The Burkert profile is a competing candidate for the analytic approximation of virialized dark halos especially when dark matter particles have a finite cross-section for elastic collisions. In this paper we reexamine its universality in massive systems, using an ensemble of 45 nearby X-ray clusters and 20 distant X-ray/lensing clusters. Despite the fact that this empirical profile turns out a great success on galactic scales and also reproduces approximately the X-ray observed surface brightness profiles of clusters, the dark matter cores of clusters predicted by the Burkert profile are too large to be reconciled with the strong gravitational lensing measurements. Specifically, the typical dark halo cores of clusters represented by the Burkert profile are about 0.2 Mpc, and only a small fraction (∼ 1/4) of clusters can have compact cores smaller than 0.1 Mpc. This will constitute a critical challenge to the Burkert profile as a universal dark matter density law over entire mass ranges.

Yan Jie Xue;向平 武

International Journal of Modern Physics D

2001-4

We present a new estimate of the projected X-ray mass of A1689 observed with Chandra X-Ray Observatory, 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 Einstein, ROSAT, and ASCA observations. Particular attention is paid to examining whether there is an offset between the X-ray centroid and the 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 A1689, 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 A1689 is a fully relaxed cluster. We thus employ the hydrostatic equilibrium hypothesis to determine the projected mass profile of A1689 and compare it with the results obtained by different lensing techniques available in the literature. Our analysis confirms the existence of a discrepancy of a factor of ∼2 between X-ray and lensing mass estimates in the central region of r ≈ 0.2 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 A1689 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 the dynamical evolution of the intracluster gas in the central regions of clusters.

Sui Jian Xue;向平 武

Astrophysical Journal

2002-9-1

Motivated by the significant overdensity of background bright quasars recently detected behind the foreground clusters of galaxies on a scale of 10′, we have investigated the possibility of attributing the quasar-cluster associations to gravitational lensing by large-scale matter inhomogeneities. Based on the conventional lensing models, we have shown that the reported quasar overdensity is unlikely to be generated by cluster matter alone. The situation does not change even if all of the clusters of galaxies that follow their spatial two-point correlation function are taken into account, while matter clumps on scales of greater than 20 Mpc are also found to be unable to provide the required mass surface density, since their density contrast is strictly limited by the anisotropy measurements of the cosmic background radiation. Moreover, we have pointed out that the influence of a nonzero cosmological constant on the quasar-cluster associations is very minor. We conclude that either the observed quasar number counts have been seriously contaminated by the magnification bias of matter inhomogeneities of the universe or there should exist some intercluster matter on a scale of less than ∼20 Mpc, e.g., from cluster-galaxy correlation, whose mean cosmic density is about an order of magnitude higher than that of clusters of galaxies.

向平 武;Li Zhi Fang

Astrophysical Journal

1996

We present HI imaging of the galaxy group IC 1459 carried out with six antennas of the Australian Square Kilometre Array Pathfinder equipped with phased-array feeds. We detect and resolve HI in 11 galaxies down to a column density of ~10²⁰ cm^-2 inside a ~6 deg² field and with a resolution of ~1 arcmin on the sky and ~8 kms^-1 in velocity. We present HI images, velocity fields and integrated spectra of all detections, and highlight the discovery of three HI clouds - two in the proximity of the galaxy IC 5270 and one close to NGC 7418. Each cloud has an HI mass of ~10⁹ M_⊙ and accounts for ~15 per cent of the HI associated with its host galaxy. Available images at ultraviolet, optical and infrared wavelengths do not reveal any clear stellar counterpart of any of the clouds, suggesting that they are not gas-rich dwarf neighbours of IC 5270 and NGC 7418. Using Parkes data, we find evidence of additional extended, low-column-density HI emission around IC 5270, indicating that the clouds are the tip of the iceberg of a larger system of gas surrounding this galaxy. This result adds to the body of evidence on the presence of intragroup gas within the IC 1459 group. Altogether, the HI found outside galaxies in this group amounts to several times 10⁹ M_⊙, at least 10 per cent of the HI contained inside galaxies. This suggests a substantial flow of gas in and out of galaxies during the several billion years of the group's evolution.

