arXiv subjects

Wen Xu

Publications and source records attributed to Wen Xu.

79 records · Page 5Linked to original sources

arXiv1999

Abundances and Correlations of Structures Beyond Galaxy Clusters

We investigated the structures on scales beyond the typical clusters of galaxies. These structures are crucial to understand the cosmic gravitational clustering in the pre-virialized stage, or quasilinear règime. Based on the multi-resolution analysis of the discrete wavelet transformation, we got statistical available ensembles of $r_{cl}$-clusters, i.e. the structures on scale $r_{cl}$, in the range $1 \leq r_{cl} \leq 24 h^{-1}$ Mpc for both N-body simulation and the IRAS 1.2 Jy galaxy survey samples. If the mass-to-light ratio on scales larger than clusters asymptotically reaches a constant, we found that the abundances and correlations of these IRAS $r_{cl}$-clusters to be basically consistent with the predictions of the flat low-density CDM model (LCDM) and the open CDM model (OCDM), except the model-predicted abundance of $r_{cl} = 24 h^{-1}$ Mpc clusters seems to be higher than IRAS data. The standard CDM (SCDM) gives too much power, and too weak correlations on all the scales. For a given $r_{cl}$, the amplitude of two-point correlation function of $r_{cl}$-clusters is increasing with their richness. However, for a given richness (defined by the mean separation of neighbor objects), the clustering strengths of both simulation and observation sample are found to be declining with $r_{cl}$ when $r_{cl}$ is larger than 3 - 4 $h^{-1}$ Mpc. Therefore, the ``universal'' increase of the correlation amplitude with the scale of objects from galaxies, groups, to poor, rich clusters is broken down for structures of $r_{cl} > 3 - 4 h^{-1}$ Mpc. Supercluster should not be a member of the ``universal'' increase family.

astro-ph↗

arXiv1999

Cosmological Constraints on the Host Halos of GRBs

The recently observed bright optical transients(OT) of high redshift GRBs indicate that they are in a violent dynamical state. We think it is reasonable to assume that the GRBs form in the environment of gravitationally collapsed halos of the cosmic matter field, and we investigate the basic parameters of the halos which are favored to host GRBs. If the harboring coefficient $f$ of GRBs per halo is weakly dependent on the mass of the halo, the redshift data of GRB OTs can yield significant constraints on the massive halos hosting GRBs. We show that, in the framework of popular cold dark matter (CDM) models, the GRB-favored environments are newly collapsed halos (i.e. their ages less than about $2 \times 10^9$ yr) with masses around $10^9$ $h^{-1}$ M$_{\odot}$. In this scenario, low redshift GRBs, if they exist, could not have the same cosmic origin as the high redshift ones. To fit with the observed rate of GRBs, we conclude that each GRB halo can host probably no more than one GRB event on average. This result implies that GRBs may be related to the merging of the halos.

astro-ph↗

arXiv1999

A Calibration Method for Wide Field Multicolor Photometric System

The purpose of this paper is to present a method to self-calibrate the spectral energy distribution (SED) of objects in a survey based on the fitting of an SED library to the observed multi-color photometry. We adopt for illustrative purposes the Vilnius (Strizyz and Sviderskiene 1972) and Gunn & Stryker (1983) SED libraries. The self-calibration technique can improve the quality of observations which are not taken under perfectly photometric conditions. The more passbands used for the photometry, the better the results. This technique has been applied to the BATC 15-passband CCD survey.

astro-ph↗

arXiv1998

Updating the $σ$-$T$ relationship for galaxy clusters

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 $σ=10^{2.47\pm0.06}T^{0.67\pm0.09}$, indicating an apparent 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.

astro-ph↗

arXiv1998

Ring structure and warp of NGC5907 -- Interaction with dwarf galaxies

The edge-on, nearby spiral galaxy NGC5907 has long been used as the prototype of a ``non-interacting'' warped galaxy. We report here the discovery of two interactions with companion dwarf galaxies that substantially change this picture. First, a faint ring structure is discovered around this galaxy that is likely due to the tidal disruption of a companion dwarf spheroidal galaxy. The ring is elliptical in shape with the center of NGC5907 close to one of the ring's foci. This suggests the ring material is in orbit around NGC5907. No gaseous component to the ring has been detected either with deep H$α$ images or in Very Large Array (VLA) HI 21-cm line maps. The visible material in the ring has an integrated luminosity <= 10^8 L_sun and its brightest part has a color R-I ~ 0.9. All of these properties are consistent with the ring being a tidally-disrupted dwarf spheroidal galaxy. Second, we find that NGC5907 has a dwarf companion galaxy, PGC54419, projected to be only 36.9 kpc from the center of NGC5907, close in radial velocity (ΔV = 45 km s^{-1}) to the giant spiral galaxy. This dwarf is seen at the tip of the HI warp and in the direction of the warp. Hence, NGC5907 can no longer be considered ``non-interacting,'' but is obviously interacting with its dwarf companions much as the Milky Way interacts with its dwarf galaxies. These results, coupled with the finding by others that dwarf galaxies tend to be found around giant galaxies, suggest that tidal interaction with companions, even if containing a mere 1% of the mass of the parent galaxy, might be sufficient to excite the warps found in the disks of many large spiral galaxies.

astro-ph↗

arXiv1998

Scale Invariance of Rich Cluster Abundance: A Possible Test for Models of Structure Formation

We investigate the dependence of cluster abundance $n(>M,r_{cl})$, i.e., the number density of clusters with mass larger than $M$ within radius $r_{cl}$, on scale parameter $r_{cl}$. Using numerical simulations of clusters in the CDM cosmogonic theories, we notice that the abundance of rich clusters shows a simple scale invariance such that $n[>(r_{cl}/r_0)^αM, r_{cl}]= n(>M,r_0)$, in which the scaling index $α$ remains constant in a scale range where halo clustering is fully developed. The abundances of scale $r_{cl}$ clusters identified from IRAS are found basically to follow this scaling, and yield $α\sim 0.5$ in the range $1.5 < r_{cl} < 4 h^{-1}$Mpc. The scaling gains further supports from independent measurements of the index $α$ using samples of X-ray and gravitational lensing mass estimates. We find that all the results agree within error limit as: $α\sim 0.5 - 0.7$ in the range of $1.5 < r_{cl} < 4 h^{-1}$Mpc. These numbers are in good consistency with the predictions of OCDM ($Ω_M=0.3$) and LCDM ($Ω_M+Ω_Λ =1$), while the standard CDM model has different behavior. The current result seems to favor models with a low mass density.

astro-ph↗

arXiv1995

Big bang simulation in superfluid 3He-B -- Vortex nucleation in neutron-irradiated superflow

We report the observation of vortex formation upon the absorption of a thermal neutron in a rotating container of superfluid $^3$He-B. The nuclear reaction n + $^3$He = p + $^3$H + 0.76MeV heats a cigar shaped region of the superfluid into the normal phase. The subsequent cooling of this region back through the superfluid transition results in the nucleation of quantized vortices. Depending on the superflow velocity, sufficiently large vortex rings grow under the influence of the Magnus force and escape into the container volume where they are detected individually with nuclear magnetic resonance. The larger the superflow velocity the smaller the rings which can expand. Thus it is possible to obtain information about the morphology of the initial defect network. We suggest that the nucleation of vortices during the rapid cool-down into the superfluid phase is similar to the formation of defects during cosmological phase transitions in the early universe.

cond-mat↗