• The Bulletin of The Korean Astronomical Society
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• v.37 no.2
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• pp.85.1-85.1
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• 2012

Energetic outflow from active galactic nuclei (AGNs) may play a critical role in galaxy evolution. We present a velocity diagnostics for detecting gas outflow in the narrow-line region of Type-2 AGNs using line-of-sight velocity offset of the [O III]${\lambda}5007$ and $H{\alpha}$ emission lines with respect to the systemic velocity of stars in host galaxies. We apply the diagnostics to nearby galaxies at 0.02 < z < 0.05: 3775 AGN-host and 907 star-forming galaxies as a comparison sample, which are selected from the Sloan Digital Sky Survey DR7. After obtaining a best-fit stellar population model for the continuum and a systemic velocity based on stellar lines, we subtract stellar component to measure velocity offsets of each emission line. We find a sample of 169 AGN-host galaxies with outflow signatures, displaying a larger velocity shift of [O III] than that of $H{\alpha}$, as expected in a decelerating outflow model. We find that the offset velocity of [O III] increases with Eddington ratio, suggesting that gas outflow depends on the energetics of AGN.

• The Bulletin of The Korean Astronomical Society
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• v.36 no.1
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• pp.60.2-60.2
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• 2011

Galactic bars are supposed to be a channel of gas inflow to the galactic center and thus possibly help nuclear star-formation and AGN activities. However, previous studies based on small local samples did not agree with this expectation. We find it necessary to examine the expectation using a large sample and so investigate the effects of bar structures on galactic nuclear activities, based on the Sloan Digital Sky Survey (SDSS) DR7. We used 6,348 late-type galaxies brighter than Mr = -19.0 in the redshift range $0.01{\leq}z{\leq}0.05$. Late-type galaxies are visually classified into barred or unbarred galaxies using SDSS color composite images. We compare the fractions of galaxies showing star-formation and AGN activities among barred and unbarred galaxies as a function of optical color, stellar mass, and black-hole mass. We have found that bar enhances nuclear star-formation activity on galaxies having low stellar mass, and low black-hole mass. This effect is stronger in redder galaxies. In the case of AGN, bar effects are higher in intermediate-mass galaxies. Bars also have an effect on the strength(!) of the star-formation and AGN activity in our sample as well. Thus, it seems that nuclear activities are powered by gas inflow from galactic bar structures perhaps not always but under certain conditions.

• The Bulletin of The Korean Astronomical Society
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• v.45 no.1
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• pp.34.2-34.2
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• 2020

Reverberation mapping is one of the best ways to investigate the physical mechanism of broad-line regions around central supermassive black holes of active galactic nuclei (AGNs). It is usually used to estimate the masses of supermassive black holes. Although spectroscopic reverberation mapping has used to study dozens of AGN, spectroscopic monitoring campaign of large sample is expansive. Here, we present results of photometric reverberation mapping with medium-band photometry. We monitored five nearby AGN which were already studied with H-alpha emission line. Observation has been performed for ~3 months with ~3 days cadence using three medium-band filters installed in LSGT (Lee Sang Gak Telescope; 0.43m). We found 0.01-0.08 magnitude variations from differential photometry. Also time-lags between continuum light-curves and H-alpha emission line light-curves are found using JAVELIN software. The result shows that our study and previous studies are consistent within uncertainty range. In the near future, medium-band photometric reverberation mapping seems useful to study large AGN samples. We will present preliminary result of following study that report new time lag measurement of six AGNs in the similar way.

• The Bulletin of The Korean Astronomical Society
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• v.45 no.1
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• pp.38.1-38.1
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• 2020

Recent integral field spectroscopy observations have found that about 11% of galaxies show star-gas misalignment. The misalignment possibly results from external effects such as gas accretion, interaction with other objects, and other environmental effects, hence providing clues to these effects. We explore the properties of misaligned galaxies using Horizon-AGN, a large-volume cosmological simulation, and compare the result with the result of the Sydney-AAO Multi-object integral field spectrograph (SAMI) Galaxy Survey. Horizon-AGN can match the overall misalignment fraction and reproduces the distribution of misalignment angles found by observations surprisingly closely. The misalignment fraction is found to be highly correlated with galaxy morphology both in observations and in the simulation: early-type galaxies are substantially more frequently misaligned than late-type galaxies. The gas fraction is another important factor associated with misalignment in the sense that misalignment increases with decreasing gas fraction. However, there is a significant discrepancy between the SAMI and Horizon-AGN data in the misalignment fraction for the galaxies in dense (cluster) environments. We discuss possible origins of misalignment and disagreement. This presentation is mainly based on the published work Khim et al. 2020, ApJ, 894, 106 (17pp).

