• Publications of The Korean Astronomical Society
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• v.7 no.1
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• pp.231-253
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• 1992

A model for the distribution of stars in the disk and the spheroid of our Galaxy is reexamined from an edge-on view of the Galaxy obtained by selecting infrared sources from the IRAS Point Source Catalog. The sources are counted as a function of galactic latitude. longitude and $12{\mu}m$ apparent magnitude. The source counts are reasonably separated into the disk component and the spheroid component contributions and each of the contributions is further interpreted as a convolution of a spatial density distribution and a luminosity function based on the least-square fit method. The spatial density of the disk component has an exponential radial scale length of $h_R{\sim}2.6\;kpc$ and the vertical distribution follows a canonical $sech^2$ law with a scale height $h_z{\sim}240\;pc$. The distribution of the spheroid component can be represented by an oblate spheriod with an axis ratio $k{\sim}0.61$ and a de Vaucouleurs' $r^{1/4}$ law with an effective radius of $R_e{\sim}120\;pc$. The steep density gradient of the spheroid component is consistent with that of late M giants in the central bulge. The luminosity functions of the disk and the spheroid component stars resemble respectively those of the K luminosity function of disk M giants (Garwood and Jones 1986) and the bolometric luminosity function of M giants in bulge fields (Frogel et al, 1990).

• The Bulletin of The Korean Astronomical Society
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• v.43 no.2
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• pp.35.3-36
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• 2018

One of the most prevalent knowledge about disk galaxies, which dominate the population of the local Universe, is that they consist of stellar structures with different kinematics, such as thin disk, bulge, and halo. Therefore, investigating when and how these components develop in a galaxy is the key to understanding the evolution of galaxies. Using the NewHorizon simulation, we can resolve the detailed structures of galaxies, in the field environment, from the early Universe where star formation and mergers were most active. We first decompose stellar particles in a galaxy into a disk and a dispersion-dominated, spheroidal, component based on their orbits and then see how these components evolve in terms of mass and structure. At high redshift z~3, galaxies are mostly dispersion-dominated as stars are formed misaligned with the galactic rotational axis. At z=1~2, massive galaxies start to dominantly form disk stars, while less massive galaxies do much later. Furthermore, massive galaxies are forming thinner and larger disks with time, and the preexistent disks are heated or even disrupted to become a part of dispersion-dominated component. Thus, the mass growth of spheroidal components at later epochs is dominated by disrupted stars with disk origins and accreted stars at large radii.

• The Bulletin of The Korean Astronomical Society
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• v.44 no.2
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• pp.73.1-73.1
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• 2019

We investigate the evolution of the galactic spin of spiral galaxies in various dynamical situations using the N-body/SPH simulations. To do this we first construct a Milky Way-like galaxy model. Then we perform both prograde and retrograde encounters between the spiral galaxy pair. We also conduct a simulation with our galaxy model in isolation for comparison. We find that the circular motion of the disk stars in the inner region of the galaxy decrease clearly when the galaxy experiences strong prograde interactions. Such decrease has not found when the galaxy experiences weak or no interactions. We compare our simulation results with recent observational studies on the galactic spins.

• The Bulletin of The Korean Astronomical Society
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• v.42 no.2
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• pp.46.1-46.1
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• 2017

Late-type galaxies falling into a cluster would evolve being influenced by the interactions with both the cluster and the nearby cluster member galaxies. Most numerical studies, however, tend to focus on the effects of the former with little work done on those of the later. We thus perform numerical study on the evolution of a late-type galaxy falling radially toward the cluster center interacting with neighbouring early-type galaxies, using N-body, hydrodynamical simulations. Based on the information about the typical galaxy encounters obtained by using the galaxy catalog of Coma cluster, we run the simulations for the cases where a Milky Way Galaxy-like late-type galaxy, flying either edge-on or face-on, experiences six consecutive collisions with twice more massive early-type galaxies having hot gas in their halos. Our simulations show that the evolution of the late-type galaxy can be significantly affected by the high-speed multiple collisions with the early-type galaxies, such as on the cold gas content and the star formation activity, particularly through the hydrodynamic interactions between the cold disk and the hot gas halos. By comparing our simulation results with those of others, we claim that the role of the galaxy-galaxy interactions on the evolution of late-type galaxies in clusters could be comparable with that of the galaxy-cluster interactions, depending on the dynamical history.

• Journal of The Korean Astronomical Society
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• v.44 no.1
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• pp.23-32
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• 2011

We present a deep CCD imaging in B and V bands which allows us to analyze the vertical structure of NGC 4631. We derive the scale heights of the thin and thick disks at a variety of positions along the major axis of the disk. The scale heights of the thin disk are nearly constant while those of the thick disk tend to increase with increasing galactocentric distance. The mean scale heights of the thin disk derived from B and V images are similar to each other (~ 450 pc). Instead, those of the thick disk show a strong east-west asymmetry which is caused by the diffuse stellar emission that is most prominent in the north west regions above the disk plane. The ratio of scale heights ($z_{thick}/z_{thin}$) is about 2.5 in the east side of the disk. However, this ratio is greater than 4 for the thick disk above the disk plane in the west side of the galaxy.

