Formation of subgalactic halos is suppressed in a 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 subarcsecond 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 mas 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.

Yan Jie Xue;向平 武

Astrophysical Journal

2001-3-1

The overwhelming foreground contamination is one of the primary impediments to probing the Epoch of Reionization (EoR) through measuring the redshifted 21 cm signal. Among various foreground components, radio halos are less studied and their impacts on the EoR observations are still poorly understood. In this work, we employ the Press-Schechter formalism, merger-induced turbulent reacceleration model, and the latest SKA1-Low layout configuration to simulate the SKA "observed" images of radio halos. We calculate the one-dimensional power spectra from simulated images and find that radio halos can be about 10⁴, 10³, and 10^2.5 times more luminous than the EoR signal on scales of in the 120-128, 154-162, and 192-200 MHz bands, respectively. By examining the two-dimensional power spectra inside properly defined EoR windows, we find that the power leaked by radio halos can still be significant, as the power ratios of radio halos to the EoR signal on scales of can be up to about 230%-800%, 18%-95%, and 7%-40% in the three bands when the 68% uncertainties caused by the variation of the number density of bright radio halos are considered. Furthermore, we find that radio halos located inside the far side lobes of the station beam can also impose strong contamination within the EoR window. In conclusion, we argue that radio halos are severe foreground sources and need serious treatments in future EoR experiments.

Weitian Li;Haiguang Xu;Zhixian Ma;Dan Hu;Zhenghao Zhu;Chenxi Shan;Jingying Wang;Junhua Gu;Dongchao Zheng;Xiaoli Lian;Qian Zheng;Yu Wang;杰 朱;向平 武

Astrophysical Journal

2019-7-10

The relationship between the X-ray determined temperature T of the intracluster gas and the optical measured velocity dispersion σ of the cluster galaxies is often believed to be not only a straightforward but also robust test for the dynamical properties of galaxy clusters. Here, we present the σ-T relationship using the 94 clusters drawn from the largest sample of 149 clusters in literature, for which both σ and T are observationally determined. Employment of the doubly weighted orthogonal distance regression to our sample yields σ = 10^2.47±0.06T^0.67±0.09apparent deviation of dynamical state from that predicted by the isothermal and hydrostatic equilibrium model for galaxy clusters, though the average ratio β_spec of specific energy in galaxies to that in gas is found to be in excellent agreement with unity. It shows that a nonisothermal gas distribution with a mean polytropic index of γ = 1.3 can account for the reported σ-T relationship, while overall clusters can still be regarded as dynamically-relaxed systems.

向平 武;Li Zhi Fang;Wen Xu

Astronomy and Astrophysics

1998-10-20

With the intent of correctly restoring the redshifted 21 cm signals emitted by neutral hydrogen during the cosmic reionization processes, we re-examine the separation approaches based on the quadratic polynomial fitting technique in frequency space in order to investigate whether they work satisfactorily with complex foreground by quantitatively evaluating the quality of restored 21 cm signals in terms of sample statistics. We construct the foreground model to characterize both spatial and spectral substructures of the real sky, and use it to simulate the observed radio spectra. By comparing between different separation approaches through statistical analysis of restored 21 cm spectra and corresponding power spectra, as well as their constraints on the mean halo bias b and average ionization fraction x_e of the reionization processes, at z = 8 and the noise level of 60 mK we find that although the complex foreground can be well approximated with quadratic polynomial expansion, a significant part of the Mpc-scale components of the 21 cm signals (75% for ≳ 6 h ^-1 Mpc scales and 34% for ≳ 1 h ^-1 Mpc scales) is lost because it tends to be misidentified as part of the foreground when the single-narrow-segment separation approach is applied. The best restoration of the 21 cm signals and the tightest determination of b and x _e can be obtained with the three-narrow-segment fitting technique as proposed in this paper. Similar results can be obtained at other redshifts.

Jingying Wang;Haiguang Xu;涛 安;Junhua Gu;Xueying Guo;Weitian Li;Yu Wang;Chengze Liu;Olivier Martineau-Huynh;向平 武

Astrophysical Journal

2013-2-1

Using a purely analytic approach to gaseous and dark matter halos, we study the cross-correlation between the Sunyaev-Zel'dovich (SZ) sky and galaxy surveys under flat sky approximation, in an attempt to acquire the redshift information of the SZ map. The problem can be greatly simplified when it is noticed that the signals of the SZ-galaxy correlation arise only from hot gas and galaxies inside the same massive halos (i.e. clusters), and that field galaxies make almost no contribution to the cross-correlation. Under the assumption that both the hot gas and galaxies trace the common gravitational potential of dark halos, we calculate the expected cross SZ-galaxy power spectra for the WMAP/Planck SZ maps and the SDSS galaxy sample at small scales 100 < l < 1000. It turns out, however, that it is not presently feasible to measure such small angular cross power spectra because of the high noise levels at l > 400 in the WMAP/Planck experiments. Future SZ observations with better angular resolutions and sufficiently wide sky coverage will be needed if this technique is to be applied for the statistical measurement of the redshift distribution of the SZ sources.

