49 research outputs found
ALMA polarimetry measures magnetically aligned dust grains in the torus of NGC 1068
The obscuring structure surrounding active galactic nuclei (AGN) can be
explained as a dust and gas flow cycle that fundamentally connects the AGN with
their host galaxies. This structure is believed to be associated with dusty
winds driven by radiation pressure. However, the role of magnetic fields, which
are invoked in almost all models for accretion onto a supermassive black hole
and outflows, is not thoroughly studied. Here we report the first detection of
polarized thermal emission by means of magnetically aligned dust grains in the
dusty torus of NGC 1068 using ALMA Cycle 4 polarimetric dust continuum
observations (, pc; 348.5 GHz, m). The polarized torus
has an asymmetric variation across the equatorial axis with a peak polarization
of \% and position angle of (B-vector) at
pc east from the core. We compute synthetic polarimetric observations of
magnetically aligned dust grains assuming a toroidal magnetic field and
homogeneous grain alignment. We conclude that the measured 860 m continuum
polarization arises from magnetically aligned dust grains in an optically thin
region of the torus. The asymmetric polarization across the equatorial axis of
the torus arises from 1) an inhomogeneous optical depth, and 2) a variation of
the velocity dispersion, i.e. variation of the magnetic field turbulence at
sub-pc scales, from the eastern to the western region of the torus. These
observations and modeling constrain the torus properties beyond spectral energy
distribution results. This study strongly supports that magnetic fields up to a
few pc contribute to the accretion flow onto the active nuclei.Comment: 19 pages, 11 figures (Accepted for Publication to ApJ
On the difference of torus geometry between hidden and non-hidden broad line active galactic nuclei
We present results from the fitting of infrared (IR) spectral energy
distributions of 21 active galactic nuclei (AGN) with clumpy torus models. We
compiled high spatial resolution (-- arcsec) mid-IR -band
spectroscopy, -band imaging and nuclear near- and mid-IR photometry from the
literature. Combining these nuclear near- and mid-IR observations, far-IR
photometry and clumpy torus models, enables us to put constraints on the torus
properties and geometry. We divide the sample into three types according to the
broad line region (BLR) properties; type-1s, type-2s with scattered or hidden
broad line region (HBLR) previously observed, and type-2s without any published
HBLR signature (NHBLR). Comparing the torus model parameters gives us the first
quantitative torus geometrical view for each subgroup. We find that NHBLR AGN
have smaller torus opening angles and larger covering factors than those of
HBLR AGN. This suggests that the chance to observe scattered (polarized) flux
from the BLR in NHBLR could be reduced by the dual effects of (a) less
scattering medium due to the reduced scattering volume given the small torus
opening angle and (b) the increased torus obscuration between the observer and
the scattering region. These effects give a reasonable explanation for the lack
of observed HBLR in some type-2 AGN.Comment: 13 pages, 5 figures, accepted for publication in Ap
About AGN ionization echoes, thermal echoes, and ionization deficits in low redshift Lyman-alpha blobs
We report the discovery of 14 Lyα blobs (LABs) at z ⌠0.3, existing at least 4â7 billion years later in the Universe than all other LABs known. Their optical diameters are 20â70 kpc, and GALEX data imply Lyα luminosities of (0.4â6.3) Ă 1043 erg sâ1. Contrary to high-z LABs, they live in low-density areas. They are ionized by AGN, suggesting that cold accretion streams as a power source must deplete between z = 2 and 0.3. We also show that transient AGN naturally explain the ionization deficits observed in many LABs. Their Lyα and X-ray fluxes decorrelate below âČ106 years because of the delayed escape of resonantly scattering Lyα photons. High Lyα luminosities do not require currently powerful AGN, independent of obscuration. Chandra X-ray data reveal intrinsically weak AGN, confirming the luminous optical nebulae as impressive ionization echoes. For the first time, we also report mid-infrared thermal echoes from the dusty tori. We conclude that the AGN have faded by three to four orders of magnitude within the last 104â5 years, leaving fossil UV, optical and thermal radiation behind. The host galaxies belong to the group of previously discovered Green Bean galaxies (GBs). Gemini optical imaging reveals smooth spheres, mergers, spectacular outflows and ionization cones. Because of their proximity and high flux densities, GBs are perfect targets to study AGN feedback, mode switching and the Lyα escape. The fully calibrated, co-added optical FITS images are publicly available
Torus and AGN properties of nearby Seyfert galaxies: Results from fitting IR spectral energy distributions and spectroscopy
We used the CLUMPY torus models and a Bayesian approach to fit the infrared
spectral energy distributions (SEDs) and ground-based high-angular resolution
mid-infrared spectroscopy of 13 nearby Seyfert galaxies. This allowed us to put
tight constraints on torus model parameters such as the viewing angle, the
radial thickness of the torus Y, the angular size of the cloud distribution
sigma_torus, and the average number of clouds along radial equatorial rays N_0.
