• Journal of The Korean Astronomical Society
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• v.48 no.6
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• pp.399-412
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• 2015

We study the radio properties at 1.4 GHz of Seyfert galaxies with strong forbidden highionization lines (FHILs), selected from the Sloan Digital Sky Survey - a large-sized sample containing nearly equal proportion of diverse range of Seyfert galaxies showing similar redshift distributions compiled by using the Very Large Array survey images. The radio detection rate is low, 49%, which is lower than the detection rate of several other known Seyfert galaxy samples. These galaxies show low star formation rates and the radio emission is dominated by the active nucleus with ≤10% contribution from thermal emission, and possibly, none show evidence for relativistic beaming. The radio detection rate, distributions of radio power, and correlations between radio power and line luminosities or X-ray luminosity for narrow-line Seyfert 1 (NLS1), Seyfert 1 and Seyfert 2 galaxies are consistent with the predictions of the unified scheme hypothesis. Using correlation between radio and [O III] λ 5007 Å luminosities, we show that ∼8% sample sources are radio-intermediate and the remaining are radio-quiet. There is possibly an ionization stratification associated with clouds on scales of 0.1-1.0 kpc, which have large optical depths at 1.4GHz, and it seems these clouds are responsible for free-free absorption of radio emission from the core; hence, leading to low radio detection rate for these FHIL-emitting Seyfert galaxies

• Journal of The Korean Astronomical Society
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• v.38 no.2
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• pp.223-226
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• 2005

We present the results of a 21cm radio continuum aperture synthesis mosaic of the Large Magellanic Cloud (LMC), made by combining data from 1344 separate pointing centers using the Australia Telescope Compact Array (ATCA) and the 64-m Parkes single-dish telescope. The resolution of the mosaicked images is 55' ( 10 pc, using a distance to the LMC) and a region $10^{\circ}{\times}12^{\circ}$ is surveyed.

• Journal of The Korean Astronomical Society
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• v.37 no.5
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• pp.517-525
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• 2004

We present an update on our proposal that during the 'quasar era' (1.5 $\le$ z $\le$ 3), powerful radio galaxies could have played a major role in the enhanced global star-formation, and in the widespread magnetization and metal pollution of the universe. A key ingredient of this proposal is our estimate that the true cosmological evolution of the radio galaxy population is likely to be even steeper than what has been inferred from flux-limited samples of radio sources with redshift data, when an allowance is made for the inverse Compton losses on the cosmic microwave background which were much greater at higher redshifts. We thus estimate that a large fraction of the clumps of proto-galactic material within the cosmic web of filaments was probably impacted by the expanding lobes of radio galaxies during the quasar era. Some recently published observational evidence and simulations which provide support for this picture are pointed out. We also show that the inverse Compton x-ray emission from the population of radio galaxies during the quasar era, which we inferred to be largely missing from the derived radio luminosity function, is still only a small fraction of the observed soft x-ray background (XRB) and hence the limit imposed on this scenario by the XRB is not violated.

• The Bulletin of The Korean Astronomical Society
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• v.39 no.2
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• pp.71.1-71.1
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• 2014

The black hole occupation fraction in dwarf galaxies can provide an important clue for understanding the black hole seed formation. As a pilot feasibility study, we performed a KVN radio monitoring campaign over 8 months for 4 dwarf galaxies. Two galaxies (IC10 and NGC1569) are detected at 22 GHz, respectively with 39 mJy, 83 mJy. The measured flux (rms) variability is 13% and 8%, respectively for IC10 and NGC1569, while the mean flux uncertainty is 25% and 12%. Thus, the detection of the radio flux variability is at best marginal. Detecting flux variability of faint sources (i.e., 22 GHz flux < 200 mJy) seems challenging with the KVN single dishes. Combining with the 1.4 GHz flux measurements from the NVSS, we find that these two galaxies have a steep spectrum, supporting that the radio sources are AGNs. Instead of a monitoring, single-epoch multi-band observations can be effective for identifying radio AGNs by providing the constraint of the radio continuum slope.

