• Journal of The Korean Astronomical Society
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• v.35 no.2
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• pp.87-95
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• 2002

We have conducted VI CCD photometry of the open cluster NGC 6819 in order to understand the effects of dynamical evolution in old open clusters. Our photometry covers 18' $\times$ 18' on the sky, centered on the cluster, which seems to cover the whole cluster field. Our photometry reaches down to V $\approx$ 20.5, which allows us to analyze the luminosity function and spatial distribution of stars brighter than Mv $\approx$ 8.5. There is a clear evidence for mass segregation in NGC 6819, i.e., the giants and upper main-sequence stars are concentrated in the inner regions, whereas the lower main-sequence stars distribute almost uniformly throughout the cluster. The luminosity function of the main-sequence stars of NGC 6819 is almost flat. The flat luminosity function indicates that a large number of low mass stars has escaped from the cluster unless its initial mass function is much different from the Salpeter type (${\phi}(m){\propto} m^{-(1+x)},x = 1.35$).

• The Bulletin of The Korean Astronomical Society
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• v.36 no.2
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• pp.74.2-74.2
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• 2011

We probe the tidal perturbation parameter f of Virgo cluster galaxies. The goal is to measure the strength of tidal fields around individual galaxies to get better understanding gravitational processes that can affect galaxy evolution in the cluster environment. The f-value is defined as a logarithmic ratio between the net internal gravitational force within a galaxy and the external tidal force exerted by a neighboring galaxy. Hence, it provides one way to quantify the tidal field strength of galaxies, in particular, due to galaxy neighbors. In this study, we determine f-values of the VIVA galaxies, samples of the VLA Imaging study of Virgo galaxies in Atomic gas, using the Extended Virgo Cluster Catalog (EVCC) which is the most complete Virgo cluster catalog to date. With diagnostics based on the HI gas and R-band morphologies, we discuss the impact of the tidal fields on the evolution of the VIVA sample. Also, we compare the tidal field strength to the intra cluster medium (ICM) pressure for each sample galaxy to pin down environmental processes at work.

• The Bulletin of The Korean Astronomical Society
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• v.42 no.1
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• pp.52.1-52.1
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• 2017

Galaxy clusters are the most massive gravitationally bound systems and thus probably the most recent objects to form. One of promising routes to understand the assembly history of galaxy clusters is to measure observable quantities of components in clusters that are sensitive to the evolutionary state of the cluster. Recent deep observations on the nearby clusters show distinct diffuse intracluster light (ICL), that the light from stars are not bound any individual cluster galaxy, however until now this component has not been well studied due to its faint nature, with typical brightness of ~100 times fainter than the sky background. As shown in galaxy cluster simulation studies, the ICL abundance increases during various dynamical exchanges of galaxies such as the disruption of dwarf galaxies, major mergers between galaxies and the tidal stripping of galaxies. Thus, the ICL is an effective tool to measure the evolutionary stage of galaxy clusters. Moreover, the investigation of the ICL evolution mechanism will allow us understand the galaxy evolution process therein. In this pilot study, we target the Coma cluster, where the existing ICL studies are limited only in the central region. With large and uniform deep optical images from the Subaru telescope, available only recently (Okabe et al. 2014), we are developing a robust ICL measurement technique, extracting the ICL surface brightness and color profiles, which will allow us to study the origin of the ICL and its connection to the evolutionary history of the Coma cluster. For the next phase, we plan to utilize the plenty of spectroscopy data from the MMT telescope to compare ICL properties with the star formation history of the brightest cluster galaxies (BCG), and discuss the ICL formation mechanism of the Coma cluster by comparing the distribution of cluster galaxies with the distribution of diffuse light inside the Coma cluster.

• The Bulletin of The Korean Astronomical Society
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• v.40 no.1
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• pp.77.3-78
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• 2015

It is known that the galaxy evolution by direct interaction between galaxies is most active in a galaxy group. As a result, the satellite galaxies are closely related to their central galaxy in properties such as morphology, color and star formation rate (so-called 'galactic conformity'). However, it is not clear yet whether such conformity between galaxies is found in a galaxy cluster. Recently, Lee et al. (2014) have found a measurable correlation between the colors of bright galaxies and the mean colors of their faint companions in a cluster WHL J085910.0+294957 at z = 0.3, using the photometrically-selected cluster members. They suggest that such correlation may be the vestige of infallen groups in the cluster as one possibility. In order to confirm the small-scale conformity in galaxy clusters with higher reliability, we study the Virgo cluster using the Extended Virgo Cluster Catalog (EVCC). The cluster members are selected spectroscopically unlike in WHL J085910.0+294957. We examine the galactic conformity in two distinct areas of the Virgo cluster: the inner X-ray emission region and its outer region. We find a marginal conformity in color (> $2{\sigma}$ significance to bootstrap uncertainty) in the outer region, while no meaningful signal of small-scale conformity is detected in the X-ray emission region. We discuss the implication of this result, focusing on cluster mass assembly and cluster environmental effects on galaxy evolution.

• Journal of The Korean Astronomical Society
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• v.35 no.1
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• pp.29-34
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• 2002

We have conducted UBVI CCD photometry of an intermediate-age open cluster NGC 559 to investigate the effect of dynamical evolution on the stellar distributions in NGC 559. Our photometry allows better estimates of distance and age of the cluster owing to much deeper photometry (V $\le$ 21) than previous ones. It is found that the luminosity function and mass function as well as the spatial stellar distributions are affected by the dynamical evolution. Mass segregation leads to the central concentration of the high mass stars, which results in the flattened mass function inside the half mass radius.

