arXiv subjects

Z. L. Wen

Publications and source records attributed to Z. L. Wen.

At least 19 recordsLinked to original sources

arXiv2026

Solitary dwarf galaxy groups as tracers of dark matter halos in the local Universe

In $Λ$CDM cosmology, galaxies and clusters form within dark matter halos and merge in the hierarchical assembly paradigm to form massive systems. Using the released optical survey data, we searched for groups composed solely of dwarf galaxies, each with a stellar mass $M_*<10^{9.5}~M_{\odot}$. We identified 14 dwarf galaxy groups with at least 5 dwarf galaxies, all located within a projected radius of 200 kpc and with a line-of-sight velocity of $\pm$300 km s$^{-1}$. We checked photometric and imaging data and found that these 14 dwarf galaxy groups are solitary, with no neighboring massive galaxies with $M_*>10^{10}~M_{\odot}$ within 500 kpc and within $\pm$1200 km s$^{-1}$. These dwarf galaxies are gravitationally bound within halos with a dynamical mass of around $M_{\rm dyn} \sim 10^{12}~M_{\odot}$ and a virial radius of less than 400 kpc. The stellar mass fractions of these dwarf galaxy groups with $M_{\rm dyn}>10^{12}~M_{\odot}$ are one magnitude lower than predicted by the canonical stellar mass and halo mass relation. These dwarf galaxy groups, therefore, are indications of dark matter halos in the local Universe that host only a few newly formed dwarf galaxies.

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arXiv2026

A rare sextuple-merging brightest cluster galaxy system in a disturbed galaxy cluster observed with the Einstein Probe Follow-up X-ray Telescope

The evolutionary processes of galaxy clusters influence the properties of their member galaxies. We present a joint X-ray--optical analysis of the galaxy cluster WHY J050106.2+013714 at $z_{\rm c}=0.151$. X-ray observations with the Einstein Probe Follow-up X-ray Telescope indicate that the cluster is dynamically young. The cluster displays an average X-ray temperature of $2.8^{+0.4}_{-0.3}$ keV and a total luminosity of 9.4$\pm0.3\times10^{43}$ erg s$^{-1}$, consistent with the scaling relation of typical disturbed clusters. Remarkably, the cluster hosts a multi-merging brightest cluster galaxy (BCG) system composed of six massive galaxies, with a total stellar mass of $1.16\times10^{12}M_{\odot}$. We detected a well-defined intracluster light component extending to a size of 310 kpc. A systematic search for merging BCGs in the DESI Legacy Surveys reveals that this sextuple-merging BCG is extremely rare in the local Universe. Additionally, other merging BCGs are also likely to form in moderately disturbed clusters, which provides valuable insights into the formation of BCGs.

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arXiv2025

A sample of 3403 galaxy clusters identified in XMM-Newton X-ray images

Currently, the number of galaxy clusters identified using galaxy data has far exceeded the number derived from intracluster medium data. In this study, we used positional information from large optical cluster catalogues to search for previously unrecognized X-ray galaxy clusters in archival XMM-Newton data. We successfully identified 1490 galaxy clusters in X-ray images for the first time. By incorporating 1913 previously known X-ray clusters, we constructed a sample of 3403 galaxy clusters observed by XMM-Newton. Our cluster mass estimates show broad consistency with previous measurements. Comparative analyses between the known and newly identified subsamples revealed that new X-ray clusters exhibit systematically higher redshifts, lower masses, and smaller X-ray-to-optical mass ratios, but show no systematic differences in dynamical properties. The newly identified X-ray clusters are a valuable addition to previous X-ray samples and are important for future statistical studies.

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arXiv2025

Evolution of Cluster Alignments as Evidence of Large-scale Structure Formation in the Universe

The universe's large-scale structure forms a vast, interconnected network of filaments, sheets, and voids known as the cosmic web. For decades, astronomers have observed that the orientations of neighboring galaxy clusters within these elongated structures are often aligned over separations of tens of Mpc. Using the largest available catalog of galaxy clusters, we show for the first time that clusters orientations are correlated over even larger scales, up to 200-300 comoving Mpc, and such alignments are seen to redshifts of at least z = 1. Comparison with numerical simulations suggests that coherent structures on similar scales may be expected in LCDM models.