The viewing angle is not the only parameter controlling the classification of a
galaxy into a type 1 or a type 2. In principle type 2s could be viewed at any
viewing angle as long as there is one cloud along the line of sight. A more
relevant quantity for clumpy media is the probability for an AGN photon to
escape unabsorbed. In our sample, type 1s have relatively high escape
probabilities, while in type 2s, as expected, tend to be low. Our fits also
confirmed that the tori of Seyfert galaxies are compact with torus model radii
in the range 1-6pc. The scaling of the models to the data also provided the AGN
bolometric luminosities, which were found to be in good agreement with
estimates from the literature. When we combined our sample of Seyfert galaxies
with a sample of PG quasars from the literature to span a range of
L_bol(AGN)~10^{43}-10^{47}erg/s, we found plausible evidence of the receding
torus. That is, there is a tendency for the torus geometrical covering factor
to be lower at high AGN luminosities than at low AGN luminosities. This is
because at low AGN luminosities the tori appear to have wider angular sizes and
more clouds along radial equatorial rays. We cannot, however rule out the
possibility that this is due to contamination by extended dust structures not
associated with the dusty torus at low AGN luminosities, since most of these in
our sample are hosted in highly inclined galaxies. (Abridged)Comment: Accepted for publication in Ap
Torus and Active Galactic Nucleus Properties of Nearby Seyfert Galaxies: Results from Fitting Infrared Spectral Energy Distributions and Spectroscopy
We used the CLUMPY torus models and a Bayesian approach to fit the infrared spectral energy distributions and ground-based high angular resolution mid-infrared spectroscopy of 13 nearby Seyfert galaxies. This allowed us to put tight constraints on torus model parameters such as the viewing angle i, the radial thickness of the torus Y, the angular size of the cloud distribution Ïtorus, and the average number of clouds along radial equatorial rays N0. We found that the viewing angle i is not the only parameter controlling the classification of a galaxy into type 1 or type 2. In principle, type 2s could be viewed at any viewing angle i as long as there is one cloud along the line of sight. A more relevant quantity for clumpy media is the probability for an active galactic nucleus (AGN) photon to escape unabsorbed. In our sample, type 1s have relatively high escape probabilities, Pesc ~ 12%-44%, while type 2s, as expected, tend to have very low escape probabilities. Our fits also confirmed that the tori of Seyfert galaxies are compact with torus model radii in the range 1-6 pc. The scaling of the models to the data also provided the AGN bolometric luminosities Lbol(AGN), which were found to be in good agreement with estimates from the literature. When we combined our sample of Seyfert galaxies with a sample of PG quasars from the literature to span a range of Lbol(AGN) ~ 1043-1047 erg s-1, we found plausible evidence of the receding torus. That is, there is a tendency for the torus geometrical covering factor to be lower (f2 ~ 0.1-0.3) at high AGN luminosities than at low AGN luminosities (f2 ~ 0.9-1 at ~{}1043-1044 erg s-1). This is because at low AGN luminosities the tori appear to have wider angular sizes (larger Ïtorus) and more clouds along radial equatorial rays. We cannot, however, rule out the possibility that this is due to contamination by extended dust structures not associated with the dusty torus at low AGN luminosities, since most of these in our sample are hosted in highly inclined galaxies
A Deep Look at the Nuclear Region of UGC 5101 Through High Angular Resolution Mid-IR Data with GTC/CanariCam
We present an analysis of the nuclear infrared (IR, 1.6â18 ÎŒm) emission of the ultraluminous IR galaxy UGC 5101 to derive the properties of its active galactic nucleus (AGN) and its obscuring material. We use new mid-IR high angular resolution (0.3â0.5 arcsec) imaging using the Si-2 filter (λC = 8.7âÎŒm) and 7.5â13 ÎŒm spectroscopy taken with CanariCam (CC) on the 10.4 m Gran Telescopio CANARIAS. We also use archival Hubble Space Telescope/NICMOS and Subaru/COMICS imaging and Spitzer/IRS spectroscopy. We estimate the near- and mid-IR unresolved nuclear emission by modelling the imaging data with GALFIT. We decompose the Spitzer/IRS and CC spectra using a power-law component, which represents the emission due to dust heated by the AGN, and a starburst component, both affected by foreground extinction. We model the resulting unresolved near- and mid-IR, and the starburst subtracted CC spectrum with the CLUMPY torus models of Nenkova et al. The derived geometrical properties of the torus, including the large covering factor and the high foreground extinction needed to reproduce the deep 9.7âÎŒm silicate feature, are consistent with the lack of strong AGN signatures in the optical. We derive an AGN bolometric luminosity Lbol ~ 1.9 Ă 1045âerg sâ1 that is in good agreement with other estimates in the literature
