• The Bulletin of The Korean Astronomical Society
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• v.41 no.2
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• pp.30.1-30.1
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• 2016

We have estimated a metallicity map of the Large Magellanic Cloud (LMC) using the Magellanic Cloud Photometric Survey (MCPS) and Optical Gravitational Lensing Experiment (OGLE III) photometric data. This is a first of its kind, high-spatial resolution map of metallicity up to a radius of $4^{\circ}-5^{\circ}$, derived using large area photometric data and calibrated using spectroscopic data of Red Giant Branch (RGB) stars. The RGB is identified in the V, (V - I) colour- magnitude diagrams of small subregions of varying sizes in both data sets. The slope of the RGB is used as an indicator of the mean metallicity of a subregion, and it is calibrated to metallicity using spectroscopic data for field and cluster red giants in selected subregions. The mean metallicity of the LMC is found to be [Fe/H] = -0.37 dex (${\sigma}[Fe/H]=0.12$) from MCPS data, and [Fe/H] = -0.39 dex (${\sigma}[Fe/H]=0.10$) from OGLE III data. The bar is found to have an uniform and higher metallicity compared to the disk, and is indicative of an active bar in the past. Both the data sets suggest a shallow radial metallicity gradient up to a radius of 4 kpc ($-0.049{\pm}0.002$ dex kpc-1 to $-0.066{\pm}0.006$ dex kpc-1). This metallicity gradient of the LMC disk, though shallow, resembles the gradient seen in spiral galaxies, and similar to that found in our Galaxy.

• The Bulletin of The Korean Astronomical Society
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• v.41 no.2
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• pp.39.1-39.1
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• 2016

We report the discovery of a kiloparsec-size supershell in the outskirts of the Milky Way with the compact high-velocity cloud, HVC 040+01-282 (hereafter, CHVC040), at its geometrical center using the "Inner-Galaxy Arecibo L-band Feed Array" HI 21 cm survey data. Supershells are large gaseous shells, which could be produced by one of most energetic activities with an explosion energy more than $3{\times}1052erg$. The most promising origin is the explosion of multiple supernovae in OB associations, or alternatively, the impact of HVCs falling into the Galactic disk. We found the association between CHVC040 and the Galactic supershell by analysis of their morphological and physical properties. Our results imply that some compact HVCs can survive their trip through the Galactic halo and inject energy and momentum into the Milky Way disk.

• The Bulletin of The Korean Astronomical Society
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• v.35 no.2
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• pp.82.1-82.1
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• 2010

We have investigated the spatial configuration of stars within the tidal radius of metal poor globular cluster NGC 6626 in the bulge direction. Data were obtained in near-IR J,H,Ks bands with wide-field ($20'\times20'$) detector, WIRCam at CFHT. To trace the stellar density around target cluster, we sorted cluster's member stars by using a mask filtering algorithm and weighting the stars on the color-magnitude diagram. From the weighted surface density map, we found that the stellar spatial distributions within the tidal radius appear asymmetric and distorted features. Especially, we found that more prominent over-density features are extending toward the direction of Galactic plane rather than toward the directions of the Galactic center and its orbital motion. This orientation of the stellar density distribution can be interpreted with result of disk-shock effect of the Galaxy that the cluster had been experienced. Indeed, this over-density feature are well represented in the radial surface density profile for different angular sections. As one of the metal poor globular clusters with extended horizontal branch (EHB) in the bulge direction, NGC 6626 is kinematically decoupled from the normal clusters and known to have disk motion of peculiar motion. Thus, our result will be able to add further constraints to understand the origin of this cluster and the formation of bulge region in early universe.

• The Bulletin of The Korean Astronomical Society
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• v.44 no.1
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• pp.34.2-34.2
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• 2019

Several observational results show a tighter pitch angle at wavelengths of optical and near-infrared than those that are associated with star formation, which is in agreement with the prediction of the density wave theory. In my recent numerical studies, the dependence of the shock positions relative to the potential minima is due to the tendency that stronger shocks form farther downstream. This causes a systematic variation of the perpendicular Mach number, with radius and makes the pitch angle of the gaseous arms smaller than that of the stellar arms, which supports the prediction of the density-wave theory, independently. However, some observations still give controversial results which show similar pitch angles at wavelengths, and there is no statistical study comparing observations and numerical models directly. By analyzing optical image of disk galaxies in the Carnegie-Irvine Galaxy Survey (CGS), I measured the physical values of stellar and gaseous arms such as their strength, length, and pitch angles. For direct comparison with numerical results, I analyzed more than 30 additional numerical models with varying the initial parameters in model galaxies. In this talk, I will present results both of observational and numerical samples and discuss the physical properties of spiral structures based on the density-wave theory.

