arXiv subjects

Xiang-Ping Wu

Publications and source records attributed to Xiang-Ping Wu.

At least 55 records · Page 3Linked to original sources

arXiv2002

Chandra observation of Abell 1689: New determination of mass distribution and comparison to lensing measurements

We present a new estimate of the projected X-ray mass of Abell 1689 observed with Chandra, 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 {\it Einstein}, {\it ROSAT} and {\it ASCA} observations. A particular attention is paid to examining if there is an offset between X-ray centroid and 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 Abell 1689, 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 Abell 1689 is a fully-relaxed cluster. We thus employ hydrostatic equilibrium hypothesis to determine the projected mass profile of Abell 1689, and compare it with the results obtained by different lensing techniques available in the literature. Our analysis confirms the existance of the discrepancy of a factor of $\sim2$ between X-ray and lensing mass estimates in the central region of $r\approx0.2$ 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 Abell 1689 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 dynamical evolution of the intracluster gas in the central regions of clusters.

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arXiv2002

Radio Confusion on Cluster Sunyaev-Zel'dovich Effect

We examine the expected radio confusion on the thermal Sunyaev-Zel'dovich (SZ) effect of galaxy clusters at 28.5 GHz and 90 GHz based on the cluster radio luminosity function (CRLF) at 1.4 GHz. With the observationally determined spectral index ($α$, where $S_{\rmν}\proptoν^{-α}$) distribution, instead of a single average index $<α>$, we convert the cluster radio luminosity function at 1.4 GHz to the high frequency ones and estimate the total radio flux in a cluster. At 28.5 GHz, radio confusion is up to 10~100% for small clusters ($M\sim 2\times 10^{14}M_{\odot}$) below redshift z=1, with more severe confusion for smaller and lower redshift clusters. By contrast, at 90 GHz the confusion is less than 10% for small clusters even at low redshifts.

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arXiv2001

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 an 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, insensitive to the X-ray temperature, and 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.

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arXiv2001

Constraints on the Interaction between Dark Matter and Baryons from Cooling Flow Clusters

Other nongravitational heating processes are needed to resolve the disagreement between the absence of cool gas components in the centers of galaxy clusters revealed recently by Chandra and XMM observations and the expectations of conventional radiative cooling models. Here we propose that the interaction between dark matter particles and ordinary baryonic matter may act as an alternative for the reheating of intracluster medium (ICM) in the inner regions of clusters, in which kinetic energy of dark matter is transported to ICM to balance the radiative cooling. Using the Chandra and XMM data of typical clusters, we set a useful constraint on the dark matter-baryon cross-section: $σ_{xp}/m_x \sim 1 \times 10^{-25}$ cm$^2$GeV$^{-1}$, where $m_x$ is the mass of dark matter particles.

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arXiv2001

The X-ray background from groups and clusters of galaxies

We present an estimate of the X-ray background (XRB) spectrum from the warm-hot intergalactic medium (IGM) associated with groups and clusters of galaxies, using purely the observationally determined X-ray luminosity function (XLF) and X-ray luminosity (Lx) - temperature (T) relations for groups and clusters. As compared with previous semi-analytic models based on the Press-Schechter mass function, our approach provides a much simpler and more realistic way to evaluate the XRB from groups and clusters in the sense that we make no assumption about the dynamical and heating properties of the IGM, and the intrinsic dispersion in the Lx-T relations due to different physical mechanisms among different groups and clusters can be also included. It shows that the resulting 0.1-10 keV XRB spectrum by summing up the X-ray emission from all groups and clusters is consistent with current upper limits placed on the contribution from diffuse gas to the XRB. This may have profound implications for our understanding of the missing baryons in the universe.

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arXiv2001

Observational tests of the delta_c-M_vir relation in hierarchical clustering models

Observational determinations of the correlation between the characteristic density delta_c and the virial mass M_vir of dark halos constitute a critical test for models of hierarchical structure formation. Using the dynamical properties of dark halos reconstructed from the galaxy distributions in massive systems (groups/clusters) and the rotation curves in less massive systems (dwarf, low surface brightness and spiral galaxies) drawn from the literature, we confirm the existence of the delta_c-M_vir relation over a broad mass range from 10**10 to 10**15 solar masses, which is in gross consistency with the prediction of a flat cosmological model with Omega_M=0.3 and Omega_Lambda=0.7. It is pointed out that previous analyses based on the measurements of X-ray emitting gas and the hydrostatic equilibrium hypothesis, which claimed a shallower scale-free spectrum of n=-1 for initial density fluctuations, may suffer from nongravitational heating influence especially in low-mass systems.

