• 천문학회보
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• 제46권2호
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• pp.66.1-66.1
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• 2021

Extinction curves observed toward individual Active Galactic Nuclei (AGN) usually show a steep rise toward Far-Ultraviolet (FUV) wavelengths and can be described by the Small Magellanic Cloud (SMC)-like dust model. This feature suggests the dominance of small dust grains of size a < 0.1 ㎛ in the local environment of AGN, but the origin of such small grains is unclear. In this paper, we aim to explain this observed feature by applying the RAdiative Torque Disruption (RATD) to model the extinction of AGN radiation from FUV to Mid-Infrared (MIR) wavelengths. We find that in the intense radiation field of AGN, large composite grains of size a > 0.1 ㎛ are significantly disrupted to smaller sizes by RATD up to dRATD > 100 pc in the polar direction and dRATD ~ 10 pc in the torus region. Consequently, optical-MIR extinction decreases, whereas FUV-near-Ultraviolet extinction increases, producing a steep far-UV rise extinction curve. The resulting total-to selective visual extinction ratio thus significantly drops to RV < 3.1 with decreasing distances to AGN center due to the enhancement of small grains. The dependence of RV with the efficiency of RATD will help us to study the dust properties in the AGN environment via photometric observations. In addition, we suggest that the combination of the strength between RATD and other dust destruction mechanisms that are responsible for destroying very small grains of a <0.05 ㎛ is the key for explaining the dichotomy observed "SMC" and "gray" extinction curve toward many AGN.

• 천문학회지
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• 제36권3호
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• pp.167-175
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• 2003

There is accumulating evidence for a strong link between nuclear starbursts and AGN. Molecular gas in the central regions of galaxies plays a critical role in fueling nuclear starburst activity and feeding central AGN. The dense molecular ISM is accreted to the nuclear regions by stellar bars and galactic interactions. Here we describe recent observational results for the OB star forming regions in M51 and the nuclear star burst in Arp 220 - both of which have approximately the same rate of star formation per unit mass of ISM. We suggest that the maximum efficiency for forming young stars is an Eddington-like limit imposed by the radiation pressure of newly formed stars acting on the interstellar dust. This limit corresponds to approximately 500 $L_{\bigodot} / M_{\bigodot}$ for optically thick regions in which the radiation has been degraded to the NIR. Interestingly, we note that some of the same considerations can be important in AGN where the source of fuel is provided by stellar evolution mass-loss or ISM accretion. Most of the stellar mass-loss occurs from evolving red giant stars and whether their mass-loss can be accreted to a central AGN or not depends on the radiative opacity of the mass-loss material. The latter depends on whether the dust survives or is sublimated (due to radiative heating). This, in turn, is determined by the AGN luminosity and the distance of the mass-loss stars from the AGN. Several AGN phenomena such as the broad emission and absorption lines may arise in this stellar mass-loss material. The same radiation pressure limit to the accretion may arise if the AGN fuel is from the ISM since the ISM dust-to-gas ratio is the same as that of stellar mass-loss.

• 천문학회보
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• 제43권1호
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• pp.37.2-37.2
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• 2018

The Wide-field Infrared Survey Explorer (WISE) mission has been efficient in selecting Active Galactic Nuclei (AGN) with high luminosities and large obscuration. According to the merger driven AGN powering scenarios, luminous and obscured AGN are in a stage where they go through feeding of gas accretion into the central black hole, and feedback to the host galaxy through outflows. We report the rest-frame UV-optical spectra of 11 Hot Dust Obscured Galaxies (Hot DOGs) at z~2, WISE color-selected to be extremely reddened AGN. A fraction of the targets show blueshifted and broadened [OIII] profiles indicative of ionized gas outflows. We present the occurrence and strength of the outflows, and discuss what impact these AGN activity could give on their hosts.

• 천문학회보
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• 제43권2호
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• pp.55.2-55.2
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• 2018

The Wide-field Infrared Survey Explorer (WISE) mission enabled efficient selection of Active Galactic Nuclei (AGN) with high luminosities and large obscuration. According to the merger driven AGN powering scenarios, luminous and obscured AGN are in a stage where they go through feeding of gas accretion into the central black hole, and feedback to the host galaxy through outflows. We report the rest-frame UV-optical spectra of Hot Dust Obscured Galaxies (Hot DOGs) at z~2, WISE color-selected to be extremely reddened AGN. Most of the targets show blueshifted and broadened [OIII] line profiles indicative of ionized gas outflows. We present the occurrence and strength of the outflows, and discuss the impact of luminous, obscured AGN activity on their hosts.

• 천문학회보
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• 제46권1호
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• pp.54.1-54.1
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• 2021

Observing sub-mm continuum emission from cold dust can play an important role in measuring star formation rates of galaxies, especially in the case of AGN host ones, since AGNs contaminate FIR fluxes by dust heating. To measure star formation rates, we observed total 49 local AGN host galaxies(z<0.2) by SCUBA-2 camera at James Clerk Maxwell Telescope(JCMT) at 450㎛ and 850㎛. We performed several tests with the observed images to determine whether each source is detected, and adopted 3s as the flux upper limit in non-detection cases. Using these measurements and FIR archival data, we modeled spectral energy distributions of the galaxies to estimate star formation rates. The effect of AGN activity on host galaxy star formation will be discussed.