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arXiv2024

A catalogue of merging clusters of galaxies: cluster partners, merging subclusters, and post-collision clusters

Clusters of galaxies are merging during the formation of large-scale structures in the Universe. Based on optical survey data, we identify a large sample of pre-mergers of galaxy clusters and merging subclusters in rich clusters. We find 39,382 partners within a velocity difference of 1500 km/s and a projected separation of 5r_{500} around 33,126 main clusters, where r_{500} is the radius of the main cluster. Based on the galaxy distribution inside rich clusters with more than 30 member galaxy candidates, we identify subclusters by modeling the smoothed optical distribution with a two-component profile, and a coupling factor is obtained for merging subclusters in 7845 clusters. In addition, we find 3446 post-collision mergers according to the deviations of brightest cluster galaxies from other member galaxies, most of which have been partially validated by using the Chandra and XMM-Newton X-ray images. Two new bullet-like clusters have been identified by using the optical and X-ray images. The large samples of merging clusters of galaxies presented here are important databases for studying the hierarchical structure formation, cluster evolution, and the physics of intergalactic medium.

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arXiv2024

A catalog of 1.58 million clusters of galaxies identified from the DESI Legacy Imaging Surveys

Based on the DESI Legacy Imaging Surveys released data and available spectroscopic redshifts, we identify 1.58 million clusters of galaxies by searching for the overdensity of stellar mass distribution of galaxies within redshift slices around pre-selected massive galaxies, among which 877,806 clusters are found for the first time. The identified clusters have an equivalent mass of M_{500}> 0.47*10^{14}~Msolar with an uncertainty of 0.2 dex. The redshift distribution of clusters extends to z~1.5, and 338,841 clusters have spectroscopic redshifts. Our cluster sample includes most of the rich optical clusters in previous catalogs, more than 95% massive Sunyaev-Zeldovich clusters and 90% ROSAT and eROSITA X-ray clusters. From the light distributions of member galaxies, we derive the dynamical state parameters for 28,038 rich clusters and find no significant evolution of the dynamical state with redshift. We find that the stellar mass of the brightest cluster galaxies grows by a factor of 2 since z=1.

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arXiv2023

More relaxed intracluster gas than galaxies in clusters in quasi-equilibrium

During cluster mergers, the intracluster gas and member galaxies undergo dynamic evolution, but at different timescales and reach different states. We collect 24 galaxy clusters in quasi-equilibrium state as indicated by the X-ray image, and calculate the cluster orientations and three kinds of dynamical parameters, i.e., the normalized centroid offset, the sphere index and the ellipticity, for these clusters from the distributions of member galaxies and also the intracluster gas. We find consistent alignments for the orientations estimated from the two components. However, the three kinds of dynamical parameters indicated by member galaxies are systematically larger than those derived from the gas component, suggesting that the gas component is more relaxed than member galaxies. Differences of dynamical features between the intracluster gas and member galaxies are independent of cluster mass and concentration. We conclude that the intracluster gas reaches the dynamic equilibrium state earlier than the almost collisionless member galaxies.

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arXiv2022

The alignment between brightest cluster galaxies and host clusters

The alignment between brightest cluster galaxies (BCGs) and host clusters can reveal the mystery of formation and evolution for galaxy clusters. We measure cluster orientations in optical based on the projected distribution of member galaxies and in X-ray by fitting the morphology of intra-cluster medium (ICM). Cluster orientations determined in the two wavelengths are generally consistent. The orientation alignment between BCGs and host clusters is confirmed and more significant than previous works. We find that BCGs are more aligned with cluster orientations measured in X-ray than those from optical data. Clusters with a brighter BCG generally show a stronger alignment. We argue that the detected redshift evolution of the alignment is probably caused by observational bias rather than intrinsic evolution. The alignment is not related to the ellipticity of BCGs, and the richness, ellipticity and dynamical state of host clusters. The strong alignment between BCGs and morphology of ICMs may be the consequence of the co-evolution between the central massive galaxy and host clusters.

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arXiv2022

Clusters of galaxies up to z=1.5 identified from photometric data of the Dark Energy Survey and unWISE

Using photometric data from the Dark Energy Survey and the Wide-field Infrared Survey Explorer, we estimate photometric redshifts for 105 million galaxies using the nearest-neighbour algorithm. From such a large data base, 151,244 clusters of galaxies are identified in the redshift range of 0.1 1. We cross-match these clusters with those in the catalogues identified from the X-ray surveys and the Sunyaev--Zel'dovich (SZ) effect by the Planck, South Pole Telescope and Atacama Cosmology Telescope surveys, and get the redshifts for 45 X-ray clusters and 56 SZ clusters. More than 95 percent SZ clusters in the sky region have counterparts in our catalogue. We find multiple optical clusters in the line of sight towards about 15 percent of SZ clusters.

