• The Bulletin of The Korean Astronomical Society
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• v.40 no.1
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• pp.46.1-46.1
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• 2015

M85 is a merger remnant galaxy in the Virgo Cluster, showing complex merging features. Globular clusters in M85 are a good tracer of its merging history. To investigate globular cluster system of M85, we obtain deep and wide field images of M85 in ugi filters covering one square degree using CFHT/MegaCam. We discover about 1,000 globular cluster candidates in these images. The color distribution of the globular cluster candidates within r < 5' from M85 does not show a clear bimodality and blue globular cluster candidates are more than red ones. These features are different from those in massive early-type galaxies. The spatial distribution of the globular cluster candidates is elongated along the faint stellar light of M85. We also investigate the spatial distribution of sub-populations of the globular cluster candidates with different color and brightness and estimate their ages based on their color. We discuss these results in relation with the formation history of M85.

• The Bulletin of The Korean Astronomical Society
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• v.46 no.2
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• pp.72.1-72.1
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• 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
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• v.36 no.1
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• pp.57.1-57.1
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• 2011

It has been suggested that merging plays an important role in the formation and the evolution of early-type galaxies (ETGs). Optical-NIR color gradients of ETGs in high density environments are found to be less steep than those of ETGs in low density environments, hinting frequent merger activities in ETGs in high density environments. In order to examine if the flat color gradients are the result of dry mergers, we studied the relations between merging features, color gradient, and environments of 281 low redshift ETGs selected from Sloan Digital Sky Survey (SDSS) Stripe82. The sample contains 222 relaxed ETGs, 38 ETGs with tidal features, 10 galaxies with dust features and 11 galaxies with tidal and dust features, and Near Infrared (NIR) images are taken from UKIRT Infrared Deep Sky Survey (UKIDSS) Large Area Survey (LAS). We find that r-K color gradients of field sample galaxies are steeper than those of sample ETGs within cluster environments. For the field sample galaxies, a relatively large number of galaxies with peculiar features contribute to the steeper color gradients, while the absence of these peculiar early-type galaxies make color gradients of the cluster sample galaxies intact. In high density environment, ETGs are already evolved and relaxed, resulting flat color gradients. However, in low density environments, a majority of ETGs undergone merging recently which makes the color gradients steep.

• Publications of The Korean Astronomical Society
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• v.27 no.4
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• pp.331-334
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• 2012

We investigate the role of galaxy environment in the evolution of individual galaxies through the AKARI observations of the merging galaxy cluster A2255. MIR diagnostics using N3-S11 colors are adopted to select star-forming galaxies and galaxies in transition between star-forming galaxies and quiescent galaxies. We do not find particular enhancement of star formation rates as a function of galaxy environment, reflected in cluster-centric distance and local surface density of galaxies. Instead, the locations of intermediate MIR-excess galaxies (-1.2 < N3 - S11 < 0.2) show that star-forming galaxies are transformed into passive galaxies in the substructures of A2255, where the local surface density of galaxies is relatively high.

• Journal of The Korean Astronomical Society
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• v.38 no.2
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• pp.141-144
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• 2005

Based on optical galaxy data, we executed a systematic search for galaxy clusters around the 15 steady unidentified EGRET GeV gamma-ray sources in high Galactic-latitude sky ([b] > $30^{\circ}$). We found a strong correlation with 3.7$\sigma$ level between close cluster pairs (merging cluster candidates) and the unidentified EGRET sources, though, in contrast, no correlation with single clusters. This result implies that merging clusters of galaxies are a possible candidate for the origin of high galactic-latitude, steady unidentified EGRET gamma-ray sources.

• Journal of the Korean Institute of Intelligent Systems
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• v.14 no.3
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• pp.369-374
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• 2004

A cluster merging algorithm that merges convex clusters resulted by the Fuzzy Convex Clustering(FCC) method into non-convex clusters was proposed. This was achieved by proposing a fast and reliable distance measure between two convex clusters using Support Vector Machines(SVM) to improve accuracy and speed over other existing conventional methods. In doing so, it was possible to reduce cluster number without losing its representation of the data. In this paper, results for several data sets are given to show the validity of our distance measure and algorithm.

• The Bulletin of The Korean Astronomical Society
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• v.46 no.1
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• pp.32.2-32.2
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• 2021

Radio relics, Mpc-size elongated diffuse radio emissions found at galaxy cluster outskirts, are known as the result of shock acceleration during the cluster merger. Theories have claimed that low Mach number shocks are too inefficient to create the observed properties of radio relics. Alternative scenarios such as fossil cosmic ray electrons (CRes) from AGNs are required to explain the observations. However, how exactly the fossil CRes from AGNs can supply the Mpc-size radio relic is still an open question. In this study, we present our recent uGMRT radio observation results of the merging galaxy cluster Abell 514. We found three remarkable AGN jet tails that may have undergone multiple reorientations and extend nearly 800 kpc. Using multi-frequency data, we have performed spectral analysis along the AGN tails and track how the tails lose or gain energy as they propagate in the intracluster medium. We will discuss whether these AGN jets can provide sufficient seed CRes to radio relics.

• The Bulletin of The Korean Astronomical Society
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• v.29 no.2
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• pp.25.1-25.1
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• 2004

• Journal of the Korean Institute of Intelligent Systems
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• v.14 no.5
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• pp.517-524
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• 2004

The fuzzy set theory has been wide used in clustering of machine learning with data mining since fuzzy theory has been introduced in 1960s. In particular, fuzzy C-means algorithm is a popular fuzzy clustering algorithm up to date. An element is assigned to any cluster with each membership value using fuzzy C-means algorithm. This algorithm is affected from the location of initial cluster center and the proper cluster size like a general clustering algorithm as K-means algorithm. This setting up for initial clustering is subjective. So, we get improper results according to circumstances. In this paper, we propose a cluster merging using enhanced density based fuzzy C-means clustering algorithm for solving this problem. Our algorithm determines initial cluster size and center using the properties of training data. Proposed algorithm uses grid for deciding initial cluster center and size. For experiments, objective machine learning data are used for performance comparison between our algorithm and others.

• Journal of The Korean Astronomical Society
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• v.37 no.5
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• pp.433-438
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• 2004

Clusters of galaxies generally form by the gravitational merger of smaller clusters and groups. Major cluster mergers are the most energetic events in the Universe since the Big Bang. The basic properties of cluster mergers and their effects are discussed. Mergers drive shocks into the intracluster gas, and these shocks heat the intracluster gas. As a result of the impulsive heating and compression associated with mergers, there is a large transient increase in the X-ray luminosities and temperatures of merging clusters. These merger boost can affect X-ray surveys of clusters and their cosmological interpretation. Similar boosts occur in the strong lensing cross-sections and Sunyaev-Zeldovich effect in merging clusters. Merger shock and turbulence associated with mergers should also (re)accelerate nonthermal relativistic particles. As a result of particle acceleration in shocks and turbulent acceleration following mergers, clusters of galaxies should contain very large populations of relativistic electrons and ions. Observations and models for the radio, extreme ultraviolet, hard X-ray, and gamma-ray emission from nonthermal particles accelerated in these shocks will also be described. Gamma-ray observations with GLAST seem particularly promising.