• The Bulletin of The Korean Astronomical Society
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• v.42 no.2
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• pp.37.2-37.2
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• 2017

Since the discovery of quasars about a half century ago, it has been an open question what triggers supermassive black holes at the center of galaxies. I will talk about AGNs and their host galaxies in the context of galaxy assembly and evolution. In particular, I will focus on the fundamental, but unanswered question whether black holes play a key role in galaxy evolution from my personal perspective.

• Publications of The Korean Astronomical Society
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• v.30 no.2
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• pp.451-452
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• 2015

We performed statistical analysis for a nearby (0.01 < z < 0.05) volume limited ($M_r$ < -19) sample of galaxies via visual inspection and the definition of galaxy pair systems based on the Sloan Digital Sky Survey Data Release 7 in order to confirm the effects of galaxy interaction on AGN activities. We found that local environmental effects such as galaxy interaction have an influence on the enhancement of the frequency and the strength of the AGN activity. This article is the summary of the paper which will be submitted soon.

• The Bulletin of The Korean Astronomical Society
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• v.40 no.1
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• pp.76.2-76.2
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• 2015

We present the results of the Westerbork Synthesis Radio Telescope (WSRT) HI imaging study of seven late-type galaxies. They are located in the outskirts of the Virgo cluster, possibly along a filament connected to Virgo from the north-west. Most galaxies in this region are found to be HI-rich, containing more HI gas compared to field galaxies with similar size and optical luminosity. The positions of the sample with respect to the cluster and their high HI mass-to-light ratios suggest that the selected galaxies might be accreting more gas from their surroundings while falling into the cluster. By high-resolution HI imaging, we aim to find evidence that galaxies are pre-processed by gas accretion from the intergalactic medium and/or gas-rich neighbors. We probe the detailed HI morphology/kinematics and the star formation properties of the sample. All of these galaxies are found with a large HI disk which is quite extended compared to their stellar disk. Together with kinematical peculiarities, this strongly suggests that cold gas accretion is responsible for active star formation in these galaxies.

• The Bulletin of The Korean Astronomical Society
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• v.37 no.1
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• pp.32.1-32.1
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• 2012

Radial color gradient of disk galaxies has been a key tool for diagnosing the ages and metallicities of the stars and gas of these galaxies, and thus, the formation process of these disks. In many cases, observational data support the 'inside-out' picture of disk galaxy formation proposed by Larson (1976). In this scenario, gas within dark matter halos cools and accretes on to the outer disk while enhancing star formation in the disk. Recent discoveries of "extended ultra-viloet" (XUV) disks also show that majority of disk galaxy experience active star formation within out disks where gas surface density is quite low (Thilker et al. 2007; Gil de Paz et al. 2007). However, neither gas, nor stars stay put within galaxies. They rather migrate into bulges, disperse throughout galaxies, or flow into and out of galaxies via various mechanisms. There have been a few notable studies to investigate how radial star formation and metal abundance gradients vary across populations of disk galaxies systematically. However, the mechanisms driving gas transport are still poorly understood. Cross-matching various galaxy catalogs including KVAGC and UKIDSS, we are investigating if color gradients of late-type galaxies depend on their physical properties, especially on environmental properties. We will present the result from the pilot study on Karachentsev isolated galaxy catalog.

• The Bulletin of The Korean Astronomical Society
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• v.43 no.2
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• pp.35.3-36
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• 2018

One of the most prevalent knowledge about disk galaxies, which dominate the population of the local Universe, is that they consist of stellar structures with different kinematics, such as thin disk, bulge, and halo. Therefore, investigating when and how these components develop in a galaxy is the key to understanding the evolution of galaxies. Using the NewHorizon simulation, we can resolve the detailed structures of galaxies, in the field environment, from the early Universe where star formation and mergers were most active. We first decompose stellar particles in a galaxy into a disk and a dispersion-dominated, spheroidal, component based on their orbits and then see how these components evolve in terms of mass and structure. At high redshift z~3, galaxies are mostly dispersion-dominated as stars are formed misaligned with the galactic rotational axis. At z=1~2, massive galaxies start to dominantly form disk stars, while less massive galaxies do much later. Furthermore, massive galaxies are forming thinner and larger disks with time, and the preexistent disks are heated or even disrupted to become a part of dispersion-dominated component. Thus, the mass growth of spheroidal components at later epochs is dominated by disrupted stars with disk origins and accreted stars at large radii.

