• 천문학회보
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• 제36권2호
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• pp.76.1-76.1
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• 2011

We present ultraviolet (UV) photometric properties of galaxies in two clusters, the Fornax and Virgo, with different dynamical conditions. We construct UV color-magnitude relations (CMRs) of galaxies in the Fornax and Virgo clusters using GALEX UV data matching with optical B band data. Elliptical and lenticular galaxies locate on red sequence in UV CMRs and show UV upturn phenomenon in both clusters. While dwarf lenticular galaxies (dS0s) in the Fornax also follow the extension of red sequence of giant early type galaxies, they are redder than dS0s in the Virgo at a given magnitude. We also investigated the effect of neighbor galaxies and cluster environment to the UV properties. In the space of projected clustercentric radius and projected nearest neighbor galaxy distance, we found that red (NUV-B>3) galaxy fraction of the Fornax depends entirely on clustercentric radius. However, in the case of Virgo, galaxy colors are also affected by interactions between galaxies outside the cluster virial radius. We suggest that UV properties of early-type galaxies in the Fornax cluster is likely consistent with its dynamically evolved system compared to the Virgo cluster.

• 천문학회보
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• 제42권1호
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• pp.30.1-30.1
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• 2017

In the hope to detect low-surface brightness features (${\mu}_{r^{\prime}}{\sim}27\;mag\;arcsec^{-2}$), we carried out KASI-Yonsei Deep Imaging Survey for Clusters (KYDISC) targeting 14 local clusters at 0.016 < z < 0.145 using Magellan/IMACS telescope and CFHT/MegaCam. Out of 1450 cluster galaxies, 18% of galaxies show the signatures of galaxy mergers. We explore merger-driven changes from various point-of-view. We first examine color-magnitude relations, and find that galaxies related to recent mergers are populated more on blue color than their counterparts. Besides, we find the extremely low frequency of mergers on low-mass red-sequence galaxies, suggesting a migration of red galaxies into the green-valley region through merger-driven star-formation. We also study the mass-size relation of our sample, finding a larger galaxy size in galaxies related to recent mergers. Our results suggest that mergers can simultaneously change properties of galaxies, making outliers on galactic scaling relations.

• 천문학회보
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• 제39권1호
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• pp.39.2-39.2
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• 2014

We present a photometric study of star clusters in the nuclear starburst region of NGC 253 using gVI, YJ, and H band images in the Hubble Space Telescope archive. We find about one thousand star clusters in about 200"x200" field by visual inspection with I<21. We also find about ten thousand star clusters in the same field by automated classification method with magnitude range of 21< I $$\leq_-$$ 24. Ages and masses of star clusters are estimated using spectral energy distribution fitting method. Age distribution of star cluster shows two distinguished young populations with peak ages at 3.5 Myr and 18 Myr. Old populations (>100 Myr) are exist, but their number is small. About thirty young massive star clusters (<10 Myr, ) are found in nuclear region of NGC 253 which are regarded as a result of the recent starburst. Mass function of young star clusters in NGC 253 is somewhat different with those of star clusters in other galaxies. This result suggests that initial cluster mass functions (ICMFs) for star clusters are not universial. Especially ICMF in starburst galaxies may be distinguishable compared with those in normal spiral galaxies. We discuss the implications of these results.

• 천문학회보
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• 제44권1호
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• pp.49.2-49.2
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• 2019

Clusters of galaxies shape up through a series of hierarchical mergers. It is believed that major mergers lead to cluster-wide shock waves, which are manifested as radio relics. The 1RXS J0603.0+4213 and CIZA J2242.8+5301 clusters, for instance, contain Mpc-size giant radio relics in the outskirts. Synchrotron emission from these radio relics reveals the presence of relativistic electrons and the magnetic fields of a few ${\mu}G$ strength. The presence of such magnetic fields in the ICM has been explained by the so-called small-scale turbulent dynamo. To get quantitative measures for magnetic fields in clusters of galaxies, we investigate the development of turbulence and the follow-up amplification of magnetic fields through three-dimensional numerical magnetohydrodynamical (MHD) simulations. The turbulence is induced in highly stratified cluster media, and driven sporadically by major mergers. We here present the results, aiming to answer whether the turbulence dynamo scenario can explain the observed strength and scale of magnetic fields in clusters. Also, we discuss whether the observed properties of giant radio relics can be reproduced in our simulations.

• 천문학회보
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• 제41권2호
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• pp.60.3-61
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• 2016

Galaxy morphology involves complex effects from both secular and non-secular evolution of galaxies. Although it is a final product of galaxy evolution, it gives a clue to the processes that the a galaxy has gone through. Galaxy clusters are the sites where the most massive galaxies are found, and thus the most dramatic merger histories are embedded. Our deep imaging program (${\mu}{\sim}28\;mag\;arcsec^{-2}$), KASI-Yonsei Deep Imaging Survey for Clusters (KYDISC), targets 14 Abell clusters at z = 0.016 - 0.14 using IMACS/Magellan telescope and MegaCam/CFHT to investigate cluster galaxies especially on low surface brightness features related to galaxy interactions. We visually classify galaxy morphology based on criteria related to secular or merger related evolution and find that the morphological mixture of galaxies varies considerably from cluster to cluster. Moreover it depends on the characteristics (e.g. cluster mass) of cluster itself which implies that environmental effects in cluster scale is also an important factor to the evolution of galaxies together with intrinsic (secular) and galaxy merger. Our deep imaging survey for morphological inspection of cluster galaxies with low surface brightness is expected to be a useful basis to understand the nature of cluster galaxies and their internal/external evolutionary path.

