• The Bulletin of The Korean Astronomical Society
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• v.38 no.2
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• pp.50.2-50.2
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• 2013

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 at high redshfit (Kang & Im 2009, Durret et al. 2011, Tashikawa et al. 2012), and evolution of galaxies in cluster has not been fully understood. Finding galaxy cluster candidates at z > 1 in wide, deep imaging survey data will enable us to solve such issues of modern extragalactic astronomy. We report new candidates of galaxy clusters in the wide and deep survey fields, European Large Area ISO Survey North1(ELAIS-N1) and North2(ELAIS-N2) fields, covering sky area of $8.75deg^2$ and $4.85deg^2$ each. We also suggest a new useful colour-colour selection technique to separate 1 < z < 2 galaxies from low-z galaxies by combining multi-wavelength data from the UKIRT Infrared Deep Sky Survey Deep Extragalactic Survey (UKIDSS DXS, JK bands), Spitzer Wise-area InfraRed Extragalactic survey (SWIRE, Optical-Infrared bands), Canada France Hawaii Telescope (CFHT, z band) and Infrared Medium-deep Survey(IMS, J band).

• The Bulletin of The Korean Astronomical Society
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• v.39 no.2
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• pp.58.2-58.2
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• 2014

We present the results of the KVN observations of central galaxies in cool-core and non cool-core clusters. The goal is to study how cooling environments affect the AGN activities in the core where their host galaxies are embedded. From the HIghest FLUx Galaxy Cluster Sample (HIFLUGCS), we have selected 19 radio bright AGNs located in the center of clusters with various cooling timescale. In our pilot study, we have obtained 22 and 43 GHz fluxes and morphologies of the sample using the Korean VLBI network. We find that 22/43 GHz fluxes do not strongly depend on the presence of a cool gas flow. However, an intriguing fact is that most AGNs in the cool-core clusters show the hint of a pc-scale jet component while the ones in the non cool-core clusters do not. Based on these results, we discuss the role of cooling flows in the central cluster AGNs and their co-evolution.

• The Bulletin of The Korean Astronomical Society
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• v.37 no.1
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• pp.46.2-46.2
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• 2012

The black hole mass-stellar velocity dispersion ($M_{BH}-{\sigma}_*$) relation observed in the present-day universe has motivated numerous studies on the black hole-galaxy co-evolution. It is crucial to define the$M_{BH}-{\sigma}_*$ local active galaxies since cosmic evolution of the correlations is calibrated based on the local relation. However, stellar velocity dispersion is difficult to measure in active galaxies due to much higher AGN continuum than stellar pseudo-continuum, resulting in a small sample with reliable velocity dispersion measurements for studying the AGN $M_{BH}-{\sigma}_*$ relation. To increase the sample size and improve the measurements, we obtained high S/N near-IR spectra for 3 local AGNs, i.e., NGC 3227, Akn 120, 3C 390.3, for which reverberation black hole masses are measured, using the TripleSpec at the Palomar 5-m Telescope. By investigating aperture effect and correcting for rotation component, we determine the luminosity-weighted ${\sigma}_*$, based on the spatially resolved kinematics and compare them with optical measurements from literature. Combining our new measurements with literature data, we present an improved $M_{BH}-{\sigma}_*$ relation for the enlarged sample of reverberation-mapped AGNs.

• The Bulletin of The Korean Astronomical Society
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• v.43 no.1
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• pp.62.1-62.1
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• 2018

Galaxies in the cluster environment interact with the intracluster medium (ICM), losing the interstellar medium (ISM) and alternating their evolution. Observational evidences of the extraplanar ISM stripped by the ICM's ram pressure are prevalent in HI imaging studies of cluster galaxies. However, current theoretical understanding of the ram pressure stripping (or ICM-ISM interaction in general) is still limited mainly due to the lack of numerical resolution at ISM scales in large-scale simulations. Especially, self-consistent modeling of the turbulent, multiphase ISM is critical to understand star formation in galaxies interacting with the ICM. To achieve this goal, we utilize the TIGRESS simulation suite, simulating a local patch of galactic disks with high resolution to resolve key physical processes in the ISM, including cooling/heating, self-gravity, MHD, star formation, and supernova feedback. We then expose the ISM disk to ICM flows and investigate the evolution of star formation rate and the properties of the ISM. By exploring ICM parameter space, we discuss an implication of the simple ram pressure stripping condition (so called the Gunn-Gott condition) to the realistic ISM.