• Journal of Astronomy and Space Sciences
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• v.20 no.3
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• pp.175-184
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• 2003

To investigate the SEDs of galaxies considering the dust extinction processes in the galactic disks, we present the population synthesis models for normal galaxies with dusty disks. We use PEGASE (Fioc & Rocca-Volmerange 1997) to model them with standard input parameters for stars and new dust parameters. We find that the model results are strongly dependent on the dust parameters as well as other parameters (e.g. star formation history). We compare the model results with the observations and discuss about the possible explanations. We find that the dust opacity functions derived from studies of asymptotic giant branch stars are useful for modeling a galaxy with a dusty disk.

• The Bulletin of The Korean Astronomical Society
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• v.35 no.1
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• pp.81.2-81.2
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• 2010

The origin of galactic angular momentum is commonly explained as a result of tidal torques of neighbouring protogalaxies on the forming galactic halo. In this context, the environment plays a preponderant role establishing the total angular momentum of present day galaxies. For the last four decades, most of the observational studies focused their attention on the spatial orientation of galaxies in filaments, groups or clusters, leaving behind the magnitude of the angular momentum. We have implemented a simple model to account for the spin of disk galaxies that allow us to obtain an estimate for any galaxy requiring a minimum of information. Applying this method to a sample of galaxies extracted from the Sloan Digital Sky Survey, we have been studying angular momentum distributions of galaxies in different environments. In this talk I will present some results for galaxies immersed in different environments, spanning three orders of magnitude in environmental density, galaxies having nearby companions and clustered galaxies.

• The Bulletin of The Korean Astronomical Society
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• v.35 no.1
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• pp.73.1-73.1
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• 2010

We use three-dimensional hydrodynamic simulations to investigate the characteristics of turbulence driven in rotating, vertically-stratified disk. Our models are isothermal, and local in the in-plane direction while global in the vertical direction. We allow localized regions with density larger than the threshold value to explode and inject kinetic energy to the surrounding medium in the real space rather than Fourier space, mimicking supernova explosions thought to be the dominant turbulence source. This work extends our previous study where we studied turbulence in a non-rotating, uniform environment. We find that the galaxy rotation does not make a significant difference in the turbulence level at saturation, since the associated shear velocity is much smaller than the explosion velocity. We analyze the properties of turbulence in our models and compare them with those from the uniform-density models. We also discuss the astrophysical implication of our findings.

• The Bulletin of The Korean Astronomical Society
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• v.35 no.1
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• pp.56.2-56.2
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• 2010

M87, a cD galaxy in the Virgo cluster, has 3-10 times larger enclosed mass than the Milky Way at any given galactocentric radius. Thus the globular cluster (GC) system in M87 is expected to have undergone a more significant dynamical evolution than that of the Milky Way if it had started from the same initial mass function (MF) and radial distribution (RD) as the Milky Way. The evolution of MF and RD of the M87 GC system has been calculated using an advanced, realistic Fokker-Planck (FP) model that considers dynamical friction, disk/bulge shocks, and eccentric cluster orbits. We perform hundreds of FP calculations with different initial cluster conditions, and then search a wide parameter space for the best-fit initial GC MF and RD that evolves into the observed present-day GC MF and RD. We also find best-fit initial MFs and RDs for blue and red GC groups, separately.

• Journal of The Korean Astronomical Society
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• v.34 no.4
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• pp.341-343
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• 2001

Here were continue the MHD study started by Santillan et al (1999) for the interaction of high-velocity clouds (HVCs) with the magnetized thick gaseous disk of our Galaxy. We use the MHD code ZEUS-3D and perform 3D-numerical simulations of this interaction, and study the formation of head-tail structures in HVCs. Our results show that clouds located above 2 kpc from mindplane present velocity and column density gradients with a cometary structure that is similar to those observed in 21 cm emission

• The Bulletin of The Korean Astronomical Society
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• v.36 no.2
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• pp.57.2-57.2
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• 2011

To study the formation and evolution of sub-galactic scale structures, we have added SPH (Smoothed Particle Hydrodynamics) method into an existing cosmological PMTree code, GOTPM. To follow the evolution of gas particles, we consider heating/cooling processes, star formation, and energy & metal feedback by supernova explosion. We have performed various tests for the new code and found that the results reproduce observed quantities or follow the known analytic solutions. We present a test simulation of isolated disk galaxy with a focus on whether the star formation reproduces the observed features.