• 천문학회보
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• 제45권1호
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• pp.32.2-32.2
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• 2020

Massive Compact Elliptical Galaxies (MCEGs) found in the local universe are as massive as normal galaxies but extremely compact (M∗ > 1011 Msun, Reff < 1.5 kpc). They are considered to be the relics of red nugget galaxies found at high redshift. They are not likely to have undergone many mergers, keeping their original mass and size. Moreover, it is expected that they host a dominant population of red (metal-rich) globular clusters rather than blue (metal-poor) ones. Indeed, Beasley et al. (2018) found that the color distribution of the cluster system of NGC 1277 is unimodal, showing only a red population. However, NGC 1277 is the only case whose cluster system was studied among MCEGs. In this study, we investigate globular cluster systems of 14 nearby MCEGs with a homogeneous data set of HST/WFC3 F814W/F160W archive images. We detect tens to hundreds of globular clusters in each galaxy and examine their color distributions. Surprisingly, the fractions of red globular clusters are similar to those of normal galaxies, and are much lower than that of NGC 1277. We additionally obtain Gemini/GMOS-N g'r'i' images of PGC 70520, one of the 14 nearby MCEGs, to detect more globular clusters from deeper and wider images. We will discuss the results from the Gemini data combined with the results from the HST data in relation with the formation of MCEGs.

• 천문학회지
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• 제45권3호
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• pp.71-84
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• 2012

We present a photometric study of the globular clusters (GCs) in the Virgo giant elliptical galaxy M86 based on Washington $CT_1$ images. The colors of the GCs in M86 show a bimodal distribution with a blue peak at ($C-T_1$) = 1.30 and a red peak at ($C-T_1$) = 1.72. The spatial distribution of the red GCs is elongated similar to that of the stellar halo, while that of the blue GCs is roughly circular. The radial number density profile of the blue GCs is more extended than that of the red GCs. The radial number density profile of the red GCs is consistent with the surface brightness profile of the M86 stellar halo. The GC system has a negative radial color gradient, which is mainly due to the number ratio of the blue GCs to the red GCs increasing as galactocentric radius increases. The bright blue GCs in the outer region of M86 show a blue tilt: the brighter they are, the redder their mean colors get. These results are discussed in comparison with other Virgo giant elliptical galaxies.

• 천문학회보
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• 제37권1호
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• pp.56.2-56.2
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• 2012

We present a study of the asymptotic giant branch (AGB) contribution to the total Ks band luminosity of star clusters in the Large Magellanic Cloud (LMC) as a function of age. AGB stars, a representative intermediate-age population, are a strong source of NIR to MIR emission so that they are a critical component for understanding the near-to-mid infrared observation of galaxies. Current calibration of IR emission in evolutionary population synthesis (EPS) models for galaxies is mainly based on a small number of LMC star clusters. However, each LMC star cluster with intermediate age contains only a few AGB stars so that it suffers from a stochastic effect. Therefore a large number of them are needed for solid calibration of the EPS models. We study physical properties of a large number of LMC star clusters to estimate the Ks band luminosity fraction of AGB stars in star clusters as a function of age. We discuss the stochastic effect in calibrating models, and the importance of this calibration for studying the evolution of not only nearby galaxies but also of high-z galaxies.

• 천문학회보
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• 제46권1호
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• pp.54.2-54.2
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• 2021

Galaxy clusters are the largest structures in the universe located at the top of the cosmological hierarchical model, so the evolution of the universe can be understood by studying clusters of galaxies. Therefore, finding a larger number of galaxy clusters plays an important role in exploring how the universe evolves. A large number of catalogs for galaxy clusters in the northern sky have been published; however, there are few catalogs in the southern sky due to the lack of wide sky survey data. KMTNet Synoptic Survey of Southern Sky(KS4) project, which observes a wide area of the southern sky about 7000 deg² with KMTNet telescopes for two years, is in progress under the SNU Astronomy Research Center. We use the KS4 multi-wavelength optical data and measure photometric redshifts of galaxies for finding galaxy clusters at redshift z<1. Currently, the KS4 project has observed approximately 33% of the target region, and a pipeline that measures photometric redshifts of galaxies has been created. When the project is completed, we expect to find more than a hundred thousand galaxy clusters, and this will improve the study of galaxy clusters in the southern sky.

• 천문학회지
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• 제54권1호
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• pp.17-35
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• 2021

We study galaxies undergoing ram pressure stripping in the Virgo cluster to examine whether we can identify any discernible trend in their star formation activity. We first use 48 galaxies undergoing different stages of stripping based on H i morphology, H i deficiency, and relative extent to the stellar disk, from the VIVA survey. We then employ a new scheme for galaxy classification which combines H i mass fractions and locations in projected phase space, resulting in a new sample of 365 galaxies. We utilize a variety of star formation tracers, which include g - r, WISE [3.4]-[12] colors, and starburstiness that are defined by stellar mass and star formation rates to compare the star formation activity of galaxies at different stripping stages. We find no clear evidence for enhancement in the integrated star formation activity of galaxies undergoing early to active stripping. We are instead able to capture the overall quenching of star formation activity with increasing degree of ram pressure stripping, in agreement with previous studies. Our results suggest that if there is any ram pressure stripping induced enhancement, it is at best locally modest, and galaxies undergoing enhancement make up a small fraction of the total sample. Our results also indicate that it is possible to trace galaxies at different stages of stripping with the combination of H i gas content and location in projected phase space, which can be extended to other galaxy clusters that lack high-resolution H i imaging.