• The Bulletin of The Korean Astronomical Society
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• v.44 no.2
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• pp.38.2-38.2
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• 2019

In the history of universe, galaxies are consistently affected by surrounding medium and neighbor galaxies. These effects control galaxy evolution, making properties of galaxies diverse and dependent on environments. We investigate environments of various types of galaxies and how they affect galaxy properties, such as bar structures and galaxy sizes, etc. First, we present the observational evidence that bars can form from a cluster-cluster interaction. The evidence indicates that bars can form due to a large-scale violent phenomenon, and cluster-cluster interaction should be considered as an important channel for bar formation. Second, we discover for the first time that local early-type galaxies heavier than 1011.2 Msol show a clear environmental dependence in mass-size relation, in such a way that galaxies are as much as 20 - 40% larger in densest environments than in underdense environments. This result suggests that mergers played a significant role in the growth of massive galaxies in dense environments as expected in theory. Lastly, we investigate environments of the most massive galaxies and extremely massive quasars. By doing so, we find that massive galaxies are a much better signpost for galaxy clusters than massive quasars.

• The Bulletin of The Korean Astronomical Society
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• v.38 no.2
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• pp.36-36
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• 2013

Globular clusters (GCs) are among the oldest stellar objects in the universe and provide valuable constraints on many aspects of galaxy evolution. GC systems typically exhibit bimodal color distributions the phenomenon of which has been a major topic in the area of GC research. GC color bimodality established a paradigm where scenarios to explain its origin require two GC groups with different formation origins. The GC division, asserted mainly by photometric color bimodality so far, has been viewed as the presence of two distinct metallicity subgroups within individual galaxies. In this study, we make use of spectroscopy of GC systems associated with two giant galaxies, M31 (the Andromeda) and M87 (NGC 4486), to investigate the GC bimodality and the underlying metallicity distributions. Recent spectroscopy on the globular cluster (GC) system of M31 with unprecedented precision witnessed a clear bimodality in absorption-line index distributions of old GCs. Given that spectroscopy is a more detailed probe into stellar population than photometry; the discovery of index bimodality may point to the very existence of dual GC populations. However, here we show that the observed spectroscopic dichotomy of M31 GCs emerges due to the nonlinear nature of metallicity-to-index conversion and thus one does not necessarily have to invoke two separate GC subsystems. We present spectra of 130 old globular clusters (GCs) associated with the Virgo giant elliptical galaxy M87, obtained using the Multi-Object Spectrography (MOS) mode of Faint Object Camera and Spectrograph (FOCAS) on the Subaru telescope. M87 GCs with reliable metallicity measurements exhibit significant inflection along the color-metallicity relations, through which observed color bimodality is reproduced from a broad, unimodal metallicity distribution. Our findings lend further support to this new interpretation of the GC color bimodality, which could change much of the current thought on the formation of GC systems and their host galaxies.

• The Bulletin of The Korean Astronomical Society
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• v.43 no.2
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• pp.40.2-40.2
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• 2018

NGC 5024 and NGC 5053 are among the most metal-poor globular clusters in the Milky Way. Both globular clusters are considered to be accreted from dwarf galaxies (like Sagittarius dwarf galaxy or Magellanic clouds), and common stellar envelope and tidal tails between globular clusters are also detected. We present a search for extra-tidal cluster member candidates around these globular clusters from APOGEE survey data. Using 20 chemical elements (e.g., Fe, C, Mg, Al) and radial velocities, t-distributed stochastic neighbour embedding (t-SNE), which identifies an optimal mapping of a high-dimensional space into fewer dimensions, was explored, and we find that globular cluster stars are well separated from the field stars in 2-dimensional map from t-SNE. We also find that some stars selected in t-SNE map are placed outside of the tidal radius of the clusters. The proper motion of stars outside tidal radius is also comparable to that of globular clusters, which suggest that these stars are tidally decoupled from the globular clusters. We manually measure chemical abundances for the clusters and extra-tidal stars, and discuss the association of extra-tidal stars with the clusters.

• The Bulletin of The Korean Astronomical Society
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• v.45 no.1
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• pp.64.1-64.1
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• 2020

Galaxy groups are the place where many galaxies feel the impact of the surroundings (e.g., merging, tidal interaction, ram pressure stripping) before joining bigger structures like (sub)clusters. A significant fraction of galaxies is quenched in the group environment. Such "pre-processing" of galaxies in groups is likely to affect galaxy evolution tremendously. To better understand how environmental processes in galaxy groups affect molecular gas, star formation activity, and galaxy evolution, we carried out CO imaging observations of group galaxies, using the Atacama Compact Array (ALMA/ACA). We selected all the targets that have been detected in the GEMS-HI survey for two groups, making the sample of 40 galaxies (18 galaxies in IC 1459 group and 22 galaxies in NGC 4636 group). Our ALMA/ACA observation is the first CO imaging survey for two groups. In this work, we present CO images of group galaxies, together with their star formation maps and HI images. Our ACA CO data show the asymmetric distribution of molecular gas in some of our samples. We discuss the impact of the group environment on molecular gas and star formation activity.