• The Bulletin of The Korean Astronomical Society
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• v.39 no.1
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• pp.48.1-48.1
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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 at z>1 in a wide, deep imaging survey data will enable us to solve the such issues of modern extragalactic astronomy. We report new candidates of galaxy clusters and their physical properties in one of the wide and deep survey fields, European Large Area ISO Survey North1(ELAIS-N1) and North2(ELAIS-N2) fields, covering sky area of and each. We also suggest a new useful color selection technique to separate 1 < z < 2 galaxies from low-z galaxies by combining multi-wavelength data from the UKIRT Infrared Deep Sky Survey Deep Extragalactic Survey (UKIDSS DXS/J and K band), Spitzer Wise-area InfraRed Extragalactic survey (SWIRE/two mid-infrared bands), Canada France Hawaii Telescope (CFHT/z band), Issac Newton Telescope(INT/ u, g, r, i, z band) and Infrared Medium-deep Survey(IMS/J band).

• Publications of The Korean Astronomical Society
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• v.30 no.2
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• pp.261-263
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• 2015

The metallicity distribution of globular clusters (GCs) provides a crucial clue for the star formation history of their host galaxy. With the assumption that GCs are generally old, GC colors have been used as a proxy for GC metallicities. Bimodal color distributions of GCs observed in most large galaxies have, for decades, been interpreted as bimodal metallicity distributions, indicating the presence of two populations within a galaxy. However, the conventional view has been challenged by a new theory that non-linear GC color-metallicity relations can cause a bimodal color distribution even from a single-peaked metallicity distribution. Using photometric and spectroscopic data of NGC 5128 GCs in combination with stellar population simulation models, we examine the effect of non-linearity in GC color-metallicity relations on transformation of the color distributions into the metallicity distributions. Although in some colors offsets are present between observations and models for the color-metallicity relations, their overall shape agrees well for various colors. After the offsets are corrected, the observed spectroscopic metallicity distribution is well reproduced via modeled color-metallicity relations from various color distributions having different morphologies. We discuss the implications of our results.

• Journal of The Korean Astronomical Society
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• v.30 no.1
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• pp.83-94
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• 1997

We have identified the candidates for the primordial galaxies in the process of formation in the Hubble Deep Field (hereafter HDF). In order to select these objects we have removed objects brighter than 29-th magnitude in the HDF images and smoothed the maps with the Gaussian filters with the FWHM of 0.8' and 4' to obtain the difference maps. This has enabled us to find. very faint diffuse structures close to the sky level. Peaks are identified in the difference map for each of three HDF chips with three filters (F450W, F606W, and F814W). They have the apparent AB magnitudes typically between 29 and 31. The objects identified in different wavelengths filters have a strong cross-correlations. The correlation lengths are about 0.8'. This means that an object found in one filter can be also found as a peak within 0.8' separation in another filter, thus telling the reality of the identified objects. This angular scale is also the size of the primordial galaxies which have strong color fluctuations on their surfaces. Their large-scale distribution quite resembles that of nearby galaxies, supporting the idea that these objects are ancestors of the present bright galaxies forming at statistically high density regions. Inspections on individual objects show that these primordial galaxy candidates have tiny multiple glares embedded in diffuse backgrounds. Their radial light distributions are quite different from that of nearby bright galaxies. We may be now looking at the epoch of galaxy formation.

• The Bulletin of The Korean Astronomical Society
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• v.42 no.2
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• pp.47.1-47.1
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• 2017

We study the alignment of kinematic position angles (PAkin) of early-type galaxies in the Virgo cluster using Atlas3D data. The PAkin represent the direction of the angular momentum of the galaxies better than the photometric position angles. Therefore, the alignment of their PA$_{kin}$ is a useful tool to trace the momentum direction. The early-type galaxies in the Virgo cluster have been known to be distributed as filamentary structures inside the cluster. We found that their PAkin are aligned to two directions of 20degree and -80degree. This fact is confirmed using the bootstrap test, and that is, the two alignment angles are statistically significant. Besides, these two angles are surprisingly aligned parallel to the filamentary structures inside the cluster. These results suggest that early-type galaxies were formed by major merging in the filament structures and then fall into the Virgo cluster while maintaining their position angles.

• Journal of The Korean Astronomical Society
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• v.41 no.5
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• pp.109-119
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• 2008

We present an optical-infrared photometric study of galaxies in six nearby clusters of galaxies at $z=0.041{\sim}0.098$ (A1436, A1773, A1809, A2048, A2142, and A2152). Using BV I photometry obtained at the Bohyunsan Optical Astronomical observatory and $JHK_S$ photometry extracted from the 2-Micron All-Sky Survey catalog, we investigate the colors of galaxies in the clusters. Using the (B - V) versus ($I\;-\;K_S$) color-color diagrams in comparison with the simple stellar population model, we estimate the ages and metallicities of bright early-type member galaxies. Early-type galaxies in each cluster show the color-magnitude relation. Ages and metallicities of early-type members show little dependence on their velocity dispersions. Mean ages of early-types in the clusters range from 3 Gyr to 20 Gyr, showing a large dispersion, and mean metallicities range from Z = 0.03 to 0.05 above the solar value, showing a negligible dispersion.

• 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.