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arXiv2022

Dynamical state of galaxy clusters evaluated from X-ray images

X-ray images of galaxy clusters often show disturbed structures that are indication of cluster mergers. As a complementary to our previous work on dynamical state of 964 clusters observed by the Chandra, we process the X-ray images for 1308 clusters from XMM-Newton archival data, together with the images of 22 clusters newly released by the Chandra, and evaluate their dynamical state from these X-ray images. The concentration index $c$, the centroid shift $ω$, the power ratio $P_3/P_0$ are calculated in circular regions with a certain radius of 500 kpc, and the morphology index $δ$ is estimated within elliptical regions adaptive to the cluster size and shape. In addition, the dynamical parameters for 42 clusters previously estimated from Chandra images are upgraded based on the newly available redshifts. Good consistence is found between dynamical parameters derived from XMM-Newton and Chandra images for the overlapped sample of clusters in the two datasets. The dependence of mass scaling relations on dynamical state is shown by using data of 388 clusters.

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arXiv2020

Photometric redshifts for galaxies in the Subaru Hyper Suprime-Cam and unWISE and a catalogue of identified clusters of galaxies

We first present a catalogue of photometric redshifts for 14.68 million galaxies derived from the 7-band photometric data of Hyper Suprime-Cam Subaru Strategic Program and the Wide-field Infrared Survey Explorer using the nearest-neighbour algorithm. The redshift uncertainty is about 0.024 for galaxies of z<0.7, and steadily increases with redshift to about 0.11 at z~2. From such a large data set, we identify 21,661 clusters of galaxies, among which 5537 clusters have redshifts z>1 and 642 clusters have z>1.5, significantly enlarging the high redshift sample of galaxy clusters. Cluster richness and mass are estimated, and these clusters have an equivalent mass of M_{500} > 0.7*10^{14} Msun. We find that the stellar mass of the brightest cluster galaxies (BCGs) in each richness bin does not significantly evolve with redshift. The fraction of star-forming BCGs increases with redshift, but does not depend on cluster mass.

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arXiv2018

A sample of 1959 massive galaxy clusters at high redshifts

We identify a sample of 1959 massive clusters of galaxies in the redshift range of 0.7 0.75. By comparing them with clusters at lower redshifts, we confirm that richer clusters host more luminous brightest cluster galaxies (BCGs) also at high redshifts, and that the fraction of blue galaxies is larger in clusters at higher redshifts. A small fraction of BCGs show ongoing star formation or active nuclei. The number density profile of member galaxies in stacked samples of clusters shows no significant redshift evolution.

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arXiv2018

A catalogue of clusters of galaxies identified from all sky surveys of 2MASS, WISE, and SuperCOSMOS

We identify 47,600 clusters of galaxies from photometric data of Two Micron All Sky Survey (2MASS), Wide-field Infrared Survey Explorer (WISE), and SuperCOSMOS, among which 26,125 clusters are recognized for the first time and mostly in the sky outside the Sloan Digital Sky Survey (SDSS) area. About 90% of massive clusters of M_{500}>3*10^{14} M_{\odot} in the redshift range of 0.025<z<0.3 have been detected from such survey data, and the detection rate drops down to 50% for clusters with a mass of M_{500}~1*10^{14} M_{\odot}. Monte Carlo simulations show that the false detection rate for the whole cluster sample is less than 5%. By cross-matching with ROSAT and XMM-Newton sources, we get 779 new X-ray cluster candidates which have X-ray counterparts within a projected offset of 0.2 Mpc.