P. Serra;B. Koribalski;V. Kilborn;J. R. Allison;S. W. Amy;L. Ball;K. Bannister;M. E. Bell;D. C.J. Bock;R. Bolton;M. Bowen;B. Boyle;S. Broadhurst;D. Brodrick;M. Brothers;J. D. Bunton;J. Chapman;W. Cheng;A. P. Chippendale;Y. Chung;F. Cooray;T. Cornwell;D. DeBoer;P. Diamond;R. Forsyth;R. Gough;N. Gupta;G. A. Hampson;L. Harvey-Smith;S. Hay;D. B. Hayman;I. Heywood;A. W. Hotan;S. Hoyle;B. Humphreys;B. Indermuehle;C. Jacka;C. A. Jackson;S. Jackson;K. Jeganathan;S. Johnston;J. Joseph;P. Kamphuis;M. Leach;E. Lenc;E. Lensson;S. Mackay;M. Marquarding;J. Marvil;N. McClure-Griffiths;D. McConnell;M. Meyer;P. Mirtschin;S. Neuhold;A. Ng;R. P. Norris;J. O'Sullivan;J. Pathikulangara;S. Pearce;C. Phillips;A. Popping;R. Y. Qiao;J. E. Reynolds;P. Roberts;R. J. Sault;A. E.T. Schinckel;R. Shaw;T. W. Shimwell;L. Staveley-Smith;M. Storey;A. W. Sweetnam;E. Troup;A. Tzioumis;M. A. Voronkov;T. Westmeier;M. Whiting;C. Wilson;O. I. Wong;向平 武

Monthly Notices of the Royal Astronomical Society

2015-10-27

Using the Monte Carlo simulation method, we investigate the time-dependent behaviour of Lyα photon transfer in an optically thick medium of the concordance ΛCDM universe. At high redshifts, the Lyα photon escaping from the optically thick medium has a time-scale as long as the age of the luminous object, or even comparable to the age of the Universe. In this case, time-independent or stationary solutions of the Lyα photon transfer with resonant scattering will overlook important features of the escaped Lyα photons in physical and frequency spaces. More importantly, the expansion of the Universe means that the time-independent solutions of the Lyα photon transfer might not exist. We show that time-dependent solutions are sometimes essential for understanding Lyα emission and absorption at high redshifts. For Lyα photons from sources at redshift 1 +z= 10, which are surrounded by the neutral hydrogen intergalactic medium (IGM) of the ΛCDM universe, the escape coefficient is found to be always less, or much less, than one, regardless of the age or lifetime of the sources. In such an environment, we also find that even when the Lyα photon luminosity of the sources is stable, the mean surface brightness gradually increases in the first 10⁶ yr, and then decreases with a power law of time. However, it never approaches a stable, time-independent state. That is, all 1 +z= 10 sources in a neutral Hubble expanding IGM with Lyα luminosity L have their maximum of mean surface brightness ∼10^-21(L/10⁴³ erg s^-1) erg s^-1 cm^-2 arcsec^-2 at the age of about 10⁶ yr. We also address the time-dependent effects on the red damping wing profile.

Wen Xu;向平 武;Li Zhi Fang

Monthly Notices of the Royal Astronomical Society

2011-12

Current measurements of the Hubble constant H₀ on a scale less than ∼100 Mpc appear to be controversial, while the observations made at high redshift seem to provide a relatively low value. On the other hand, the Hubble expansion is driven by the matter content of the universe. The dynamical analysis on a scale of ∼10 Mpc indicates that the matter density Ω₀ is only ∼0.2-0.3, which is significantly smaller than Ω₀ = 1 predicted in the standard inflation model. This might support the tendency of the Hubble constant to decrease with distance. In this paper, we discuss the influence of a possible variant Hubble constant on two fundamental relations in astronomy: the magnitude-redshift (m-z) and the number-magnitude relations. Using a distant Type la supernova at z = 0.458, we show that neither the deceleration parameter q₀ nor Ω₀ can be determined from the m-z relation at moderate/high redshift unless the variation of the Hubble constant is measured a priori. It is further demonstrated that the number density of distant sources would be underestimated when, their local calibration is employed, which may partially account for the number excess of the faint blue galaxies observed at moderate/high redshift.