• 천문학논총
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• 제27권4호
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• pp.265-270
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• 2012

Many observations have found evidence of the presence of a large number of heavily obscured Active Galactic Nuclei (AGNs). However, the nature of this population is only poorly understood because heavy obscuration by dust prevents one from finding them at optical wavelengths. Mid-infrared AGN searches can overcome this obstacle by penetrating through dust and by detecting direct emission from the dust torus. Thus, we can identify most of the AGN population, including type-2 and buried AGNs. Using the AKARI mid-infrared all-sky survey, we performed an AGN search in the nearby universe. Utilizing the 2MASS photometry, we selected mid-infrared-excess sources and carried out near-infrared spectroscopic observations in the AKARI Phase 3. During these follow-up observations, we have found three galaxies that show strong near-infrared red continuum from hot dust with a temperature of about 500 K, but do not show any AGN features in other wavelengths. The most suitable explanation of near-infrared continuum is the presence of central AGNs. Therefore, we conclude that they are AGNs obscured by dust. We performed X-ray observations of the two galaxies with SUZAKU. No detections in the 0.4-10 keV suggest that the column density may be much higher than $N_H=10^{23.5}cm^{-2}$. Comparing the masses of the host galaxies with those of the SDSS AGNs, we find that the host galaxies of the dusty AGNs discovered with AKARI are less massive populations than those of optically selected AGNs.

• 천문학회보
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• 제46권2호
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• pp.40.2-40.2
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• 2021

The impact of AGN on star formation is one of the main questions in AGN-galaxy coevolution studies. However, direct evidence of AGN feedback is still rare. One of the main obstacles is that various star formation rate (SFR) indicators are contaminated by AGN contribution. We present IR-based SFR measurements of a sample of 52 local (z<0.3) AGNs, which were selected based on kinematical properties of ionized gas outflows, using SED analysis with JCMT/SCUBA-2 data. First, we will compare IR-based SFR with other SFR indicators to check the reliability of the SFR indicators. Second, we will discuss the contribution of Mid-IR emission from hot dust of AGN torus by comparing SED fitting results with and without including AGN dust component. Finally, we will report the correlation between specific SFR (sSFR) and AGN activity (e.g., outflow strength or Eddington ratio) as evidence of no instantaneous feedback and discuss the implications of these results

• 천문학회보
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• 제43권1호
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• pp.37.3-37.3
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• 2018

Theoretical simulation studies suggest that dust-obscured AGNs appear for a certain period when merger-driven star-forming galaxies evolve to unobscured type 1 AGNs. The dust-obscured AGNs would have red colors due to the dust extinction in their host galaxies, and they are expected to have higher accretion rates than unobscured type 1 AGNs. Red AGNs are found by selecting type 1 AGNs with very red colors, and they have been suspected as the intermediate-stage, dusty AGNs. However, it is not yet clear if red AGNs really correspond to the dusty AGNs due to a lack of intrinsic properties of red AGNs. For unveiling intrinsic properties of red AGNs, we study the NIR and MIR spectra of unobscured type 1 AGNs and red AGNs. There are three main themes: (i) derivation of NIR and MIR BH mass estimators can be used for red AGN study; (ii) investigation of red AGN selection methods to test its usefulness to identify dusty red AGNs; and (iii) investigation of the accretion rates of red AGNs to see if they have the properties as predicted in the simulation studies.

• 천문학회보
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• 제37권1호
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• pp.47.2-47.2
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• 2012

Using deep images taken at Maidanak 1.5m telescope, at McDonald 2.1m telescope and Canada-France-Hawaii Telescope, Dupont 2.5m telescope we investigated the fraction of merging galaxies in hosts of 39 AGN which are brighter than M = -22 mag and nearer than z = 0.3. We found that 16 to 17 of 39 AGN host galaxies show the evidence of mergers like tidal tail, shell via careful visual inspection. We also studied with the merging fraction of a control sample, SDSS Stripe82 early type galaxies of which surface brightness limit and bulge magnitude are similar to that of the AGN sample. We found that merging fraction of the AGN sample is higher than that of early type galaxy samples in the whole range of bulge magnitude. This result implies that AGN activity may be correlated with merging. We also investigated the detailed morphology of merging feature. At least - 1/4 of control samples classified as a tidal and tidal+dust are shell structures. On the other hand only one (5.9%) of AGN sample classified as merger shows shell structures, and almost all merging AGNs show tidal tail features. From point of view that tidal tail may be at the early stage of merging, and shell may be at the late stage of mergers, this result suggests that AGN might be evolved into early-type galaxies after merging.

• 천문학회보
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• 제36권2호
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• pp.79-79
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• 2011

Using deep images taken at Maidanak 1.5m telescope, at McDonald 2.1m telescope and Canada-France-Hawaii Telescope, we investigated the fraction of merging galaxies in hosts of 26 AGN which are brighter than M = -22.2 mag and nearer than z = 0.2. We found that 9 to 12 of 26 AGN host galaxies show the evidence of mergers like tidal tail, shell via visual inspection. We also studied with the merging fraction of a control sample, SDSS Stripe82 galaxies. Surface brightness limit and magnitude are similar to that of the AGN sample. We found that merging fraction of the AGN sample is higher than that of normal galaxy samples. This result implies that AGN activity may be correlated with merging. We also investigated the detailed morphology of merging feature. About ~1/4 of control sample classified as a tidal and tidal+dust are shell structures. On the other hand only one of the AGN sample shows shell structures. Almost all merging AGNs show tidal tail features. From point of view that tidal tail may be at the early stage of merging, and shell may be at the late stage of mergers, this result implies that AGN may be evolved into early-type galaxies after merging.