• The Bulletin of The Korean Astronomical Society
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• v.37 no.1
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• pp.39.2-39.2
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• 2012

We present our study on a comparison of dark matter halo merger history from the runs using different numerical simulation codes. To analyze the uncertainty caused by the use of different N-body calculation methods, we compare the results from two cosmological hydrodynamic simulation codes GADGET-2 and RAMSES, which use a TreePM algorithm and the Adaptive Mesh Refinement(AMR) technique respectively. We perform cosmological dark matter-only simulations with the same parameter set and initial condition for both. The dark matter halo mass functions from two simulation runs correspond well with each other, except for lower mass haloes. The discrepancy on the low-mass haloes in turn causes a notable difference in halo merger rate, especially for the case of extremely minor merger. The result from GADGET-2 predicts that most haloes undergo more number of mergers with small haloes than that from RAMSES, independent of halo mass and environment. However, in the context of the study on galaxy evolution, such extreme minor mergers generally do not have strong effects on galaxy properties such as morphology or star formation history. Hence, we suggest that this uncertainty could be quantitatively negligible, and the results from two simulations are reliable even with only minor difference in merger history.

• The Bulletin of The Korean Astronomical Society
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• v.37 no.1
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• pp.47.2-47.2
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• 2012

Using deep images taken at Maidanak 1.5m telescope, at McDonald 2.1m telescope and Canada-France-Hawaii Telescope, Dupont 2.5m telescope we investigated the fraction of merging galaxies in hosts of 39 AGN which are brighter than M = -22 mag and nearer than z = 0.3. We found that 16 to 17 of 39 AGN host galaxies show the evidence of mergers like tidal tail, shell via careful visual inspection. We also studied with the merging fraction of a control sample, SDSS Stripe82 early type galaxies of which surface brightness limit and bulge magnitude are similar to that of the AGN sample. We found that merging fraction of the AGN sample is higher than that of early type galaxy samples in the whole range of bulge magnitude. This result implies that AGN activity may be correlated with merging. We also investigated the detailed morphology of merging feature. At least - 1/4 of control samples classified as a tidal and tidal+dust are shell structures. On the other hand only one (5.9%) of AGN sample classified as merger shows shell structures, and almost all merging AGNs show tidal tail features. From point of view that tidal tail may be at the early stage of merging, and shell may be at the late stage of mergers, this result suggests that AGN might be evolved into early-type galaxies after merging.

• 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.

• The Bulletin of The Korean Astronomical Society
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• v.36 no.2
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• pp.79-79
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• 2011

Using deep images taken at Maidanak 1.5m telescope, at McDonald 2.1m telescope and Canada-France-Hawaii Telescope, we investigated the fraction of merging galaxies in hosts of 26 AGN which are brighter than M = -22.2 mag and nearer than z = 0.2. We found that 9 to 12 of 26 AGN host galaxies show the evidence of mergers like tidal tail, shell via visual inspection. We also studied with the merging fraction of a control sample, SDSS Stripe82 galaxies. Surface brightness limit and magnitude are similar to that of the AGN sample. We found that merging fraction of the AGN sample is higher than that of normal galaxy samples. This result implies that AGN activity may be correlated with merging. We also investigated the detailed morphology of merging feature. About ~1/4 of control sample classified as a tidal and tidal+dust are shell structures. On the other hand only one of the AGN sample shows shell structures. Almost all merging AGNs show tidal tail features. From point of view that tidal tail may be at the early stage of merging, and shell may be at the late stage of mergers, this result implies that AGN may be evolved into early-type galaxies after merging.

• The Bulletin of The Korean Astronomical Society
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• v.39 no.1
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• pp.38.1-38.1
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• 2014

A non-axisymmetric mass distribution in the galactic bulge (or bar) causes gas flow from the disk to the nuclear region, inducing intense star formation in the nucleus. We investigate the relation between the ellipticity of the bulge and the presence of a nuclear starburst by using a volume-limited sample of galaxies. We use 1,680 spiral galaxies with Mr < -19.5 at 0.02 <= z < 0.05 in the Sloan Digital Sky Survey Data Release 7. We find that the occurrence of nuclear starburst has a moderate correlation with bulge ellipticity in intermediate-type spiral galaxies (morphology classes Sab~Sb) in low galaxy number density environments. In high galaxy number density environments, close encounters and mergers between galaxies can cause gas inflow to the nuclear region even without the presence of non-axisymmetric bulges.

• The Bulletin of The Korean Astronomical Society
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• v.37 no.2
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• pp.71.2-71.2
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• 2012

Fly-by interactions of galaxies are hidden drivers of galaxy evolution: The impulsive encounters are by far more frequent than and thus as important as direct mergers, yet hard to identify observationally. Here we present the key characteristics of fly-bys that are examined theoretically via cosmological N-body simulations. In particular, we use the simulations generated by a particle-mesh tree code, GOTPM, and investigate the statistics of galactic fly-by interactions, which are defined by the total energy of two halos of interest being positive and their minimum distances escaping mergers. We discuss (1) the rate of fly-by interactions (the Fly-by Rate, $R_f$) as functions of(a) redshifts, (b) halo masses and mass ratios, and (c) environments, and (2) their impact on galaxy evolution in terms of morphology and star-formation rate, in comparison to that of direct mergers.