• Publications of The Korean Astronomical Society
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• v.8 no.1
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• pp.179-183
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• 1993

We analyze the evolution of active galactic nuclei for the decreasing accretion rate case. Our analysis is based on the unified theory of active galactic nuclei which entirely depends on the accretion rates of the central supermassive black holes. Our discussion leads us to conclude that active galactic nuclei may evolve from QSOs into the nuclei of Seyfert or radio galaxies.

• Journal of The Korean Astronomical Society
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• v.47 no.5
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• pp.195-199
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• 2014

We present a GUI-based interactive Python program, VIMAP, which generates radio spectral index maps of active galactic nuclei (AGN) from Very Long Baseline Interferometry (VLBI) maps obtained at different frequencies. VIMAP is a handy tool for the spectral analysis of synchrotron emission from AGN jets, specifically of spectral index distributions, turn-over frequencies, and core-shifts. In general, the required accurate image alignment is difficult to achieve because of a loss of absolute spatial coordinate information during VLBI data reduction (self-calibration) and/or intrinsic variations of source structure as function of frequency. These issues are overcome by VIMAP which in turn is based on the two-dimensional cross-correlation algorithm of Croke & Gabuzda (2008). In this paper, we briefly review the problem of aligning VLBI AGN maps, describe the workflow of VIMAP, and present an analysis of archival VLBI maps of the active nucleus 3C 120.

• Publications of The Korean Astronomical Society
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• v.27 no.4
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• pp.295-296
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• 2012

We provide a new physical insight on the hot molecular clouds near the nucleus of the heavily obscured AGN IRAS 01250+2832, based on the results of near-infrared high-resolution spectroscopy of gaseous CO ro-vibrational absorption lines with Subaru/IRCS. The detected CO absorption lines up to highly excited rotational levels reveal that hot dense molecular clouds exist around the AGN under the peculiar physical conditions.

• The Bulletin of The Korean Astronomical Society
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• v.42 no.1
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• pp.28.2-28.2
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• 2017

Using numerical hydrodynamics code RAMSES, we perform idealized galaxy merger simulations and study the star formation of merging disk galaxies. In our simulations, we consider the active galactic nucleus (AGN) feedback effect. In order to investigate the star formation influenced by AGN, we run ~60 simulations with various initial conditions. We confirm that star formation is more efficiently suppressed in merging galaxies than in isolated galaxies. In the mergers, AGN effect is more significant when the masses of two galaxies are similar. Furthermore, we find that bulge fraction does not affect the star formation when the AGN effect is considered. We discuss the implications on semi-analytic galaxy formation models and the limitation of the current AGN prescriptions.

• Publications of The Korean Astronomical Society
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• v.27 no.4
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• pp.275-279
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• 2012

We investigate the mid-infrared (MIR) to far-infrared (FIR) properties of a nearly complete sample of local active galactic nuclei (AGNs) detected in the Swift/Burst Alert telescope (BAT) all-sky hard X-ray (14-195 keV) survey, based on the cross correlation with the infrared survey catalogs of AKARI, IRAS and WISE. Out of 135 non-blazar AGNs in the Swift/BAT 9-month catalog, we obtain the MIR photometric data for 128 sources in either the 9, 12, 18, 22, and $25{\mu}m$ band. We find a good correlation between their hard X-ray and MIR luminosities ranging three orders of magnitude (42 < log ${\lambda}L_{\lambda}$(9, $18{\mu}m$) < 45), which is tighter than that with the FIR luminosities at $90{\mu}m$. Both X-ray unabsorbed and absorbed AGNs follow the same correlation, implying isotropic infrared emission, as expected in clumpy dust tori models rather than homogeneous ones.