letter, using a simple analytic model, we show that radiative cooling of the hot intragroup/intracluster gas may allow one to reproduce the observed scale dependence of the global stellar and gas mass fractions and mass-to-light ratio of groups and clusters, provided that about half of the cooled gas is converted into stars. Together with the recent success in the recovery of the entropy excess and the steepening of the X-ray luminosity-temperature relations detected in groups and clusters, radiative cooling provides a simple, unified scheme for the evolution of hot gas and the formation of stars in the largest virialized systems of the universe.

向平 武;Yan Jie Xue

Astrophysical Journal

2002-6-10

Heavy particles in galaxy clusters tend to be more centrally concentrated than light ones according to the Boltzmann distribution. An estimate of the drift velocity suggests that it is possible that the helium nuclei may have entirely or partially sedimented into the cluster core within the Hubble time. We demonstrate this scenario using the Navarro-Frenk-White profile as the dark matter distribution of clusters and assuming that the intracluster gas is isothermal and in hydrostatic equilibrium. We find that a greater fraction of baryonic matter is distributed at small radii than at large radii, which challenges the prevailing claim that the baryon fraction increases monotonically with cluster radius. It shows that the conventional mass estimate using X-ray measurements of intracluster gas along with a constant mean molecular weight may have underestimated the total cluster mass by ∼20%, which in turn leads to an overestimate of the total baryon fraction by the same percentage. Additionally, it is pointed out that the sedimentation of helium nuclei toward cluster cores may at least partially account for the sharp peaks in the central X-ray emissions observed in some clusters.

Bo Qin;向平 武

Astrophysical Journal

2000-1-20

In this Letter we present the radial temperature profiles of three X-ray clusters (A119, A2255 and A2256) reconstructed from a combination of the X-ray surface brightness measurements and the universal density profile as the underlying dark matter distribution. Our algorithm is based on the hydrostatic equilibrium for intracluster gas and the universality of the total baryon fraction within the virial radius. The scaled temperature profiles of these three clusters appear to be remarkably similar in shape, reflecting the underlying structural regularity, although they are inconsistent with either isothermality or a significant decline with increasing radius. Nevertheless, we find a good agreement between our derived temperature profiles and the recent analysis of 11 clusters observed with BeppoSAX (Irwin & Bregman 2000), which provides a useful clue to resolving the temperature profile discrepancy raised recently in literature. A comparison of our derived temperature profiles with future spatially-resolved spectral measurements may constitute a critical test for the standard model of structure formation and the conventional scenario for dynamical properties of clusters.

Y. J. Xue;向平 武

Astronomy and Astrophysics

2000

By analyzing the gas temperature maps created from the Chandra archive data, we reveal the prevailing existence of temperature substructures on ∼ 100 h ^-1 ₇₀kpc 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.

Liyi Gu;Haiguang Xu;Junhua Gu;Yu Wang;Zhongli Zhang;Jingying Wang;Zhenzhen Qin;Haijuan Cui;向平 武

Astrophysical Journal

2009