• The Bulletin of The Korean Astronomical Society
• /
• 제40권1호
• /
• pp.79.3-80
• /
• 2015

Galaxy overdensities such as galaxy clusters and superclusters are the largest gravitationally bound systems in the Universe. Since they contain many different levels of local densities, they are excellent places to test galaxy evolution models in connection to the environments. The environment studies of galaxies at z ~ 1 are important because the environmental quenching seems to be an important mechanism to reduce star formation activities in galaxies at z < 1. However, there have been not many studies about high redshift galaxy clusters at z ~ 1 because of the lack of wide and deep multi-wavelength data. We have used the multi-wavelength data from the UKIDSS DXS (J and K band), the SWIRE (4 IRAC bands), and the PAN-STARRS (g, r, i, z, y bands) in the ELAIS-N1 field. We identified galaxy cluster candidates at 0.2 < z < 1.6 using the multi-wavelength data. We found several superclusters where cluster candidates are concentrated on few tens of Mpc scale. Interestingly, some of the supercluster candidates consist of galaxy clusters which have high blue galaxy. We will present high redshift galaxy cluster and supercluster candidates in ELAIS-N1 field and galaxy properties in different environments including dense clusters and fields.

• The Bulletin of The Korean Astronomical Society
• /
• 제43권2호
• /
• pp.35.1-35.1
• /
• 2018

Bars are commonly found in disk galaxies. However, how bars form is yet unclear. There are two common pictures for the bar formation mechanism. Bars form through a physical process inherent in galaxies, or through and external process like galaxy-galaxy interaction. In this paper, we present the observational evidence that bars can form from another channel, namely a cluster-cluster interaction. We examined 105 galaxy clusters at 0.015

• The Bulletin of The Korean Astronomical Society
• /
• 제46권2호
• /
• pp.68.2-68.2
• /
• 2021

Galaxy clusters are the largest structures in the universe located at the top of the cosmological hierarchical model, so the evolution of the universe can be understood by studying clusters of galaxies. Therefore, finding a larger number of galaxy clusters plays an important role in exploring how the universe evolves. A large number of catalogs for galaxy clusters in the northern sky have been published; however, there are few catalogs in the southern sky due to the lack of wide sky survey data. KMTNet Synoptic Survey of Southern Sky(KS4) project, which observes a wide area of the southern sky about 7000 deg² with KMTNet telescopes for two years, is in progress under the SNU Astronomy Research Center. We use the KS4 multi-wavelength optical data and measure photometric redshifts of galaxies for finding galaxy clusters at redshift z<1. Currently, the KS4 project has observed approximately 50% of the target region, and a pipeline that measures photometric redshifts of galaxies has been created. When the project is completed, we expect to find more than a hundred thousand galaxy clusters, and this will improve the study of galaxy clusters in the southern sky.

• The Bulletin of The Korean Astronomical Society
• /
• 제43권1호
• /
• pp.59.2-59.2
• /
• 2018

Recent studies of galaxy clusters have shown that the galaxy clusters in dense environment tend to have lower star formation rate in local universe with z < 1. However, this correlation is not significant in galaxy clusters with z > 1. The study of galaxy clusters around z=1 can yield insight into cosmological galaxy evolution. Nevertheless, the identification of galaxy clusters beyond the scope of immediate local universe requires wide field data in optical and near-infrared bands. By incorporating data from Canada-France-Hawaii Telescope Legacy Survey(CFHTLS) and Infrared Medium-Deep Survey(IMS), the photometric redshifts of galaxies in CFHTLS W2 field were calculated. Using spatial distribution and photometric redshifts, the galaxies in the field were divided into redshift bins. The image of each redshift bin was analyzed by measuring the number density within proper distance of 1Mpc. By comparing high density regions in consecutive redshift bins, we identified the cluster candidates and mapped the large-scale structure within the CFHTLS W2 field.

• Publications of The Korean Astronomical Society
• /
• 제30권2호
• /
• pp.393-395
• /
• 2015

Weak gravitational lensing is an efficient technique for detecting galaxy clusters and probing their mass distribution. We present a weak gravitational lensing analysis of a large sample of galaxy clusters. We have built a nearly complete sample of 50 optically rich clusters, located in the redshift range 0.1 < z < 0.6 and observed in the Canada France Hawaii Telescope Legacy Survey (CFHT-LS). We used weak gravitational lensing to measure, for each galaxy cluster, the density radial profile, the total mass and the mass-to-light ratio (by comparing with the total luminosity of the member galaxies). This project is a preliminary step towards the next analysis of the weak lensing galaxy clusters in the surveys KiDS and VOICE, which are currently collecting data with the VLT Survey Telescope, in Chile.