• The Bulletin of The Korean Astronomical Society
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• v.46 no.1
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• pp.28.2-28.2
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• 2021

We investigate the impact of ram pressure stripping due to the intracluster medium (ICM) on star-forming disk galaxies with a multiphase interstellar medium maintained by strong stellar feedback. We carry out radiation-hydrodynamic simulations of an isolated disk galaxy embedded in a 1011 M⦿ dark matter halo with various ICM winds mimicking the cluster outskirts (moderate) and the central environment (strong). We find that both star formation quenching and triggering occur in ram pressure-stripped galaxies, depending on the strength of the winds. HI and H2 in the outer galactic disk are significantly stripped in the presence of moderate winds, whereas turbulent pressure provides support against ram pressure in the central region, where star formation is active. Moderate ICM winds facilitate gas collapse, increasing the total star formation rates by ~40% when the wind is oriented face-on or by ~80% when it is edge-on. In contrast, strong winds rapidly blow away neutral and molecular hydrogen gas from the galaxy, suppressing star formation by a factor of 2 within ~200 Myr. Dense gas clumps with nH≳10 M⦿ pc-2 are easily identified in extraplanar regions, but no significant young stellar populations are found in such clumps. In our attempts to enhance radiative cooling by adopting a colder ICM of T=106K only a few additional stars are formed in the tail region, even if the amount of newly cooled gas increases by an order of magnitude.

• Journal of The Korean Astronomical Society
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• v.29 no.1
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• pp.63-73
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• 1996

The previous study of chemical evolution of the Galaxy is extended to the radial properties of the Galactic disk. The present model includes radial dependency of the time-dependent bimodal IMF, radial flow of material in the disk, and the change of type I supernova explosion rate with radial distance from the disk center as model parameters and observed gas and stellar density distributions and metallicity abundance gradient as observational constraints. The results of two models in this study explain the observed gas and stellar density distributions well, with the slope of the gas density gradient in the region of 4.5 kpc$Y_1$ and -0.123dex/kpc in model $Y_2$, respectively, which fit well the observed gradient of -0.l1dex/kpc. The abundance gradient reproduced in model $Y_1$ is getting flatter with decreasing radius, while that in model $Y_2$ is getting steeper, which fits better the observed abundance gradient. This result shows the necessity of exponentially increasing type I supernova explosion rate with decreasing radius in order to explain the observed abundance gradient in the disk. The fitness of observed density distribution and star formation rate distribution justifies the reliability of time-dependent bimodal IMF as a compound quantitative chemical evolution model of the Galaxy. The temporal variations of metallicity gradients for carbon, nitrogen and oxygen are also shown.

• The Bulletin of The Korean Astronomical Society
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• v.36 no.2
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• pp.117.2-117.2
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• 2011

We probe 12CO J=2-1 and 13CO J=1-0 properties of a Virgo disk galaxy, NGC 4402 which is located near the cluster center. Our goal is to study the impact of intra cluster medium (ICM) on the molecular gas of a galaxy in the cluster environment. It has been believed that cluster galaxies are deficient in atomic hydrogen gas (HI gas) compared to their field counterparts and now there is much evidence that low density ISM can be easily removed by ram pressure caused by ICM wind. Meanwhile, no significant molecular gas deficiency of the cluster galaxy population has been found yet they show overall lower star formation rate than galaxies in the field, and it is still controversy whether dense ISM can be also stripped by the ICM wind or not. NGC 4402 with truncated HI disk($D_{HI}/D_{opt}$ ~ 0.75 and only 36%of HI gas compare to field galaxies of a similar size) and a disturbed gas morphology, appears to have strong ongoing ram pressure. Using high resolution 12 and 13CO data of NGC 4402 from a Sub Millimeter Array (SMA), we probe the molecular gas properties under strong ICM pressure. We discuss how its star formation activity and hence the global color of NGC4402 would be changed in the future.

• Journal of The Korean Astronomical Society
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• v.24 no.2
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• pp.151-159
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• 1991

Using the photographic imagery of the SO galaxy NGC 5102, the equivalent luminosity profile and the fractional integrated luminosity curve have been derived. Several photometric parameters of the galaxy, such as a total magnitude ($B_T$), equivalent radius ($r^*_e$), effective surface brightness (${\mu}^*_e$), and concentration indices ($C_{21}$, $C_{32}$) were derived from the luminosity profiles. According to the decomposition method from the nonlinear least squares fitting, photometric parameters of the bulge (${\mu}_e$, $r_e$) and the exponential disk (${\mu}(0)$, ${\alpha}^{-1}$), and the bulge to total luminosity ratio (B/T) were obtained. The derived central disk surface brightness (${\mu} (0)\;=\;22.06\;{\pm}\;0.18\;mag/\Box"$) and the evaluated B/T ratio (= 0.52) of the NGC 5102 are close to the mean values (${\mu} (0)\;=\;21.65\;{\pm}\;0.3\;mag/\Box"$, B/T=0.63) of the SO galaxies. Analysis showed that a lens-like hump is embedded in the equivalent luminosity profile of the NGC 5102.

• The Bulletin of The Korean Astronomical Society
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• v.43 no.2
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• pp.32.1-32.1
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• 2018

The galactic warp is almost ubiquitous among disk galaxies and suspected to be an imprint of recent interactions with other galaxies. The detailed evolutionary course, however, is still uncertain due to the lack of observational evidence. To address this issue, we construct a new extensive catalog of 412 conspicuously warped disks at z = 0.01 ~ 0.05, based on SDSS DR7. We classify the warp morphology through a visual inspection from the Zooniverse Project and our new automated scheme for the warp measurement. We find an interesting color difference between S-and U-shaped warps. The U-type warp galaxies exhibits considerable color offset towards blue compared to both the S-type warps and the control sample of un-warped galaxies. The effect is even more pronounced for galaxies (a) with the greater warp amplitude and (b) with lower luminosity. This is the first piece of observational evidence that the S- and U-shaped warps are on different evolutionary phases in terms of not only dynamics but stellar populations as well. We discuss the implications in the context of the warp evolution theory.