The Emission and Distribution of Dust of the Torus of NGC 1068
We present observations of NGC 1068 covering the 19.7â53.0 ÎŒm wavelength range using FORCAST and HAWC+ on board SOFIA. Using these observations, high-angular-resolution infrared (IR) and submillimeter observations, we find an observational turnover of the torus emission in the 30â40 ÎŒm wavelength range with a characteristic temperature of 70â100 K. This component is clearly different from the diffuse extended emission in the narrow line and star formation regions at 10â100 ÎŒm within the central 700 pc. We compute 2.2â432 ÎŒm 2D images using the best inferred CLUMPY torus model based on several nuclear spectral energy distribution (SED) coverages. We find that when 1â20 ÎŒm SED is used, the inferred result gives a small torus size (<4 pc radius) and a steep radial dust distribution. The computed torus using the 1â432 ÎŒm SED provides comparable torus sizes, 5.1^(+0.4)_(-0.4) pc radius, and morphology to the recently resolved 432 ÎŒm Atacama Large Millimeter Array observations. This result indicates that the 1â20 ÎŒm wavelength range is not able to probe the full extent of the torus. The characterization of the turnover emission of the torus using the 30â60 ÎŒm wavelength range is sensitive to the detection of cold dust in the torus. The morphology of the dust emission in our 2D image at 432 ÎŒm is spatially coincident with the cloud distribution, while the morphology of the emission in the 1â20 ÎŒm wavelength range shows an elongated morphology perpendicular to the cloud distribution. We find that our 2D CLUMPY torus image at 12 ÎŒm can produce comparable results to those observed using IR interferometry
The Differences in the Torus Geometry Between Hidden and Non-Hidden Broad Line Active Galactic Nuclei
We present results from the fitting of infrared (IR) spectral energy distributions of 21 active galactic nuclei (AGNs) with clumpy torus models. We compiled high spatial resolution (~0.3â0.7 arcsec) mid-IR (MIR) N-band spectroscopy, Q-band imaging, and nuclear near- and MIR photometry from the literature. Combining these nuclear near- and MIR observations, far-IR photometry, and clumpy torus models enables us to put constraints on the torus properties and geometry. We divide the sample into three types according to the broad line region (BLR) properties: type-1s, type-2s with scattered or hidden broad line region (HBLR) previously observed, and type-2s without any published HBLR signature (NHBLR). Comparing the torus model parameters gives us the first quantitative torus geometrical view for each subgroup. We find that NHBLR AGNs have smaller torus opening angles and larger covering factors than HBLR AGNs. This suggests that the chance to observe scattered (polarized) flux from the BLR in NHBLR could be reduced by the dual effects of (a) less scattering medium due to the reduced scattering volume given the small torus opening angle and (b) the increased torus obscuration between the observer and the scattering region. These effects give a reasonable explanation for the lack of observed HBLR in some type-2 AGNs
The Emission and Distribution of Dust of the Torus of NGC 1068
We present observations of NGC 1068 covering the 19.7â53.0 ÎŒm wavelength range using FORCAST and HAWC+ on board SOFIA. Using these observations, high-angular-resolution infrared (IR) and submillimeter observations, we find an observational turnover of the torus emission in the 30â40 ÎŒm wavelength range with a characteristic temperature of 70â100 K. This component is clearly different from the diffuse extended emission in the narrow line and star formation regions at 10â100 ÎŒm within the central 700 pc. We compute 2.2â432 ÎŒm 2D images using the best inferred CLUMPY torus model based on several nuclear spectral energy distribution (SED) coverages. We find that when 1â20 ÎŒm SED is used, the inferred result gives a small torus size (<4 pc radius) and a steep radial dust distribution. The computed torus using the 1â432 ÎŒm SED provides comparable torus sizes, 5.1^(+0.4)_(-0.4) pc radius, and morphology to the recently resolved 432 ÎŒm Atacama Large Millimeter Array observations. This result indicates that the 1â20 ÎŒm wavelength range is not able to probe the full extent of the torus. The characterization of the turnover emission of the torus using the 30â60 ÎŒm wavelength range is sensitive to the detection of cold dust in the torus. The morphology of the dust emission in our 2D image at 432 ÎŒm is spatially coincident with the cloud distribution, while the morphology of the emission in the 1â20 ÎŒm wavelength range shows an elongated morphology perpendicular to the cloud distribution. We find that our 2D CLUMPY torus image at 12 ÎŒm can produce comparable results to those observed using IR interferometry