• The Bulletin of The Korean Astronomical Society
• /
• 제46권2호
• /
• pp.42.3-43
• /
• 2021

At high redshift, unlike local, many galaxy clusters are still at their stages of building. Likewise, they show a wide range in their star formation properties: some are still forming stars actively unlike their local counterparts, while others have very low level of star formation already. Here we report the two high-redshift (z~1) galaxy clusters, confirmed via Magellan MOS observation. While existing at similar redshift and having similar mass, these two clusters show very different quiescent galaxy fraction. The origin of this difference is investigated, and will be presented in the presentation.

• The Bulletin of The Korean Astronomical Society
• /
• 제46권2호
• /
• pp.72.1-72.1
• /
• 2021

Galaxy mergers are known to have been one of the main drivers in galaxy evolution in a wide range of environments. However, in galaxy clusters, high-speed encounters have been believed to undermine the role of mergers as a driver in galaxy evolution. Nonetheless, a high fraction (~38% in Sheen et al. 2012 and ~20% in Oh et al. 2018) of galaxies with post-merging features have been reported in deep (>~28 mag/arcsec2) optical surveys of cluster galaxies. The authors argue that these galaxies could have merged outside of the cluster and, later, fallen into the cluster, sustaining their long-lasting post-merging features. On the other hand, when galaxy clusters interact, galaxy orbits might be destabilized resulting in a higher galaxy merger rate. To test this idea, we measure the ongoing-merger fraction of galaxies in deep DECam mosaic data of seven Abell clusters (A754, A2399, A2670, A3558, A3574, A3659 and A3716) with a variety of dynamical states (0.016

• The Bulletin of The Korean Astronomical Society
• /
• 제36권2호
• /
• pp.144.2-144.2
• /
• 2011

A dwarf irregular galaxy IC10 in the Local Group is the nearest starburst galaxy, playing an important role revealing the details of starburst. It is located close to the Galactic plane so that it suffers from severe foreground reddening. Therefore much less is known about the property of this galaxy compared with other galaxies in the Local Group. So are star clusters in this galaxy. We present a photometric study of the star clusters in IC10. 57 star clusters are already found from HST images in previous studies, and we newly found 15 star clusters using Local Group Survey data and SUBARU/Suprime-Cam data. We derive UBVRI integrated photometry of these star clusters from the images from Local Group Survey data and JHKs photometry taken with SUBARU/MOIRCS. Then we derive age and mass of these clusters using the spectral energy distribution fitting with the simple stellar population models. We discuss the photometric and physical properties of these star clusters and its implication.

• The Bulletin of The Korean Astronomical Society
• /
• 제46권2호
• /
• pp.67.4-68
• /
• 2021

In studying the dynamical evolution of galaxy clusters, one intriguing approach is to compare the spatial distributions of various components, such as the dark matter, the member galaxies, the gas, and the intracluster light (ICL; the diffuse light from stars, which are not bound any individual cluster galaxy). If we find a visible component whose spatial distribution coincides with the dark matter distribution, then we could draw a dark matter map without requiring laborious weak lensing analysis. Furthermore, if the component traces the dark matter distribution better for more relaxed galaxy cluster, we could use the similarity as a dynamical stage estimator of the galaxy cluster. We present a novel new methodology to quantify the similarity of two or more 2-dimensional spatial distributions. We apply the method to a sample of galaxy clusters at different dynamical stages simulated within N-cluster Run, which is an N-body simulation using the galaxy replacement technique. Among the various components (stellar particles, galaxies, ICL), the velocity defined ICL+ brightest cluster galaxy (BCG) component traces the dark matter best. Between the sample galaxy clusters, the relaxed clusters show stronger similarity of the spatial distribution between the dark matter and ICL+BCG than the dynamically young clusters.

• Journal of The Korean Astronomical Society
• /
• 제38권3호
• /
• pp.345-355
• /
• 2005

We present a photometric study of the star cluster system in the merging galaxy NGC 1487, based on the BI photometry obtained from the F450W and F814W images in the HST /WFPC2 archive data. We have found about 560 star cluster candidates in NGC 1487, using the morphological parameters of the objects. We have investigated several photometric characteristics of the clusters: color-magnitude diagrams (CMDs), color distribution, spatial distribution, age, size and luminosity function. The CMD of the bright clusters with 18.5 < B < 24 mag in NGC 1487 shows three major populations of clusters: a blue cluster population with $(B-I){\le}0.45$, an intermediate-color cluster population with $0.45<(B-I){\le}1.55$, and a red cluster population with (B - I) > 1.55. The intermediate-color population is the most dominant among the three populations. The brightest clusters in the blue and intermediate- color populations are as bright as $B{\approx}18mag$ ($M_B{\approx}-12mag$), which are three magnitudes brighter than those in the red population. The blue and intermediate-color clusters are strongly concentrated on the bright condensations, while the red clusters are relatively more scattered over the galaxy. The CMD of these clusters is found to be remarkably similar to that of the clusters in the famous interacting system M51. From this we suggest that the intermediate-color clusters were, probably, formed during the merging process which occurred about 500 Myrs ago.