向平 武;Bo Qin;Li Zhi Fang

Astrophysical Journal

1996

We present a quantitative estimate of the contamination of cluster radio halos and galaxies in the measurement of the angular power spectrum of the thermal Sunyaev-Zel'dovich (SZ) effect. To achieve the goal, we use a purely analytic approach to both radio sources and dark matter of clusters by incorporating empirical models and observational facts together with some theoretical considerations. It is shown that the correction of cluster radio halos and galaxies to the measurement of the thermal SZ angular power spectrum is no more than 20% at l > 2000 for observing frequencies ν > 30 GHz. This eliminates the concern that the SZ measurement may be seriously contaminated by the existence of cluster radio sources.

Wei Zhou;向平 武

Astrophysical Journal

2004-1-10

We present a catalog of 624 radio sources detected around the North Celestial Pole (NCP) with the 21 Centimeter Array (21CMA), a radio interferometer dedicated to the statistical measurement of the epoch of reionization (EoR). The data are taken from a 12 hr observation made on 2013 April 13, with a frequency coverage from 75 to 175 MHz and an angular resolution of ∼4′. The catalog includes flux densities at eight sub-bands across the 21CMA bandwidth and provides the in-band spectral indices for the detected sources. To reduce the complexity of interferometric imaging from the so-called "w" term and ionospheric effects, the present analysis is restricted to the east-west baselines within 1500 m only. The 624 radio sources are found within 5° around the NCP down to ∼0.1 Jy. Our source counts are compared, and also exhibit a good agreement, with deep low-frequency observations made recently with the GMRT and MWA. In particular, for fainter radio sources below ∼1 Jy, we find a flattening trend of source counts toward lower frequencies. While the thermal noise (∼0.4 mJy) is well controlled to below the confusion limit, the dynamical range (∼10⁴) and sensitivity of current 21CMA imaging are largely limited by calibration and deconvolution errors, especially the grating lobes of very bright sources, such as 3C061.1, in the NCP field, which result from the regular spacings of the 21CMA. We note that particular attention should be paid to the extended sources, and their modeling and removal may constitute a large technical challenge for current EoR experiments. Our analysis may serve as a useful guide to the design of next generation low-frequency interferometers like the Square Kilometre Array.

Qian Zheng;向平 武;Melanie Johnston-Hollitt;Jun Hua Gu;Haiguang Xu

Astrophysical Journal

2016-12-1

To investigate the spatial distribution of the intracluster medium temperature in galaxy clusters in a quantitative way and probe the physics behind it, we analyze the X-ray spectra from a sample of 50 clusters that were observed with the Chandra ACIS instrument over the past 15 years and measure the radial temperature profiles out to 0.45r₅₀₀. We construct a physical model that takes into consideration the effects of gravitational heating, thermal history (such as radiative cooling, active galactic nucleus feedback, and thermal conduction), and work done via gas compression, and use it to fit the observed temperature profiles by running Bayesian regressions. The results show that in all cases our model provides an acceptable fit at the 68% confidence level. For further validation, we select nine clusters that have been observed with both Chandra (out to 0.3r₅₀₀) and Suzaku (out to 1.5r₅₀₀) and fit their Chandra spectra with our model. We then compare the extrapolation of the best fits with the Suzaku measurements and find that the model profiles agree with the Suzaku results very well in seven clusters. In the remaining two clusters the difference between the model and the observation is possibly caused by local thermal substructures. Our study also implies that for most of the clusters the assumption of hydrostatic equilibrium is safe out to at least 0.5r₅₀₀ and the non-gravitational interactions between dark matter and its luminous counterparts is consistent with zero.

Zhenghao Zhu;Haiguang Xu;Jingying Wang;Junhua Gu;Weitian Li;Dan Hu;Chenhao Zhang;Liyi Gu;涛 安;Chengze Liu;Zhongli Zhang;杰 朱;向平 武

Astrophysical Journal

2016-1-10

We present a comparison of the Sunyaev-Zeldovich (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-temperature (L_X-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 nonevolving and evolving XLFs of clusters suggested by X-ray observations, we calculate the expectations for SZ surveys of clusters with X-ray luminosity L_X ≥ 3 × 10⁴⁴ ergs s^-1 and L_X ≥ 1 × 10⁴³ ergs s^-1 in the 0.5-2.0 band, respectively. The nonevolving XLF results in a significant excess of SZ cluster counts at high redshifts as compared with the evolving XLF, while a slightly steeper L_X-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.

Yan Jie Xue;向平 武

Astrophysical Journal

2001-5-10