• The Bulletin of The Korean Astronomical Society
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• v.44 no.1
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• pp.35.4-35.4
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• 2019

We have conducted a 22 GHz very long baseline interferometry (VLBI) survey of 281 local (z < 0.05) active galactic nuclei (AGNs) selected from the Swift Burst Alert Telescope (BAT) 70-month ultra hard X-ray (14-195 keV) catalog. The main goal is to investigate the relation between the strengths of black hole accretion and the parsec-scale nuclear jet, which is expected to tightly correlate but has not been observationally confirmed yet. The BAT AGN Spectroscopic Survey (BASS) provides the least biased AGN sample against obscuration including both Seyfert types, hence it makes an ideal parent sample for studying the nuclear jet properties of an overall AGN population. Using the Korean VLBI Network (KVN), the KVN and VERA Array (KaVA), and the Very Long Baseline Array (VLBA), we observed 281 objects with a 22 GHz flux > 30 mJy, detecting 11 targets (~4% of VLBI detection rate). This implies that the fraction of X-ray AGNs which are currently ejecting a strong nuclear jet is very small. Although our 11 sources span a wide range of pc-scale morphological types, from compact to complex, they lie on a tight linear relation between accretion luminosity and nuclear jet luminosity. Our finding may indicate that the power of nuclear jet is directly responsible for the amount of black hole accretion. We also have probed the fundamental plane of black hole activity in VLBI scale (e.g., few milli-arcsecond). The results from our high-frequency VLBI radio study support that the change of jet luminosity and size follows what is predicted by the AGN evolution scenario based on the Eddington ratio (ƛ$_{Edd}$) - column density ($N_H$) plane, proposed by a previous study.

• Publications of The Korean Astronomical Society
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• v.36 no.3
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• pp.47-63
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• 2021

Variability is one of the major characteristics of Active Galactic Nuclei (AGN), and it is used for understanding the energy generation mechanism in the center of AGN and/or related physical phenomena. It it known that there exists a time lag between AGN light curves simultaneously observed at different wavelengths, which can be used as a tool to estimate the size of the area that produce the radiation. In this paper, We present long term near-infrared variability of optically bright type 1 AGN using the Wide-field Infrared Survey Explorer data. From the Milliquas catalogue v6.4, 73 type 1 QSOs/AGN and 140 quasar candidates are selected that are brighter than 18 mag in optical and located within 5 degree around the ecliptic poles. Light curves in the W1 band (3.4 ㎛) and W2 band (4.6 ㎛) during the period of 2010-2019 were constructed for these objects by extracting multi-epoch photometry data from WISE and NEOWISE all sky survey database. Variability was analyzed based on the excess variance and the probability P_var. Applying both criteria, the numbers of variable objects are 19 (i.e., 26%) for confirmed AGN and 12 (i.e., 9%) for AGN candidates. The characteristic time scale of the variability (τ) and the variability amplitude (σ) were derived by fitting the DRW model to W1 and W2 light curves. No significant correlation is found between the W1/W2 magnitude and the derived variability parameters. Based on the subsample that are identified in the X-ray source catalog, there exists little correlation between the X-ray luminosity and the variability parameters. We also found four AGN with changing W1-W2 color.

• Publications of The Korean Astronomical Society
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• v.27 no.4
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• pp.291-292
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• 2012

We select infrared-luminous galaxies by cross-matching the SDSS spectroscopic sample of galaxies with the WISE all-sky survey catalog. Based on photometric data points covering from SDSS u-band to WISE $22{\mu}m$, their spectral energy distributions (SEDs) are separated into AGN, elliptical, spiral, and irregular galaxy components. The derived luminosities of spiral galaxy and AGN are well correlated with $H{\alpha}$ and [OIII] line luminosities, respectively. Most galaxies are dominated by young stellar populations even for optical AGNs, but at least 10% of optical non-AGNs appear to harbor buried AGNs. The AGN contribution increases dramatically with the total luminosity. These results show that the SED decomposition is successful and is useful to understand the true nature of dusty galaxies.

• 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.

• Publications of The Korean Astronomical Society
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• v.27 no.4
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• pp.287-288
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• 2012

Radio-loud active galaxies have been found to exhibit a close connection to galactic mergers and host galaxy star-formation quenching. We present preliminary results of an optical spectroscopic investigation of the AKARI NEP field. We focus on the population of radio-loud AGN and use photometric and spectroscopic information to study both their star-formation and nuclear activity components. Preliminary results show that radio-AGN are associated with early type, massive galaxies with relatively old stellar populations.

• The Bulletin of The Korean Astronomical Society
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• v.38 no.2
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• pp.49.2-49.2
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• 2013

We report preliminary results from a radio study of central galaxies in cool and non-cool core clusters. A cooling flow is expected to rapidly form in the center of galaxy clusters unless additional heating mechanisms such as merging with sub clusters are at work. It has been suggested that cool flows can feed the AGN in the central galaxies, increasing their power. On the other hand, the AGN feedback can also affect the surrounding medium, heating back up the gas in the cluster core region. In this study, we investigate the co-evolution of cool flows and the AGN of galaxies located in the cluster center. For this study, we have selected 13 radio bright central galaxies from clusters with a range of cooling time scale. In this work, we present results of our recent observations using the Korean VLBI Network. We discuss the properties of the sample in radio and other wavelengths.