• Journal of The Korean Astronomical Society
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• v.48 no.1
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• pp.21-55
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• 2015

If the Universe is dominated by cold dark matter and dark energy as in the currently popular ${\Lambda}CDM$ cosmology, it is expected that large scale structures form gradually, with galaxy clusters of mass $M{\geq}10^{14}M_{\odot}$ appearing at around 6 Gyrs after the Big Bang (z ~ 1). Here, we report the discovery of 59 massive structures of galaxies with masses greater than a few times $10^{13}M_{\odot}$ at redshifts between z = 0.6 and 4.5 in the Great Observatories Origins Deep Survey fields. The massive structures are identified by running top-hat filters on the two dimensional spatial distribution of magnitude-limited samples of galaxies using a combination of spectroscopic and photometric redshifts. We analyze the Millennium simulation data in a similar way to the analysis of the observational data in order to test the ${\Lambda}CDM$ cosmology. We find that there are too many massive structures (M > $7{\times}10^{13}M_{\odot}$) observed at z > 2 in comparison with the simulation predictions by a factor of a few, giving a probability of < 1/2500 of the observed data being consistent with the simulation. Our result suggests that massive structures have emerged early, but the reason for the discrepancy with the simulation is unclear. It could be due to the limitation of the simulation such as the lack of key, unrecognized ingredients (strong non-Gaussianity or other baryonic physics), or simply a difficulty in the halo mass estimation from observation, or a fundamental problem of the ${\Lambda}CDM$ cosmology. On the other hand, the over-abundance of massive structures at high redshifts does not favor heavy neutrino mass of ~ 0.3 eV or larger, as heavy neutrinos make the discrepancy between the observation and the simulation more pronounced by a factor of 3 or more.

• The Bulletin of The Korean Astronomical Society
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• v.24 no.2
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• pp.50-50
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• 1999

• The Bulletin of The Korean Astronomical Society
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• v.24 no.1
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• pp.61-61
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• 1999

• The Bulletin of The Korean Astronomical Society
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• v.23
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• pp.50-50
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• 1998

• The Bulletin of The Korean Astronomical Society
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• v.37 no.1
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• pp.49.1-49.1
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• 2012

Recently it is found that the globular clusters are not only bound in their host galaxies, but also are wandering between galaxies in Virgo and Coma clusters. The cluster-wide distribution of these intracluster globular clusters (IGCs) suggests that IGCs are an important probe to understand hierarchical structure formation. We present a survey of IGCs in Virgo cluster using HST archive images for four HST/ACS fields located from about 9 arcmin to 40 acrmin from the cluster center. We find ten new IGCs and confirm four previously known IGCs. The number density of IGCs decreases as the distance from the cluster center increases. We derive integrated photometry of IGCs. We also obtain photometry of resolved stars in the outer region of each cluster. These IGCs are fainter than $M_V{\approx}-9.5$ and mostly blue in (V-I) color. showing that they are mostly metal poor. The locations of red giant branch stars of IGCs in color-magnitude diagrams also show that they are meal-poor. We discuss the implications of these results.

• The Bulletin of The Korean Astronomical Society
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• v.39 no.2
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• pp.70.2-70.2
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• 2014

Galaxy clusters, the largest gravitationally bound systems, are an important means to place constraints on cosmological models. Moreover, they are excellent places to test galaxy evolution models in connection to the environments. To this day, massive clusters have been found unexpectedly(Kang & Im 2009, Durret et al. 2011, Tashikawa et al. 2012) and evolution of galaxies in cluster have been still controversial (Elbaz et al. 2007, Cooper et al. 2008, Tran et al. 2009). Finding galaxy cluster candidates in a wide, deep imaging survey data will enable us to solve the such issues of modern extragalactic astronomy. We have used multi-wavelength data from the UKIRT Infrared Deep Sky Survey Deep Extragalactic Survey (UKIDSS DXS/J and K bands), Spitzer Wise-area InfraRed Extragalactic survey (SWIRE/two mid-infrared bands), the Panoramic Survey Telescope and Rapid Response System (PAN-STARRS/ g, r, i, z, y bands) and Infrared Medium-deep Survey(IMS/J band). We report new candidates of galaxy clusters and properties of their member galaxies in one of the wide and deep survey fields ELAIS-N1, European Large Area ISO Survey North1, covering sky area of $8.75deg^2$.