The presence of spectral curvature resulting from the synchrotron self-absorption of extragalactic radio sources could break down the spectral smoothness feature. This leads to the premise that the bright radio foreground can be successfully removed in 21-cm experiments that search for the epoch of reionization (EoR). We present a quantitative estimate of the effect of the spectral curvature resulting from the synchrotron self-absorption of extragalactic radio sources on the measurement of the angular power spectrum of the low-frequency sky. We incorporate a phenomenological model, which is characterized by the fraction (f) of radio sources with turnover frequencies in the range of 100-1000MHz and by a broken power law for the spectral transition around the turnover frequencies ν _m, into simulated radio sources over a small sky area of 10° × 10°. We compare statistically the changes in their residual maps with and without the inclusion of the synchrotron self-absorption of extragalactic radio sources after the bright sources of S150 MHz ≥100mJy are excised. Furthermore, the best-fitting polynomials in the frequency domain on each pixel are subtracted. It has been shown that the effect of synchrotron self-absorption on the detection of the EoR depends sensitively on the spectral profiles of the radio sources around the turnover frequencies ν _m. A hard transition model, described by the broken power law with the turnover of spectral index at ν _m, would leave pronounced imprints on the residual background and would therefore cause serious confusion with the cosmic EoR signal. However, the spectral signatures on the angular power spectrum of the extragalactic foreground, generated by a soft transition model in which the rising and falling power laws of the spectral distribution around ν _m are connected through a smooth transition spanning ≥200MHz in a characteristic width, can be fitted and consequently subtracted by the use of polynomials to an acceptable degree (δT < 1mK). As this latter scenario seems to be favoured in both theoretical expectations and radio spectral observations, we conclude that the contamination of extragalactic radio sources by synchrotron self-absorption in 21-cm experiments is probably very minor.

Qian Zheng;向平 武;Jun Hua Gu;Jingying Wang;Haiguang Xu

Monthly Notices of the Royal Astronomical Society

2012-8-21

Increasing evidences provided primarily by the cluster lensing and numerical simulations of cluster formation indicate that galaxy clusters may contain compact cores that are substantially smaller than the cores revealed by the X-ray observations of hot intracluster gas. In this paper we present a model that describes how two distinct cores can grow simultaneously as a result of infall from the background dark matter. This model needs a pre-existing localized large fluctuation, which can be the non-Gaussian density peak of the primordial fluctuations, to seed the individual growing cluster. On the other hand, numerous recent observations also show that no strong evolution is detected for galaxy clusters within the redshift up to z ∼ 0.8. We therefore present a comparison of observations with the saturated cluster configuration resulting from our infall model. In the saturated state, the predicted compact core mass is about few × 10¹¹ M_⊙ and the core size about 1 kpc for a cluster mass ∼ several × 10¹⁵ M_⊙ within a radius of 3 Mpc. We have successfully reproduced the dark matter distribution revealed by the gravitational lensing, and the observed radial distributions of cluster galaxies, intracluster gas (i.e., the conventional β model) and baryon fractions in a consistent way. This model, when combined with the observed X-ray surface brightness profiles of clusters, predicts that the overall temperature of intracluster gas has a tendency of radial decline with a mean polytropic index γ ≈ 1.2. Finally, the so-called β discrepancy finds a natural explanation for such a polytropic gas in our model.

Tzihong Chiueh;向平 武

Astronomy and Astrophysics

2000

Very deep galaxy surveys have revealed a considerably large population of faint galaxies, which leads to the speculation that all distant objects are moderately magnified by the gravitational lensing effects of galaxies (Fried 1997). In this letter, we present a simple estimate of the lensing amplitudes by all galaxies up to redshift z = 2 in terms of galaxy merging and answer the question whether the sky is fully covered by the lensing cross-sections of galaxies. It is shown that, as a result of the combination of the increase of galaxy number and the decrease of galaxy velocity dispersion with lookback time, less than ∼ 1/10 of the sky to z = 2 can be moderately affected by the galaxy acting as lenses with magnification μ, > 1.1. This conclusion is independent of the galaxy limiting magnitude. In other Words, no matter how high the surface number density of faint galaxies becomes, it is unlikely that their lensing crosssections of μ > 1.1 can cover the whole sky.