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arXiv2001

The Mass-Temperature Relation of 22 Nearby Clusters

We present a new investigation of the mass-temperature (\MTrelation) relation of 22 nearby clusters based on the analysis of their ROSAT X-ray surface brightness profiles ($S_{X}(r)$) 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 $S_{X}(r)$. These two models yield essentially the same goodness of fits for $S_{X}(r)$ and the similar \MTrelation relations, with the latter demonstrating a significant departure from the simple gravitational scaling of $M_{tot}\propto T_X^{3/2}$. It is also shown that the best-fit \MTrelation relations could be reconciled with the theoretical expectation if the low-temperature clusters ($T_X<3.5$ keV) are excluded from the list, which lends support to the scenario that the intra-cluster medium is preheated in the early phase of cluster formation. Together with the entropy-temperature distribution, the existence of a similarity break at $T_X=3-4$ keV in the dynamical scaling relations for galaxy clusters has been confirmed.

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arXiv2001

Constraints on the warm dark matter model from gravitational lensing

Formation of sub-galactic halos is suppressed in warm dark matter (WDM) model due to thermal motion of WDM particles. This may provide a natural resolution to some puzzles in standard cold dark matter (CDM) theory such as the cusped density profiles of virialized dark halos and the overabundance of low mass satellites. One of the observational tests of the WDM model is to measure the gravitationally lensed images of distant quasars below sub-arcsecond scales. In this Letter, we report a comparison of the lensing probabilities of multiple images between CDM and WDM models using a singular isothermal sphere model for the mass density profiles of dark halos and the Press-Schechter mass function for their distribution and cosmic evolution. It is shown that the differential probability of multiple images with small angular separations down to 10 milliarcseconds should allow one to set useful constraints on the WDM particle mass. We discuss briefly the feasibility and uncertainties of this method in future radio surveys (e.g. VLBI) for gravitational lensing.

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arXiv2001

Expectations for SZ cluster counts: mass function versus X-ray luminosity function

We present a comparison of the SZ cluster counts predicted by the Press-Schechter (PS) mass function (MF) and the X-ray luminosity function (XLF) of clusters. The employment of the cluster XLF, together with the observationally determined X-ray luminosity(Lx)-temperature(T) relation, may allow us to estimate the SZ cluster counts in a more realistic manner, although such an empirical approach depends sensitively on our current knowledge of the dynamical properties of intracluster gas and its cosmic evolution. Using both the non-evolving and evolving XLFs of clusters suggested by X-ray observations, we calculate the expectations for SZ surveys of clusters with X-ray luminosity Lx>3X10^{44} erg/s and Lx>1X10^{43} erg/s in the 0.5 - 2.0 band, respectively. The non-evolving XLF results in a significant excess of SZ cluster counts at high redshifts as compared with the evolving XLF, while a slightly steeper Lx-T relation than the observed one is needed to reproduce the distributions of SZ clusters predicted by the standard PS formalism. It is pointed out that uncertainties in the cosmological application of future SZ cluster surveys via the standard PS formalism should be carefully studied.

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arXiv2000

Reconstruction of radial temperature profiles of galaxy clusters

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.

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arXiv2000

Radial density and temperature profiles of the intracluster gas constructed jointly from the X-ray surface brightness measurement and the universal density profile

In this paper we have made an attempt to derive the radial profiles of density and temperature of intracluster gas based on the two well-established facts at present: the X-ray observed surface brightness of clusters described by the standard beta model and the (NFW) universal density profile as the underlying dark matter distribution. We have numerically solved the hydrostatic equation by demanding that the volumed-averaged baryon fraction of a cluster should asymptotically approach the universal value at its viral radius. We have shown that the radial temperature variation derived from these constraints differs significantly from the conventional polytropic equation of state: The gas temperature profile may show a dramatic increase or decrease with outward radius, depending sensitively on the beta parameter. A large beta value (typically greater than 0.8) is required in order to ensure that the X-ray temperature makes a drop at the virial radius. This indicates that either the NFW profile is inappropriate to apply for the overall dark matter distribution of clusters or other non-gravitational heating processes may play an important role in the dynamical evolution of clusters.

