• The Bulletin of The Korean Astronomical Society
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• v.40 no.2
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• pp.38.4-39
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• 2015

We have been monitoring Sgr A*, the radio source at the center of our galaxy, continuously since G2 encounter was predicted. KaVA is a powerful High resolution imaging array at K and Q band, and it has a excellent uv-coverage for Sgr A*. Together with 1-Gbps recording, our observations have provided high-quality images of Sgr A* at Q-band. Our images reveal a scatter-broadened, elliptical Gaussian structure of the source. We found no significant flux or structural variation of Sgr A* in 2013-2014, which is consistent with recent simulations by Kawashima et al. Continuous monitoring in the coming few years would be able to capture the possible flux increase in the source caused by G2, which will lead to better understanding of the accretion process around supermassive black holes.

• Bulletin of the Korean Mathematical Society
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• v.25 no.2
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• pp.195-201
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• 1988

In general relativity, analyzing causality is central to the study of black holes, to cosmology, and to each of the major recent mathematical theorems. By causality we refer to the general question of which points in a space-time can be joined by causal curves; relativistically which events can influence (be influenced by) a given event. Various causality conditions have been developed for space-times of the problems associated with examples of causality violations (2, 4). Causally continuous space-times were defined by Hawking and Sachs (5). Budic and Sachs (3) established causal completion. A metrizable topology on the causal completion of a causally continuous space-time was studied by Beem(1). Recently the region of space-time where causal continuity is violated was studied by Ishikawa (6) and Vyas and Akolia (8). In this paper we show characterization for reflectingness in terms of continuity of set valued functions. We investigate some properties of the region related to a causally continuous space-time where distinguishingness is violated, and characterize the chronology condition in terms of distinguishing-violated region.

• The Bulletin of The Korean Astronomical Society
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• v.39 no.2
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• pp.115-115
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• 2014

First, We will briefly introduce early science results of AGN observations with KVN and KaVA. KaVA is the combined array of the Korean VLBI network (KVN) and VLBI Exploration of Radio Astronomy (VERA). These include KaVA monitoring of M87, Sgr A* and a few bright blazars and KVN Search for circular polarized Blazars. Furthermore, we will present our future plan of monitoring observation of Sgr A* and M87 with KaVA and Low Radio Power AGN multi frequency polarization survey with KVN. Because of the largeness of their centralsuper-massive black holes, we select them as top-priority sources of our key science program (KSP). The main science goals of the KaVA KSP are (1) mapping the velocity field of the M87 jet and testing magnetically-driven-jet paradigm, and (2) obtaining tightest constraints on physical properties of radio emitting region in Sgr A. High sensitivity achieved through simultaneous multifrequency phase referencing technique of KVN will allow us to explore Low Radio Power AGN cores which build majority of AGNs and therefore are important for undestanding the evolution of AGNs and of their hosts.

• Publications of The Korean Astronomical Society
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• v.25 no.3
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• pp.53-58
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• 2010

The origin of the galaxies represents an important focus of current cosmological research, both observational and theoretical. Its resolution involves a comprehensive understanding of star formation and evolution, galaxy dynamics, supermassive black holes, and the cosmology of the very early universe. In this paper, I will review our current understanding of galaxy formation and review some of the challenges that lie ahead. Specific issues that I address include the galaxy luminosity function, feedback by supernovae and by AGN, and downsizing. I argue that current evidence favours two distinct modes of star formation in the early universe, in order to account for the origin of disk and massive spheroidal galaxies. However perhaps the most urgent need is for a robust theory of star formation.

• The Bulletin of The Korean Astronomical Society
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• v.35 no.1
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• pp.77.1-77.1
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• 2010

I summarize highlights from extragalactic research activities performed with AKARI infrared space telescope. The main emphasis will be given to the works carried out by Korean astronomers. The activities span a wide range of topics, such as MIR properties of nearby galaxies in cluster environment, MIR diagnosis of star-forming galaxies at z=0 through z=2 in the North Ecliptic Pole (NEP) survey field, the Extended Groth Strip (EGS), and the First Look Survey (FLS) field, and the NIR spectroscopy of Luminous Infrared Galaxies (LIRGs) and Active Galactic Nuclei/Super-massive Black Holes at low redshift as well as near the re-ionization epoch of z~6. I describe FIR and MIR all sky data which can be used as a precious resource for extragalactic research, and other future and ongoing works with AKARI. These AKARI results will form a strong basis for future studies using other facilities, such as infrared surveys with UKIRT, FIR study of dusty universe with Herschel, and the SPICA mission.