宗宏 朱;向平 武

Astronomy and Astrophysics

1997

While both X-ray emission and Sunyaev-Zel'dovich (SZ) temperature fluctuations are generated by the warm-hot gas in dark matter halos, the two observables have different dependence on the underlying physical properties, including the gas distribution. A cross-correlation between the soft X-ray background (SXRB) and the SZ sky may allow an additional probe on the distribution of warm-hot gas at intermediate angular scales and redshifts complementing studies involving clustering within SXRB and SZ separately. Using a halo approach, we investigate this cross-correlation analytically. The two contributions are correlated mildly with a correlation coefficient of ∼0.3, and this relatively low correlation presents a significant challenge for its detection. The correlation, at small angular scales, is affected by the presence of radiative cooling or preheating and provides a probe on the thermal history of the hot gas in dark halos. While the correlation remains undetectable with CMB data from the WMAP satellite and X-ray background data from existing catalogs, upcoming observations with CMB missions such as Planck, for the SZ side, and an improved X-ray map of the large scale structure, such as the one planned with DUET mission, may provide a first opportunity for a reliable detection of this cross-correlation.

L. M. Cheng;向平 武;A. Cooray

Astronomy and Astrophysics

2004-1

Using a simple analytic approach we address the question of whether radiative cooling, nongravitational heating, and cooling plus heating models can simultaneously explain the observed global X-ray properties (entropy and X-ray luminosity distributions) of groups and clusters and the residual soft X-ray background (XRB) after discrete sources are removed. Within the framework of typical cold dark matter structure formation characterized by an amplitude of matter power spectrum σ₈ = 0.9, it is argued that while radiative cooling alone is able to marginally reproduce the entropy floor detected in the central regions of groups and clusters, it is insufficient to account for the steepening of the X-ray luminosity-temperature relation for groups and the unresolved soft XRB. A phenomenological preheating model, in which either an extra specific energy budget or an entropy floor is added to the hot gas in groups and clusters, fails in the recovery of at least one of the X-ray observed features. Finally, the soft XRB predicted by our combined model of cooling plus heating exceeds the observational upper limits by a factor of ∼2, if the model is required to reproduce the observed entropy and X-ray luminosity-temperature relationships of groups and clusters. Inclusion of the cosmic variation of metallicity and the self-absorption of the cooled gas as a result of radiative cooling in groups and clusters, or exclusion of the contribution of nearby massive clusters to the XRB, does not significantly alter the situation. If the discrepancy is not a result of the oversimplification of our analytic models, this implies that either our current understanding of the physical processes of the hot gas is still incomplete, or the normalization of the present power spectrum has been systematically overestimated. For the latter, both the X-ray properties of groups and clusters and the XRB predicted by the preheating model and the cooling plus heating model can be reconciled with the X-ray observations if a lower value of the normalization parameter σ₈ ≈ 0.7 is assumed.

Yan Jie Xue;向平 武

Astrophysical Journal

2003-2-10

A combination of the X-ray imaging and Sunyaev-Zeldovich (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 γ = 0.9 ± 0.1.

Tong Jie Zhang;向平 武

Astrophysical Journal

2000-12-10

The PrimevAl Structure Telescope (PAST), will be used to locate and study the era the of the first luminous objects, the epoch of reionization. The first stars ionized the gas around them producing a pattern of ionization that reflects the large scale density structure present at the time. The PAST array will be used to sense and study this ionization, by mapping the brightness of 21 cm neutral Hydrogen Cosmic Background (HCB) at redshift from 6 to 25. The HCB disappears on ionization, allowing the study of large scale structure and of star formation at this very early epoch.

