• 천문학회보
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• 제40권1호
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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.

• 천문학회보
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• 제46권2호
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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

• 천문학회보
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• 제36권1호
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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.

• 천문학논총
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• 제27권4호
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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.

• 천문학회지
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• 제38권2호
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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.

• 한국지능시스템학회논문지
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• 제14권3호
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• pp.369-374
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• 2004

Convex한 클러스터간의 최적의 거리와 Fuzzy Convex Clustering(FCC) 방법에 의한 효과적인 클러스터 결합 알고리즘을 제시하였다. 또한 두 convex한 클러스터간의 거리 측정 방법의 문제점인 정확성과 수행속도 개선하기 위하여 Support Vector Machines(SVM) 을 이용한 빠르고 정확한 거리 측정 방법을 제시하였다. 따라서 데이터의 부적절한 표현 없이 클러스터들의 개수를 크게 더 줄일 수 있었다. 본 논문에서는 제시한 알고리즘의 타당성을 위하여 여러 데이터에 대한 실험결과를 보여주므로서 제시한 알고리즘을 실제 영상 분할에 적용하여 다른 클러스터링 방법의 결과와 비교분석한다.

• 천문학회보
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• 제46권1호
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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.

• 천문학회보
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• 제29권2호
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• pp.25.1-25.1
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• 2004

• 한국지능시스템학회논문지
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• 제14권5호
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• pp.517-524
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• 2004

1960년대 퍼지 이론이 소개된 이후 데이터 마이닝을 포함한 기계 학습 분야의 군집화 작업에서 퍼지 이론이 폭넓게 사용되었다. 퍼지 C-평균 알고리즘은 가장 많이 사용되는 퍼지 군집화 알고리즘이다. 이 알고리즘은 하나의 데이터 개체가 서로 다른 소속 정도를 가지고 각 군집에 할당될 수 있도록 한다. 퍼지 C-평균 알고리즘도 K-평균 알고리즘과 같은 일반적인 군집화 알고리즘과 마찬가지로 초기 군집수와 군집 중심의 위치에 의해 최종 군집 결과의 성능 차이가 나타난다. 군집화를 위한 이러한 초기 설정은 주관적이며 이 때문에 적절치 못한 결과를 얻게 될 수도 있다. 본 논문에서는 이 문제를 해결할 수 있는 방법으로 주어진 학습 데이터의 속성을 기반으로 한 초기 군집수와 군집 중심을 결정하는 개선된 밀도 기반의 퍼지 C-평균 알고리즘을 제안하였다. 제안 방법은 격자를 사용하여 초기 군집 중심의 위치와 군집수를 결정하였다. 기존에 많이 이용되었던 객관적인 기계 학습 데이터를 이용하여 제안 알고리즘의 성능비교를 수행하였다.

• 천문학회지
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• 제37권5호
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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.