• Journal of The Korean Astronomical Society
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• v.23 no.2
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• pp.116-121
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• 1990

It has been suggested that there could be a large number of primordial black holes which were formed in the early universe. We analyze the growth of such a primordial black hole following two different accretion rates - the Eddington accretion rate and the Bondi accretion rate - at the center of a host star like the sun. We find that a primordial black hole with M < ${\sim}10^{17}\;g$ cannot substantially grow in any case throughout the lifetime of a host star. If M > ${\sim}10^{17}\;g$, the evolution of a host star depends entirely on the mode of accretion, but it ends as a black hole in either case. Since more stars may have primordial black holes at the center of a galaxy this may result in a cluster of such black holes, and the cluster may eventually collapse to produce a single supermassive black hole.

• The Bulletin of The Korean Astronomical Society
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• v.39 no.2
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• pp.77.2-77.2
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• 2014

Questions of how our Milky Way evolves through the interaction with its environment have been constantly raised. One particularly interesting question is how the metallicity would change as our Milky Way goes through collision with HVCs. Because of the possibility of HVCs providing fuel for star formation in the Galactic disk, we simulate the collision between HVCs and the Galactic disk. More specifically, we trace how the Galactic metallicity changes throughout the process of HVCs colliding with our Milky Way based upon a specific scenario that HVCs are primordial gas left-overs from an ancient galaxy formation. Such mixing between metal-rich gas (disk) and metal-poor HVC can be traced by running numerical simulations with the FLASH code due to its capability of tracking down the abundance change of a specific element such as carbon at each time step of the hydrodynamic evolution. As for now, we give how this mixing depends on model parameters that we choose such as collision speed, initial metallicities, temperature and so on.

• The Bulletin of The Korean Astronomical Society
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• v.39 no.2
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• pp.66.2-66.2
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• 2014

The size evolution of galaxies is one of the fascinating issues in these days. It is known that the size of a galaxy is closely related to other galactic properties, such as the central surface brightness, stellar mass, and redshift. However, we need to measure the sizes of galaxies precisely to understand those relations. We determine the sizes of early-type galaxies in the nearby clusters (z<0.2) using the GALAPAGOS. The GALAPAGOS is a tool which provides surface photometry for multiple sources, which make it fast and convenient to deal with huge image data. We run the GALAPAGOS individually to the Subaru/Suprime-Cam and the HST/ACS images for the same targets. We present and discuss the result of our size measurements for further applications.

• The Bulletin of The Korean Astronomical Society
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• v.41 no.2
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• pp.56.2-56.2
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• 2016

We present chemical abundances for about 800 red giant stars in Omega Centauri, based on medium-resolution spectra obtained using Hydra multi-fiber spectrograph at the CTIO 4-m telescope. Our sample covers 14.2 < V < 15.0, and is almost unbiased against colors (and therefore metallicity). The metallicity distribution function (MDF) constructed from our data has an overall shape and local peaks that approximately match those for brighter giant stars in Johnson et al.. We also find that more metal-rich cluster members are more concentrated in the cluster center, which is consistent with previous studies. On the other hand, we find no clear evidence for such a spatial dependence with respect to alpha elemental abundance ([${\alpha}/Fe$]).

• The Bulletin of The Korean Astronomical Society
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• v.41 no.2
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• pp.40.3-41
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• 2016

Previous HI survey data have shown that the central HI gas in the Milky Way that resides within ~1.5 kpc of the Galactic Centre (GC) is tilted by ${\sim}15^{\circ}$ with respect to the Galactic plane. Although several models, such as a tilted disk model, have been suggested to interpret the observed morphology of the HI layer, it is still unknown what causes and how it preserves its tilted structure. We study the behavior of a gas disk near the GC using an N-body / SPH code. Our galaxy model includes four components; nuclear bulge, bulge, disk and halo. We construct a HI model whose radius is 1.3 kpc, scale height is 100 pc and mass is $3.6{\times}10^6M_{\odot}$. We also assume that the gas disk is initially tilted $30^{\circ}$ with respect to the Galactic plane. Here we report our simulation results and discuss the evolution of the tilted gas disk.