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arXiv2016

A detection of baryon acoustic oscillations from the distribution of galaxy clusters

We calculate the correlation function of 79,091 galaxy clusters in the redshift region of $z \leq 0.5$ selected from the WH15 cluster catalog. With a weight of cluster mass, a significant baryon acoustic oscillation (BAO) peak is detected on the correlation function with a significance of $3.7 σ$. By fitting the correlation function with a $Λ$CDM model curve, we find $D_v(z = 0.331) r_d^{fid}/r_d = 1261.5 \pm 48$~Mpc which is consistent with the Planck 2015 cosmology. We find that the correlation function of the higher mass sub-sample shows a higher amplitude at small scales of $r < 80~h^{-1}{\rm Mpc}$, which is consistent with our previous result. The 2D correlation function of this large sample of galaxy clusters shows a faint BAO ring with a significance of $1.8σ$, from which we find that the distance scale parameters on directions across and along the line-of-sight are $α_σ = 1.02 \pm 0.06$ and $α_π = 0.94 \pm 0.10$, respectively.

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arXiv2016

Radio luminosity function of brightest cluster galaxies

By cross-matching the currently largest optical catalog of galaxy clusters and the NVSS radio survey database, we obtain the largest complete sample of brightest cluster galaxies (BCGs) in the redshift range of 0.05<z<0.45, which have radio emission and redshift information. We confirm that more powerful radio BCGs tend to be these optically very bight galaxies located in more relaxed clusters. We derived the radio luminosity functions of BCGs from the largest complete sample of BCGs, and find that the functions depend on the optical luminosity of BCGs and the dynamical state of galaxy clusters. However, the radio luminosity function does not show significant evolution with redshift.

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arXiv2016

A Newly-Discovered Radio Halo in Merging Cluster MACS J2243.3-0935

We report the discovery of a radio halo in the massive merging cluster MACSJ2243.3-0935, as well as a new radio relic candidate, using the Giant Meterwave Radio Telescope and the KAT-7 telescope. The radio halo is coincident with the cluster X-ray emission and has a largest linear scale of approximately 0.9 Mpc. We measure a flux density of $10.0\pm 2.0$ mJy at 610 MHz for the radio halo. We discuss equipartition estimates of the cluster magnetic field and constrain the value to be of the order of 1 $μ$G. The relic candidate is detected at the cluster virial radius where a filament meets the cluster. The relic candidate has a flux density of $5.2\pm 0.8$ mJy at 610 MHz. We discuss possible origins of the relic candidate emission and conclude that the candidate is consistent with an infall relic.

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arXiv2015

The scaling relations and the fundamental plane for radio halos and relics of galaxy clusters

Diffuse radio emission in galaxy clusters is known to be related to cluster mass and cluster dynamical state. We collect the observed fluxes of radio halos, relics, and mini-halos for a sample of galaxy clusters from the literature, and calculate their radio powers. We then obtain the values of cluster mass or mass proxies from previous observations, and also obtain the various dynamical parameters of these galaxy clusters from optical and X-ray data. The radio powers of relics, halos, and mini-halos are correlated with the cluster masses or mass proxies, as found by previous authors, with the correlations concerning giant radio halos being, in general, the strongest ones. We found that the inclusion of dynamical parameters as the third dimension can significantly reduce the data scatter for the scaling relations, especially for radio halos. We therefore conclude that the substructures in X-ray images of galaxy clusters and the irregular distributions of optical brightness of member galaxies can be used to quantitatively characterize the shock waves and turbulence in the intracluster medium responsible for re-accelerating particles to generate the observed diffuse radio emission. The power of radio halos and relics is correlated with cluster mass proxies and dynamical parameters in the form of a fundamental plane.

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arXiv2015

Calibration of the optical mass proxy for clusters of galaxies and an update of the WHL12 cluster catalog

Accurately determining the mass of galaxy clusters is fundamental for many studies on cosmology and galaxy evolution. We collect and rescale the cluster masses of 1191 clusters of 0.05<z<0.75 estimated by X-ray or Sunyaev-Zeldovich measurements and use them to calibrate the optical mass proxy. The total r-band luminosity (in units of L^{\ast}) of these clusters are obtained by using spectroscopic and photometric data of the Sloan Digital Sky Survey (SDSS). We find that the correlation between the cluster mass M_{500} and total r-band luminosity L_{500} significantly evolves with redshift. After correcting for the evolution, we define a new cluster richness R_{L\ast,500}=L_{500}*E(z)^{1.40} as the optical mass proxy. By using this newly defined richness and the recently released SDSS DR12 spectroscopic data, we update the WHL12 cluster catalog and identify 25,419 new rich clusters at high redshift. In the SDSS spectroscopic survey region, about 89% of galaxy clusters have spectroscopic redshifts. The mass can be estimated with a scatter of 0.17 dex for the clusters in the updated catalog.

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