• The Bulletin of The Korean Astronomical Society
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• v.36 no.1
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• pp.56.1-56.1
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• 2011

To understand the effect of the initial rotation for tidally bounded clusters with mass spectrum, we performed N-body simulations for the clusters with different degrees of initial rotation and compared to Fokker-Planck results. We confirmed that the cluster evolution is accelerated by the initial rotation as well as the mass spectrum. For the slowly rotating models, the time evolution of mass, energy and angular momentum show good agreements between N-body and Fokker-Planck calculations. On the other hand, for the rapidly rotating models, there are significant differences between two approaches at the beginning of the evolution. By investigating cluster shapes, we concluded that these differences are mainly due to secular instability that takes place for very rapidly rotating clusters. The shape of cluster for N-body simulations becomes tri-axial or even prolate, while the 2-dimensional Fokker-Planck simulation can treat only oblate type axisymmetric systems. We also founded that there is the angular momentum exchange from high mass to low mass.

• The Bulletin of The Korean Astronomical Society
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• v.46 no.1
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• pp.29.2-29.2
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• 2021

Intracluster light (ICL) is composed of the stars diffused throughout the galaxy cluster but does not bound to any galaxy. The ICL is a ubiquitous feature of galaxy clusters and occupies a significant fraction of the total stellar mass in the cluster. Therefore, the ICL components are believed to help understand the formation and evolution of the clusters. However, in the numerical study, one needs to perform the high-resolution cosmological hydrodynamic simulations, which require an expensive calculation, to trace these low-surface brightness structures (LSB). Here, we introduce the Galaxy Replacement Technique (GRT) that focuses on implementing the gravitational evolution of the diffused ICL structures without the expensive baryonic physics. The GRT reproduces the ICL structures by a multi-resolution cosmological N-body re-simulation using a full merger tree of the cluster from a low-resolution DM-only cosmological simulation and an abundance matching model. Using the GRT, we show the preliminary results about the evolution of the ICL in the on-going simulations for the various clusters.

• Journal of Industrial Convergence
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• v.14 no.1
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• pp.39-45
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• 2016

This paper deals with the consideration of mathematical models with regards to growth of cluster and firms by reviewing the Metcalf and Breuner's articles. prior studies have been argued the phenomenon of local industrial clusters and districts. Several concepts have been adopted to support the success of and changes to these clusters and firm growth. Through the review of two papers, evolution of both cluster and firm growth may be achieved in terms of utilizations of the different local aspects and mechanisms. This paper supports the theoretical back bone with regards to the regional cluster policy implementing in Korea for the purpose of regional developments. In particular, a mathematical model that, on a more abstract level, captures the fundamental dynamic structure of all the observed mechanisms. On the basis of this model, the emergence and evolution of local clusters can be described. Also this model has given that the knowledge sharing between firms has an important role to firms and cluster' growth.

• The Bulletin of The Korean Astronomical Society
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• v.43 no.1
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• pp.58.2-58.2
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• 2018

Fossil groups of galaxies have characteristic features of a dominant central elliptical galaxy (${\Delta}M_{12}$ > 2 in $0.5R_{vir}$) embedded in highly relaxed X-ray halo, which indicates dynamically stable and evolved systems. These are thought as a final stage of the evolution of galaxy groups in the hierarchical structure formation scenario. However, the formation and evolution of fossil clusters are still unclear due to lack of detailed studies. Therefore, we perform a kinematic research of a known fossil cluster Abell 2261 (A2261 hereafter) using spectroscopic data of 589 galaxies in the A2261 field. Even though A2261 is known as a fossil cluster, previous studies found several unusual features such as quite high X-ray entropy for a stable cluster, and an elongated shape, which are not expected in standard fossil clusters. Using the caustic method, we identify cluster member galaxies and discover a second bright galaxy (${\Delta}M_{12}=1.68$) at ${\sim}1.5R_{vir}$. The presence of such a bright galaxy can break the current fossil state of cluster in the near future. In addition, with two independent substructure finding methods, we confirm that the previously detected elongated galaxy distribution of the cluster is a real feature. These findings indicate that A2261 is not in a fully stable state, unlike the existing fossil definition diagnostic. We require a more stringent criterion for the fossil definition to represent a genuinely final stage of cluster evolution.

• The Bulletin of The Korean Astronomical Society
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• v.37 no.2
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• pp.84.2-84.2
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• 2012

We present wide-field multiband photometry of globular cluster (GC) systems toward the central region of the Fornax cluster of galaxies, including NGC 1399, NGC 1404, and NGC 1387. Observation was carried out through four optical passbands (U, B, V, and I) with the Mosaic II CCD imager mounted on the 4-m Blanco telescope at Cerro Tololo Inter-American Observatory (CTIO). This marks one of the widest U-band photometric studies on GC systems. GC candidates are selected among point sources based on their two color diagrams together with a magnitude cut. We investigate the radial variation of color distributions for the GC systems, focusing on the fundamental parameters that characterize bimodal distributions; the number ratio between blue and red GCs, the mean colors of the groups, and their color dispersions. We discuss the implication of our result regarding the origin of GC color bimodality.