• Journal of The Korean Astronomical Society
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• v.36 no.3
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• pp.177-187
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• 2003

Active galactic nuclei (AGNs) are distant, powerful sources of radiation over the entire electromagnetic spectrum, from radio waves to gamma-rays. There is much evidence that they are driven by gravitational accretion of stars, dust, and gas, onto central massive black holes (MBHs) imprisoning anywhere from $\~$1 to $\~$10,000 million solar masses; such objects may naturally form in the centers of galaxies during their normal dynamical evolution. A small fraction of AGNs, of the radio-loud type (RLAGNs), are somehow able to generate powerful synchrotron-emitting structures (cores, jets, lobes) with sizes ranging from pc to Mpc. A brief summary of AGN observations and theories is given, with an emphasis on RLAGNs. Preliminary results from the imaging of 10000 extragalactic radio sources observed in the MITVLA snapshot survey, and from a new analytic theory of the time-variable power output from Kerr black hole magnetospheres, are presented. To better understand the complex physical processes within the central engines of AGNs, it is important to confront the observations with theories, from the viewpoint of analyzing the time-variable behaviours of AGNs - which have been recorded over both 'short' human ($10^0-10^9\;s$) and 'long' cosmic ($10^{13} - 10^{17}\;s$) timescales. Some key ingredients of a basic mathematical formalism are outlined, which may help in building detailed Monte-Carlo models of evolving AGN populations; such numerical calculations should be potentially important tools for useful interpretation of the large amounts of statistical data now publicly available for both AGNs and RLAGNs.

• Publications of The Korean Astronomical Society
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• v.30 no.2
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• pp.429-432
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• 2015

We present a kinematic study of the parsec-scale radio jet in OJ 287, one of the most studied BL Lac objects, during ${\gamma}$-ray flares, to explore the relation between parsec-scale radio jet activity and ${\gamma}$-ray emission. The 22-GHz light curve of OJ 287 show three obvious flare events around 2011 May, 2011 October, and 2012 March. The second radio flare occurred during the ${\gamma}$-ray flaring period, and the third radio flare seemed to precede the ${\gamma}$-ray flare by one month. One jet component moved outward with respect to the core component with an apparent superluminal speed (~ 11c) from 2010 November to 2011 November. Then it changed direction, moving apparently inward in 2011 November, when the ${\gamma}$-ray flare occurred. The observed apparent inward motion of the jet at 22 GHz could be caused by a new jet component, unresolved at 22 GHz, in the innermost region.

• Journal of The Korean Astronomical Society
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• v.37 no.5
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• pp.509-515
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• 2004

We present results from an extensive synthetic observation analysis of numerically-simulated radio galaxy (RG) jets. This analysis is based on the first three-dimensional simulations to treat cosmic ray acceleration and transport self-consistently within a magnetohydrodynamical calculation. We use standard observational techniques to calculate both minimum-energy and inverse-Compton field values for our simulated objects. The latter technique provides meaningful information about the field. Minimum-energy calculations retrieve reasonable field estimates in regions physically close to the minimum-energy partitioning, though the technique is highly susceptible to deviations from the underlying assumptions. We also study the reliability of published rotation measure analysis techniques. We find that gradient alignment statistics accurately reflect the physical situation, and can uncover otherwise hidden information about the source. Furthermore, correlations between rotation measure (RM) and position angle (PA) can be significant even when the RM is completely dominated by an external cluster medium.

• Publications of The Korean Astronomical Society
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• v.32 no.1
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• pp.231-233
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• 2017

We have imaged the AKARI Deep Field with the GMRT radio telescope at 610 MHz, detecting 1224 radio components, which are optically identified with 455 optical galaxies having a mean r' magnitude brighter of 22.5 (to a completeness limit of 25.4 mag), and an average redshift ~ 0.8.

• Journal of The Korean Astronomical Society
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• v.50 no.5
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• pp.151-155
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• 2017

We investigate the radio properties of the dwarf galaxy SDSS J133245.62+263449.3 which shows optical signatures of black hole activity. Dwarf galaxies are known to host intermediate mass black holes (IMBHs) with masses $M_{BH}{\sim}10^{4-6}M_{\odot}$, some of them being radio loud. Recently, Reines et al. (2013) found dwarf galaxy candidates which show signatures of being black hole hosts based on optical spectral lines. SDSS J133245.62+263449.3 is one of them; it shows a flux density of ~ 20 mJy at 1.4 GHz, which corresponds to $L_{1.4GHz}{\sim}10^{23}W\;Hz^{-1}$. This is much brighter than other black hole host dwarf galaxies. However, star formation activity can contribute to radio continuum emission as well. To understand the nature of the radio emission from SDSS J133245.62+263449.3, we imaged this radio loud dwarf galaxy at low frequencies (325 MHz and 610 MHz) using the Giant Metrewave Radio Telescope (GMRT). We present here the high resolution images from our GMRT observations. While we detect no obvious extended emission from radio jets from the central AGN, we do find the emission to be moderately extended and unlikely to be dominated by disk star formation. VLBI observations using the Korean VLBI Network (KVN) are now being planned to understand the emission morphology and radiation mechanism.

• 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.