• Publications of The Korean Astronomical Society
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• v.25 no.3
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• pp.59-64
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• 2010

The core and global properties of the early-type ("red sequence") galaxies in the Virgo and Fornax clusters are examined using high-quality HST/ACS imaging for 143 galaxies. Rather than dividing neatly into disparate populations having distinct formation and/or evolution histories, many of the core and global properties of these galaxies show smooth and systematic variations along the galaxy luminosity function. The few examples of the rare class of compact elliptical galaxies in our sample all show properties that are strongly suggestive of tidal stripping by massive galaxies; if so, then these systems should not be viewed as populating the low-luminosity extension of so-called "normal" elliptical sequences. These results demonstrate that complete and/or unbiased samples are a pre-requisite for identifying the physical mechanisms that gave rise to the early-type galaxies we observe locally, and how these mechanisms varied with mass and environment.

• The Bulletin of The Korean Astronomical Society
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• v.37 no.1
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• pp.43.1-43.1
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• 2012

We have performed our first cosmological hydrodynamic simulation using the recently developed SPH+GOPTM code that includes radiative cooling/heating, star formation, and supernova feedback. Here we present preliminary results from the simulation $3.4{\times}10^4M_{\odot}$, thus sub-galactic structures, such as satellite galaxies and globular clusters around a host galaxy, can be resolved with more than hundred particles. We follow formation and evolution of the sub-galactic structures in view of their star formation history, merging/accretion rate, and origins.

• The Bulletin of The Korean Astronomical Society
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• v.37 no.1
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• pp.50.1-50.1
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• 2012

As a tracer of star formation history, metallicity provides crucial information for understanding galaxy evolution. In the case AGN, gas metallicity is often derived from the flux ratio of UV emission lines, i.e., NV1240 and CIV1549. To investigate the dependence of metallicity on AGN luminosity, black hole mass, and accretion rate, we measure NV1240 and CIV1549 line fluxes and derive gas metallicity of a sample of 73 local Seyfert 1 galaxies and QSOs, using archival UV spectra obtained with the HST and IUE. In this work, we will present the metallicity of local AGN and its relation with AGN properties.

• The Bulletin of The Korean Astronomical Society
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• v.42 no.2
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• pp.56.1-56.1
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• 2017

The distribution of gas on cosmological scales is vital to our understanding of galaxy formation. Using the RAMSES cosmological hydrodynamical simulation code we have explored the evolution of the gas properties in a cosmological volume. We have identified the effect of the maximum simulation force resolution, and the resolution of the initial conditions, on the gas density power spectrum, as well as artefacts due to the RAMSES algorithm. The RAMSES methodology can add spurious power on small scales, particularly in low resolution simulations. This effect can be expected to have a strong impact on the results of RAMSES simulations, because this additional power appears at specific epochs, implying a sudden change to the system.

• The Bulletin of The Korean Astronomical Society
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• v.42 no.1
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• pp.29.2-29.2
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• 2017

초기 우주의 구상성단과 같은 소규모 은하 빌딩블럭이 은하의 형성에 미치는 영향을 이해하기 위하여 우리는 완전히 새로운 개념의 화학적 진화모형을 구축하였다. 최근의 이론적 진보를 바탕으로 우리의 모델에서는 이전 모델과 달리 초신성의 폭발과 분출물이 원시 구상성단 내 잔존 가스에 아무런 영향을 끼치지 못하고 대부분 빠져나간다. 이 경우 화학적 진화는 질량이 큰 별의 윈드와 점근거성계열성의 분출물에 의해 좌우된다. 놀랍게도 우리의 모델은 오랜 난제인 구상성단 내 Na-Oxygen anticorrelation 및 다중항성종족의 기원을 자연스럽게 설명하면서 동시에 Lee, Joo, & Chung (2015) 이 최근 주장한 것처럼 은하벌지에 헬륨 함량이 매우 높은 항성이 존재할 것으로 예측한다. 이 결과는 은하의 헤일로와 벌지 형성에 매우 중요한 단서를 제공한다.