• Journal of Astronomy and Space Sciences
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• v.34 no.2
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• pp.83-97
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• 2017

We present extra-tidal features of spatial configuration of stars around three metal-poor globular clusters (NGC 6266, NGC 6273, NGC 6681) located in the Galactic bulge. The wide-field photometric data were obtained in BVI bands with the MOSAIC II camera at CTIO 4 m Blanco telescope. The derived color-magnitude diagrams (CMDs) contain stars in a total $71^{\prime}{\times}71^{\prime}$ area including a cluster and its surrounding field outside of the tidal radius of the cluster. Applying statistical filtering technique, we minimized the field star contaminations on the obtained cluster CMDs and extracted the cluster members. On the spatial stellar density maps around the target clusters, we found overdensity features beyond the tidal radii of the clusters. We also found that the radial density profiles of the clusters show departures from the best-fit King model for their outer regions which support the overdensity patterns.

• Journal of The Korean Astronomical Society
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• v.40 no.4
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• pp.171-177
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• 2007

In external galaxies, the velocity dispersion of the atomic hydrogen gas shows a remarkably flat distribution with the galactocentric radius. This has been a long-standing puzzle because if the gas velocity dispersion is due to turbulence caused by supernova explosions, it should decline with radius. After a discussion on the role of spiral arms and ram pressure in driving interstellar turbulence in the outer parts of galactic disks, we argue that the constant bombardment by tiny high-velocity halo clouds can be a significant source of random motions in the outer disk gas. Recent observations of the flaring of H I in the Galaxy are difficult to explain if the dark halo is nearly spherical as the survival of the streams of tidal debris of Sagittarius dwarf spheroidal galaxy suggests. The radial enhancement of the gas velocity dispersion (at R > 25 kpc) due to accretion of cloudy gas might naturally explain the observed flaring in the Milky Way. Other motivations and implications of this scenario have been highlighted.

• Journal of The Korean Astronomical Society
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• v.47 no.1
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• pp.1-13
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• 2014

We analyze the dependence of disk morphology (arm class, Hubble type, bar type) of nearby spiral galaxies on the galaxy environment by using local background density (${\Sigma}_n$), projected distance ($r_p$), and tidal index (T I) as measures of the environment. There is a strong dependence of arm class and Hubble type on the galaxy environment, while the bar type exhibits a weak dependence with a high frequency of SB galaxies in high density regions. Grand design fractions and early-type fractions increase with increasing ${\Sigma}_n$, $1/r_p$, and T I, while fractions of flocculent spirals and late-type spirals decrease. Multiple-arm and intermediate-type spirals exhibit nearly constant fractions with weak trends similar to grand design and early-type spirals. While bar types show only a marginal dependence on ${\Sigma}_n$, they show a fairly clear dependence on $r_p$ with a high frequency of SB galaxies at small $r_p$. The arm class also exhibits a stronger correlation with $r_p$ than ${\Sigma}_n$ and T I, whereas the Hubble type exhibits similar correlations with ${\Sigma}_n$ and $r_p$. This suggests that the arm class is mostly affected by the nearest neighbor while the Hubble type is affected by the local densities contributed by neighboring galaxies as well as the nearest neighbor.

• Journal of The Korean Astronomical Society
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• v.40 no.1
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• pp.9-16
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• 2007

We examine the morphology and luminosity distribution of a strongly warped spiral galaxy PGC 20348 by conducting a detailed BVI CCD surface photometry using BOAO 1.8m telescope. The radial surface brightness shows a break at warp radius $(r_{\omega})$ with a shallow gradient in the inner disk and a steeper gradient in the outer disk. The luminosity of east side of the disk is ${\sim}0.5$ mag fainter than the west side at r > $r_{\omega}$. The reason for the asymmetric luminosity distribution is thought to be the asymmetric flarings that result in the formation of a large diffuse region at the edge of the east disk and a smaller diffuse region at the west disk. The vertical luminosity profiles show a thick disk component whose scale heights increase with increasing galactocentric distances. The warp of PGC 20348 seems to be made by the tidal interactions with the two massive companion galaxies since the flarings and radial increase of disk scale heights are thought to be general properties of tidally perturbed disks. According to the colors of the two clumps inside the diffuse region at the edge of the east disk, they seem to be sites of active star formation triggered by tidal forces from the companion galaxies.

• Publications of The Korean Astronomical Society
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• v.30 no.2
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• pp.503-505
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• 2015

The galaxy cluster is an important object for investigating the large scale structure and evolution of galaxies. Recent wide and deep near-IR surveys provide an opportunity to search for galaxy clusters in the high redshift universe. We have identified candidate clusters of 0.8< z <1.2 from the $25deg^2$ SA22 field using an optical-near-IR dataset from merged UKIDSS DXS, IMS and CFHTLS catalogs. Using these candidates, we investigate the star forming activity of member galaxies. Consequently, at z ~1, the star forming activity of cluster galaxies is not distinguishable from those of field galaxies, which is different from members in local clusters. This means the environmental effect becomes more important for $M_{\ast}>10^{10}M_{\odot}$ galaxies at z <1.