Y. Qu;向平 武

Astronomy and Astrophysics

2004-8

向平 武

National Science Review

2017-9-1

The signature of the self-microlensing in compact binaries (white dwarfs, neutron stars and blade holes) is a flare with the characteristic time of typically a few minutes. The probability of detecting these microlensing events can be as high as 1/50 for a photometric accuracy of Am = 0.01 in magnitude. The discovery of the seif-microlensing by binaries would furnish an additional way to find the masses of the lens and the companion, and will be promising for the searches of black holes.

Bo Qin;向平 武;Zhen Long Zou

Chinese Physics Letters

1997

Using samples of structures identified by a multiscale decomposition from numerical simulations, we analyze the scale dependence of the virialization of clusters. We find that beyond the scale of full virialization there exists a radius range over which clusters are quasi-virialized, i.e., while the internal structure of an individual cluster may depart substantially from dynamical relaxation, some statistical properties of the multiscale-identified clusters are approximately the same as those for the virialized systems. The dynamical reason for the existence of quasi virialization is that some of the scaling properties of dynamically relaxed systems of cosmic gravitational clustering approximately hold beyond the full virialization regime. This scaling can also be seen from a semianalytic calculation of the mass functions of collapsed and uncollapsed halos in the Press-Schechter formalism. The "individual-statistical" duality of the quasi virialization provides an explanation of the observed puzzle that the total masses of clusters derived from virial theory are statistically the same as the masses determined from gravitational lensing, in spite of the presence of irregular configuration and substructures in individual clusters. It also explains the tight correlation between the velocity dispersion of optical galaxies and the temperature of X-ray -emitting gas. Consequently, the virial mass estimators based on the assumptions of isothermal and hydrostatic models are statistically applicable to scales on which the clusters are quasi-virialized. In the quasi-virialization regime, the temperature functions of clusters also show scaling. This feature is a useful discriminator among models. As a preliminary comparison with observation, the discriminator yields favor the models of LCDM and OCDM.

Wen Xu;Li Zhi Fang;向平 武

Astrophysical Journal

2000-4-1

In previous works, the nearby galaxy cluster Abell 3158 was reported to possess a relatively regular, relaxed morphology in the X-ray band. By analysing the Chandra and XMM-Newton archived data of Abell 3158, we have identified a bow-edge-shaped discontinuity in the X-ray surface brightness distribution about 120 h^-1₇₁ kpc west of the X-ray peak. This feature is found to be associated with a massive, off-centre cool gas clump, and actually forms the west boundary of the cool clump. By calculating the thermal gas pressures in the cool clump and in the free-stream region, we determine that the cool gas clump is moving at a subsonic velocity of 700⁺¹⁴⁰_-340 km s^-1 (M = 0.6^+0.1_-0.3) towards west on the sky plane. We exclude the possibility that this cool clump was formed by local inhomogeneous radiative cooling in the intracluster medium, because of the effectiveness of the thermal conduction on the time-scale of ∼0.3 Gyr. As no evidence for central active galactic nucleus activity has been found in Abell 3158, and this cool clump bears many similarities to the off-centre cool gas clumps detected in other merging clusters in terms of their mass, size, location and thermal properties (e.g. lower temperature and higher abundance as compared with the environment), we speculate that the cool clump in Abell 3158 was caused by a merger event, and is the remnant of the original central cool-core of the main cluster or the infalling subcluster. This idea is supported not only by the study of the line-of-sight velocity distribution of the cluster member galaxies, but also by the study of the gas entropy-temperature correlation. This example shows that the appearance of such massive, off-centre cool gas clumps can be used to diagnose the dynamical state of a cluster, especially when prominent shocks and cold fronts are absent.

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

Monthly Notices of the Royal Astronomical Society

2010-4

We present a morphological classification of 14,245 radio active galactic nuclei (AGNs) into six types, i.e., typical Fanaroff-Riley Class I/II (FRI/II), FRI/II-like bent-tailed, X-shaped radio galaxy, and ringlike radio galaxy, by designing a convolutional neural network based autoencoder, namely MCRGNet, and applying it to a labeled radio galaxy (LRG) sample containing 1442 AGNs and an unlabeled radio galaxy (unLRG) sample containing 14,245 unlabeled AGNs selected from the Best-Heckman sample. We train MCRGNet and implement the classification task by a three-step strategy, i.e., pre-training, fine-tuning, and classification, which combines both unsupervised and supervised learnings. A four-layer dichotomous tree is designed to classify the radio AGNs, which leads to a significantly better performance than the direct six-type classification. On the LRG sample, our MCRGNet achieves a total precision of ∼93% and an averaged sensitivity of ∼87%, which are better than those obtained in previous works. On the unLRG sample, whose labels have been human-inspected, the neural network achieves a total precision of ∼80%. Also, using Sloan Digital Sky Survey Data Release 7 to calculate the r-band absolute magnitude (M _opt ) and using the flux densities to calculate the radio luminosity (L _radio ), we find that the distributions of the unLRG sources on the L _radio -M _opt plane do not show an apparent redshift evolution and could confirm with a sufficiently large sample that there could not exist an abrupt separation between FRIs and FRIIs as reported in some previous works.

Zhixian Ma;Haiguang Xu;杰 朱;Dan Hu;Weitian Li;Chenxi Shan;Zhenghao Zhu;Liyi Gu;Jinjin Li;Chengze Liu;向平 武

Astrophysical Journal, Supplement Series

2019-2