Jeffery B. Peterson;Ue Li Pen;向平 武

Modern Physics Letters A

2004-5-30

The redshifted 21cm signal of neutral hydrogen from the epoch of reionization (EoR) is extremely weak and its first detection is therefore expected to be statistical with first-generation low-frequency radio interferometers. In this Letter, we propose a method to extract the angular power spectrum of the EoR from the visibility correlation coefficients p _ij(u, v), instead of the visibilities V_ij(u, v) measured directly by radio interferometers in conventional algorithm. The visibility correlation coefficients are defined as p_ij(u, v) = V_ij(u, v)/√|V_ii||V_ij| by introducing the autocorrelation terms V_ii and V_jj such that the angular power spectrum C_ℓ can be obtained through C_ℓ = T ²₀〈|p_ij(u, v)|²〉, independently of the primary beams of antennas. This also partially removes the influence of receiver gains in the measurement of C_ℓ because the amplitudes of the gains cancel each other out in the statistical average operation of 〈|p_ij(u, v)|²〉. We use the average system temperature T ₀ as a calibrator of C_ℓ, which is dominated by the Milky Way and extragalactic sources in the frequency range that we are interested in, below 200MHz. Finally, we demonstrate the feasibility of this novel method using the simulated sky maps as targets and the 21 CentiMeter Array (21CMA) as interferometer.

Qian Zheng;向平 武;Jun Hua Gu;Jingying Wang;Haiguang Xu

Astrophysical Journal Letters

2012-10-10

The theoretical average line-of-sight velocity dispersion along the radius of a spherical galaxy or cluster of galaxies tends to a variable value. Statistical lensing studies often take into account only the asymptotic value, either close to the center or at large distances, rather than the full variation: this results in significant effects in lensing properties depending on the mass density models of the lensing systems. Magnification probabilities by clusters of galaxies modeled by four well-known density profiles are calculated in this paper and the uncertainties from the variable velocity dispersions are shown. It is found that magnification probabilities span a range of about an order of magnitude among the models, which implies that it is impossible to distinguish the model differences from statistical lensing, and the present evaluation of lensing effects may also have a large uncertainty from velocity dispersions. It is suggested that a perfectly adequate estimate of lensing probabilities can be obtained by modeling galaxies and galaxy clusters as singular isothermal spheres, in which the line-of-sight velocity dispersion is in fact a constant.

向平 武

Astrophysical Journal

1993-7-10

We chose the bright compact group HCG 62, which has been found to exhibit both excess X-ray emission and high Fe abundance to the southwest of its core, as an example to study the impact of mergers on chemical enrichment in the intragroup medium. We first reanalyze the high-quality Chandra and XMM-Newton archive data to search for evidence of additional SN II yields, which is expected to be a direct result of the possible merger-induced starburst. We reveal that, similar to the Fe abundance, the Mg abundance also shows a high value in both the innermost region and the southwest substructure, forming a high-abundance plateau. Meanwhile, all the SN Ia and SN II yields show rather flat distributions in >0.1r ₂₀₀ in favor of an early enrichment. Then, we carry out a series of idealized numerical simulations to model the collision of two initially isolated galaxy groups by using the TreePM-SPH GADGET-3 code. We find that the observed X-ray emission and metal distributions, as well as the relative positions of the two bright central galaxies with reference to the X-ray peak, can be well reproduced in a major merger with a mass ratio of 3 when the merger-induced starburst is assumed. The "best-match" snapshot is pinpointed after the third pericentric passage when the southwest substructure is formed due to gas sloshing. By following the evolution of the simulated merging system, we conclude that the effects of such a major merger on chemical enrichment are mostly restricted to within the core region when the final relaxed state is reached.

Dan Hu;Haiguang Xu;熙 康;Weitian Li;Zhenghao Zhu;Zhixian Ma;Chenxi Shan;Zhongli Zhang;Liyi Gu;Chengze Liu;Qian Zheng;向平 武

Astrophysical Journal

2019-1-10