• The Bulletin of The Korean Astronomical Society
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• v.36 no.1
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• pp.63.2-63.2
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• 2011

We present a photometric study of star clusters in three nearby barred spiral galaxies NGC 1300, NGC 1672 and NGC 6217. We use the Hubble Heritage CCD images taken with Advanced Camera for Surveys (ACS) of Hubble Space Telescope (HST) in several filters. We have selected hundreds of star clusters with V $\leq$ 24 mag in each galaxy, based on the morphological parameters and visual inspection. Most of the blue star clusters with B-V $\leq$ 0.5 are strongly concentrated in spiral arms. A significant fraction of these star clusters are distributed in ansae (the joint between spiral arms and bar structure). Some of the blue star clusters are also found in the nuclear starburst region, especially in NGC 1672. A small number of star clusters are found in the bar region. In contrast, the red star clusters with B-V < 0.5 are relatively uniformly located over the entire field of galaxy, and show some central concentration around the bulge. We discuss the physical properties of these star clusters with the expected results from simulations.

• The Bulletin of The Korean Astronomical Society
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• v.44 no.1
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• pp.56.3-56.3
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• 2019

We are currently living in the era of the wide field cosmological surveys, either spectroscopic such as Dark Energy Spectrograph Instrument or photometric such as the Dark Energy Survey or the Large Synoptic Survey Telescope. By analyzing the distribution of matter clustering, we can use the growth of structure, in combination with measurements of the expansion of the Universe, to understand dark energy or to test different models of gravity. But we also live in the era of multi-tracer or multi-messenger astrophysics. In particular, during the next decades radio surveys will map the matter distribution at higher redshifts. Like in optical surveys, there are radio imaging surveys such as continuum radio surveys such as the ongoing EMU or spectroscopic by measuring the hydrogen 21cm line. However, we can also use intensity mapping as a low resolution spectroscopic technique in which we use the intensity given by the emission from neutral hydrogen from patches of the sky, at different redshifts. By cross-correlating this maps with galaxy catalogues we can improve our constraints on cosmological parameters and to understand better how neutral hydrogen populates different types of galaxies and haloes. Creating realistic mock intensity mapping catalogues is necessary to optimize the future analysis of data. I will present the mock neutral hydrogen catalogues that we are developing, using the Horizon run 4 simulations, to cross-correlate with mock galaxy catalogues from low redshift surveys and I will show the preliminary results from the first mock catalogues.

• 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.

• Journal of The Korean Astronomical Society
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• v.44 no.6
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• pp.217-234
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• 2011

We present two large cosmological N-body simulations, called Horizon Run 2 (HR2) and Horizon Run 3 (HR3), made using $6000^3$ = 216 billions and $7210^3$ = 374 billion particles, spanning a volume of $(7.200\;h^{-1}Gpc)^3$ and $(10.815\;h^{-1}Gpc)^3$, respectively. These simulations improve on our previous Horizon Run 1 (HR1) up to a factor of 4.4 in volume, and range from 2600 to over 8800 times the volume of the Millennium Run. In addition, they achieve a considerably finer mass resolution, down to $1.25{\times}10^{11}h^{-1}M_{\odot}$, allowing to resolve galaxy-size halos with mean particle separations of $1.2h^{-1}$Mpc and $1.5h^{-1}$Mpc, respectively. We have measured the power spectrum, correlation function, mass function and basic halo properties with percent level accuracy, and verified that they correctly reproduce the CDM theoretical expectations, in excellent agreement with linear perturbation theory. Our unprecedentedly large-volume N-body simulations can be used for a variety of studies in cosmology and astrophysics, ranging from large-scale structure topology, baryon acoustic oscillations, dark energy and the characterization of the expansion history of the Universe, till galaxy formation science - in connection with the new SDSS-III. To this end, we made a total of 35 all-sky mock surveys along the past light cone out to z = 0.7 (8 from the HR2 and 27 from the HR3), to simulate the BOSS geometry. The simulations and mock surveys are already publicly available at http://astro.kias.re.kr/Horizon-Run23/.

• The Bulletin of The Korean Astronomical Society
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• v.38 no.2
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• pp.58.1-58.1
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• 2013

The main idea of the Alcock-Paczynski (AP) test is that, if we use a wrong distance-redshift relation to infer the shape of a spherical object in the Universe, this object may look non-spherical. To probe the cosmic expansion history through the AP test, the key point is to find something which is known as spherical in the Universe. We propose two possible ways applying the AP test to the large scale structure (LSS): 1) Based on the observed galaxies or quasars, one built up the beta-skeleton tracing the LSS, and investigating the inhomogeneity of the connections; 2) One reconstructs the smoothed density-contrast gradient field based on LSS observations, and investigating the inhomogeneity of the gradient vectors. Compared with some existed methods probing AP effect through 2-point correlation function, galaxy pairs, or voids, our methods have various advantages: 1) The information of both the high and low dense regions of the LSS are taken into account. 2) The redshift space distortion as the main contamination to the AP effect can be easily removed.