• The Bulletin of The Korean Astronomical Society
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• v.36 no.1
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• pp.59.1-59.1
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• 2011

We report the HST pixel analysis results of the interacting S0 galaxy, NGC 5195 (M51B), using the HST/ACS images in the F435W, F555W and F814W (BVI) bands. After 4x4 binning of the HST/ACS images to secure sufficient signal-to-noise ratio for each pixel, we derive several quantities describing the pixel color-magnitude diagram (pCMD) of NGC 5195, such as blue/red color cut, red pixel sequence parameters, blue pixel sequence parameters and blue-to-red pixel ratio. Those parameters reflect the internal properties of NGC 5195 like age, metallicity, dust content and galaxy morphology. To investigate the spatial distributions of stellar populations, we divide pixel stellar populations using the pixel color-color diagram and population synthesis models. As a result, we find that the tidal interaction with NGC 5194 significantly affects the stellar populations in their dust content and mean stellar age.

• The Bulletin of The Korean Astronomical Society
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• v.45 no.1
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• pp.38.1-38.1
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• 2020

Recent integral field spectroscopy observations have found that about 11% of galaxies show star-gas misalignment. The misalignment possibly results from external effects such as gas accretion, interaction with other objects, and other environmental effects, hence providing clues to these effects. We explore the properties of misaligned galaxies using Horizon-AGN, a large-volume cosmological simulation, and compare the result with the result of the Sydney-AAO Multi-object integral field spectrograph (SAMI) Galaxy Survey. Horizon-AGN can match the overall misalignment fraction and reproduces the distribution of misalignment angles found by observations surprisingly closely. The misalignment fraction is found to be highly correlated with galaxy morphology both in observations and in the simulation: early-type galaxies are substantially more frequently misaligned than late-type galaxies. The gas fraction is another important factor associated with misalignment in the sense that misalignment increases with decreasing gas fraction. However, there is a significant discrepancy between the SAMI and Horizon-AGN data in the misalignment fraction for the galaxies in dense (cluster) environments. We discuss possible origins of misalignment and disagreement. This presentation is mainly based on the published work Khim et al. 2020, ApJ, 894, 106 (17pp).

• The Bulletin of The Korean Astronomical Society
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• v.37 no.1
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• pp.41.1-41.1
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• 2012

Interactions between the galactic interstellar medium (ISM) and the intra cluster medium (ICM) are believed to be one of the main processes affecting galaxy evolution in cluster environments. The aim of our research is to study the molecular gas properties of a galaxy under the ICM pressure in the cluster environment. It has been well known 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 is being removed by ram pressure due to 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 puzzling how the star formation could decrease without stripping of dense molecular gas. To address this issue, we probe the detailed molecular gas properties of NGC 4402, located near the cluster center, as part of a study of four spiral galaxies in the Virgo Cluster. NGC 4402 is well known undergoing ram pressure stripping with a 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. Comparing the high resolution 12CO and 13CO data of NGC 4402 from the Sub Millimeter Array (SMA) with existing other wavelength data, we probe the spatial distribution and a physical condition of molecular gas under strong ICM pressure. We discuss the star formation activity might have been altered and hence how the global color of NGC4402 would change in the future.

• The Bulletin of The Korean Astronomical Society
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• v.38 no.1
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• pp.39-39
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• 2013

We present a study of correlations between the elemental abundances and galaxy morphologies of 91 blue compact galaxies (BCGs) at z=0.20-0.35 with Sloan Digital Sky Survey (SDSS) DR7 data. We classify the morphologies of the galaxies as either 'disturbed' or 'undisturbed', by visual inspection of the SDSS images, and using the Gini coefficient and M20. We derive oxygen and nitrogen abundances using the Te method. We find that a substantial fraction of BCGs with disturbed morphologies, indicative of merger remnants, show relatively high N/O and low O/H abundance ratios. The majority of the disturbed BCGs exhibit higher N/O values at a given O/H value compared to the morphologically undisturbed galaxies, implying more efficient nitrogen enrichment in disturbed BCGs. We detect Wolf-Rayet (WR) features in only a handful of the disturbed BCGs, which appears to contradict the idea that WR stars are responsible for high nitrogen abundance. Combining these results with Galaxy Evolution Explorer (GALEX) GR6 ultraviolet (UV) data, we find that the majority of the disturbed BCGs show systematically lower values of the $H{\alpha}$ to near-UV star formation rate ratio. The equivalent width of the $H{\beta}$ emission line is also systematically lower in the disturbed BCGs. Based on these results, we infer that disturbed BCGs have undergone star formation over relatively longer time scales, resulting in a more continuous enrichment of nitrogen. We suggest that this correlation between morphology and chemical abundances in BCGs is due to a difference in their recent star formation histories.