• The Bulletin of The Korean Astronomical Society
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• v.35 no.1
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• pp.72.2-72.2
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• 2010

We use three-dimensional hydrodynamic simulations to investigate nonlinear gravitational responses of gas to, and the resulting drag force on, a massive perturber moving on a circular orbit through a uniform gaseous medium. We assume that the background medium is non-rotating and adiabatic with index 5/3, and represent the perturber using a Plummer potential with softening radius a. This work extends our previous study where we showed that the drag force on a straight-line trajectory is proportional to a0.45 if the perturber is massive enough. This indicates that the orbital decay of supermassive black holes (SMBHs) near galaxy centers may take much longer than the prediction of the linear force formula applicable for low-mass perturbers. For the circular orbits are considered, however, we find that the nonlinear drag force becomes independent of a, but dependent instead on the orbital radius R as $\varpropto$ R0.5. This suggests not only that the choices of large values of a, for resolution issues, in recent numerical experiments for mergers of SMBH, are marginally acceptable, but also that the gaseous drag indeed provides an efficient mean for the orbtial decay of SMBHs.

• The Bulletin of The Korean Astronomical Society
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• v.43 no.1
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• pp.47.3-48
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• 2018

SPHEREx is a proposed MIDEX mission, planned to conduct spectral imaging survey to cover 0.75-5 um with a spectral resolution of R~40-135. We will briefly overview the uniqueness of SPHEREx data, and how Korean community can take advantage of it. We will present extragalactic science cases that can be addressed with SPHEREx dataset. In particular, SPHEREx survey will uniquely provide the variability information of bright QSOs, both in continuum and fluxes of emission lines, which enables us to investigate the central structures of QSOs through the reverberation mapping method. SPHEREx will also allow us to understand how supermassive black holes and host galaxies co-evolve, by discovering new high-z QSOs, and investigating star formation properties in nearby QSOs.

• Publications of The Korean Astronomical Society
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• v.30 no.2
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• pp.423-427
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• 2015

Recognition of the role of radio galaxies in the universe has been increasing in recent years. Their colossal energy output over huge volumes is now widely believed to play a key role not only in the formation of galaxies and their supermassive black holes, but also in the evolution of clusters of galaxies and, possibly, the cosmic web itself. In this regard, we need to understand the inflation of radio bubbles in the hot gas atmospheres of clusters and the importance of the role that radio galaxies play in the overall energy budget of the intracluster medium. Here, we present results from X-ray and radio band observations of the hot gas atmospheres of powerful, nearby radio galaxies in poor clusters.

• Journal of the Korean Mathematical Society
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• v.46 no.5
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• pp.979-995
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• 2009

As warped product manifolds provide an excellent setting to model space time near black holes or bodies with large gravitational field, the study of these manifolds assumes significance in general. B. Y. Chen [4] initiated the study of CR-warped product submanifolds in a Kaehler manifold. He obtained a characterization for a CR-submanifold to be locally a CR-warped product and an estimate for the squared norm of the second fundamental form of CR-warped products in a complex space form (cf [6]). In the present paper, we have obtained a necessary and sufficient conditions in terms of the canonical structures P and F on a CR-submanifold of a nearly Kaehler manifold under which the submanifold reduces to a locally CR-warped product submanifold. Moreover, an estimate for the second fundamental form of the submanifold in a generalized complex space is obtained and thus extend the results of Chen to a more general setting.

• The Journal of the Convergence on Culture Technology
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• v.5 no.1
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• pp.385-388
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• 2019

Considering spherically symmetric spacetimes embedded in a 5-dimensional static Lorentzian manifold, within the large distance limit of DGP model, we study null geodesic equations. We discuss possible relations of particles following the geodesics with the gravitational waves detected recently, in comparison with the electromagnetic waves propagaing in these brane spacetimes.