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arXiv2000

A comparison of central temperatures of the intracluster gas determined from X-ray and SZ measurements

A combination of the X-ray imaging and SZ measurements of clusters permits an indirect determination of the radial temperature profiles of intracluster gas, which requires no assumption about the dynamical properties and the equation of state for clusters. A comparison of such a derived gas temperature with that given by the X-ray spectral analysis constitutes an effective probe of properties of intracluster gas. Using the available data of 31 clusters in literature, we have performed the first comparison of the central gas temperatures provided by the two methods. The good agreement between these two independent temperature estimates suggests that the distribution of intracluster gas is essentially consistent with isothermality characterized by a mean polytropic index of gamma=0.9+-0.1.

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arXiv2000

Time-Delay Effect on the cOsmic Background Radiation by Static Gravitational Potential of Clusters

We present a quantitative analysis of the time-delay effect on the cosmic background radiation (CBR) by static gravitational potential of galaxy clusters. This is primarily motivated by growing observational evidence that clusters have essentially experienced no-evolution since redshift $z\approx1$, indicating that the contribution of a time-dependent potential to CBR anisotropy discussed in literature could be rather small for the dynamically-relaxed clusters. Using the softened isothermal sphere model and the universal density profile for the mass distribution of rich clusters, we calculate the CBR anisotropy by the time-delay effect and compare it with those generated by the thermal and kinematic S-Z effects as well as by the transverse motion of clusters. While it is unlikely that the time-delay effect is detectable in the current S-Z measurement because of its small amplitude of $10^{-6}$-$10^{-7}$ and its achromaticity, it nevertheless leads to an uncertainty of $\sim10%$ in the measurement of the kinematic S-Z effect of clusters. Future cosmological application of the peculiar velocity of clusters to be measured through the S-Z effect should therefore take this uncertainty into account.

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arXiv2000

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.

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arXiv2000

Properties of the double beta model for intracluster gas

We present an extensive study of the double beta model for the X-ray surface brightness profiles of clusters, and derive analytically the gas density and total masses of clusters under the hydrostatic equilibrium hypothesis. It is shown that the employment of the double beta model instead of the conventional single beta model can significantly improve the goodness of fit to the observed X-ray surface brightness profiles of clusters, which will in turn lead to a better determination of the gas and total mass distributions in clusters. In particular, the observationally fitted beta parameter for the extended component in a double beta model may become larger. This opens a new possibility of resolving the long-standing beta discrepancy for clusters. Using an ensemble of 33 ROSAT PSPC observed clusters drawn from the Mohr, Mathiesen & Evrard (1999) sample, we find that the asymptotic value of beta_{fit} is (0.83+-0.33) at large radii, consistent with both the average spectroscopic parameter beta_{spec}=(0.78+-0.37) and the result given by numerical simulations.

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arXiv2000

On the radial density profile of intracluster gas tracing the isothermal dark halo with a finite core

The cusped NFW universal density profile suggested by typical CDM models has been challenged in recent years by the discoveries of the soft cores for a broad range of masses from dwarf galaxies to clusters of galaxies. It is thus desirable that a new, analytic model would 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 a great success on galactic scales. This stimulates us to apply the RDH profile to more massive systems like clusters of galaxies. In this paper we have made an attempt to derive the radial density profile of intracluster gas from the RDH profile, and compare it with those revealed by X-ray observations and inferred from the NFW profile. It is shown that the RDH predicted gas density can be well represented by the conventional beta model with a typical beta parameter of 0.7-0.9. Fitting the theoretically predicted X-ray surface brightness profile to an ensemble of 45 X-ray clusters, we find that the RDH and NFW profiles become to be almost indistinguishable. Yet, the RDH profile allows us to work out straightforwardly the central dark matter density of clusters from X-ray measurements. Using the 45 clusters, we find that the central densities of clusters have an average value of ~0.01 solar mass pc^{-3}, in agreement with the result estimated on galactic scales, which reinforces the claim for the presence of the soft halo cores over the entire mass range.

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arXiv2000

The Lx-T, Lx-sigma and sigma-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 Kendall's 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.

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arXiv1999

Baryon Distribution in Galaxy Clusters as a Result of Sedimentation of Helium Nuclei

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 the scenario using the NFW 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 $\sim 20%$, 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.

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