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Register a new userNuclear Infrared Spectral Energy Distribution of Type II Active Galactic Nuclei
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We present near and mid--IR observations of a sample of Seyfert II galaxies drawn from the $12mu$m Galaxy Sample. The sample was observed in the J, H, K, L, M and N bands. Galaxy Surface Brightness Profiles are modeled using nuclear, bulge, bar (when necessary) and disk components. To check the reliability of our findings the procedure was tested using {em Spitzer/} observations of M,31. Nuclear Spectral Energy Distributions (SEDs) are determined for 34 objects, and optical spectra are presented for 38, including analysis of their stellar populations using the STARLIGHT spectral synthesis code. Emission line diagnostic-diagrams are used to discriminate between genuine AGN and HII nuclei. Combining our observations with those found in the literature, we have a total of 40 SEDs. It is found that about 40% of the SEDs are characterized by an upturn in the near-IR, which we have quantified as a NIR slope $alpha < 1$ for an SED characterized as $lambda f_{lambda} propto lambda^{alpha}$. Three objects with an HII nucleus and two Seyfert nuclei with strong contamination from a circumnuclear starburst, also show an upturn. For genuine AGN this component could be explained as emission from the accretion disk, a jet, or from a very hot dust component leaking from the central region through a clumpy obscuring structure. The presence of a very compact nuclear starburst as the origin for this NIR excess emission is not favored by our spectroscopic data for these objects.
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We present results from model fitting to the Spectral Energy Distribution (SED) of a homogeneous sample of Seyfert II galaxies drawn from the $12mu$m Galaxy Sample. Imaging and nuclear flux measurements are presented in an accompanying paper (Videla et al., 2013). Here we add IRS Spitzer observations to further constrain the SEDs after careful subtraction of a starburst component. We use the library of CLUMPY torus models from Nenkova et al.~(2008ab) and also test the two-phase models recently produced by Stalevski et al.~(2012). We find that photometric and spectroscopic observations in the mid-IR (>5mu) are crucial to properly constrain the best-fit torus models. About half of our sources show clear near-IR excess of their SEDs above the best fit models. This problem can be less severe when using the Stalevski et al.~(2012) models. It is not clear what is the nature of this emission since best fitted black body temperatures are very high (~1700-2500 K) and the Type II classification of our sources would correspond to a small probability to peer directly into the hottest regions of the torus. Crucially, the derived torus parameters when using CLUMPY models are quite robust,, independently of whether the sources require an additional black body component or not. Our findings suggest that tori are characterized by N_0 > 5, sigma > 40, tau < 25, i > 40 degrees, Y < 50 and A_v^los ~ 100-300. From these we can determine that typical torus sizes and masses of 0.1-5.0 pc and 10^{4-6} M_odot. We find tentative evidence that those nuclei with a detected Hidden Broad Line Regions are characterized by lower levels of extinction than those without one. Finally, we find no correlation between the torus properties and the presence of circumnuclear or more global star-formation.
We present high-resolution mid-infrared (MIR) imaging, nuclear spectral energy distributions (SEDs) and archival Spitzer spectra for 22 low-luminosity active galactic nuclei (LLAGN; Lbol lesssim 10^42 erg/sec). Infrared (IR) observations may advance our understanding of the accretion flows in LLAGN, the fate of the obscuring torus at low accretion rates, and, perhaps, the star formation histories of these objects. However, while comprehensively studied in higher-luminosity Seyferts and quasars, the nuclear IR properties of LLAGN have not yet been well-determined. We separate the present LLAGN sample into three categories depending on their Eddington ratio and radio emission, finding different IR characteristics for each class. (I) At the low-luminosity, low-Eddington ratio (log Lbol/LEdd < -4.6) end of the sample, we identify host-dominated galaxies with strong polycyclic aromatic hydrocarbon bands that may indicate active (circum-)nuclear star formation. (II) Some very radio-loud objects are also present at these low Eddington ratios. The IR emission in these nuclei is dominated by synchrotron radiation, and some are likely to be unobscured type 2 AGN that genuinely lack a broad line region. (III) At higher Eddington ratios, strong, compact nuclear sources are visible in the MIR images. The nuclear SEDs of these galaxies are diverse; some resemble typical Seyfert nuclei, while others lack a well-defined MIR dust bump. Strong silicate emission is present in many of these objects. We speculate that this, together with high ratios of silicate strength to hydrogen column density, could suggest optically thin dust and low dust-to-gas ratios, in accordance with model predictions that LLAGN do not host a Seyfert-like obscuring torus.
We present ongoing work on the spectral energy distributions (SEDs) of active galactic nuclei (AGNs), derived from X-ray, ultraviolet, optical, infrared and radio photometry and spectroscopy. Our work is motivated by new wide-field imaging surveys that will identify vast numbers of AGNs, and by the need to benchmark AGN SED fitting codes. We have constructed 41 SEDs of individual AGNs and 80 additional SEDs that mimic Seyfert spectra. All of our SEDs span 0.09 to 30 microns, while some extend into the X-ray and/or radio. We have tested the utility of the SEDs by using them to generate AGN photometric redshifts, and they outperform SEDs from the prior literature, including reduced redshift errors and flux density residuals.
We present spectral energy distributions (SEDs) of 41 active galactic nuclei, derived from multiwavelength photometry and archival spectroscopy. All of the SEDs span at least 0.09 to 30 micron, but in some instances wavelength coverage extends into the X-ray, far-infrared and radio. For some AGNs we have fitted the measured far-infrared photometry with greybody models, while radio flux density measurements have been approximated by power-laws or polynomials. We have been able to fill some of the gaps in the spectral coverage using interpolation or extrapolation of simple models. In addition to the 41 individual AGN SEDs, we have produced 72 Seyfert SEDs by mixing SEDs of the central regions of Seyferts with galaxy SEDs. Relative to the literature, our templates have broader wavelength coverage and/or higher spectral resolution. We have tested the utility of our SEDs by using them to generate photometric redshifts for 0 < z < 6.12 AGNs in the Bootes field (selected with X-ray, IR and optical criteria) and, relative to SEDs from the literature, they produce comparable or better photometric redshifts with reduced flux density residuals.
The physics of active super massive black holes (BHs) is governed by their mass (M_BH), spin (a*) and accretion rate ($dot{M}$). This work is the first in a series of papers with the aim of testing how these parameters determine the observable attributes of active galactic nuclei (AGN). We have selected a sample in a narrow redshift range, centered on z~1.55, that covers a wide range in M_BH and $dot{M}$, and are observing them with X-shooter, covering rest wavelengths ~1200-9800 AA. The current work covers 30 such objects and focuses on the origin of the AGN spectral energy distribution (SED). After estimating M_BH and $dot{M}$ based on each observed SED, we use thin AD models and a Bayesian analysis to fit the observed SEDs in our sample. We are able to fit 22/30 of the SEDs. Out of the remaining 8 SEDs, 3 can be fit by the thin AD model by correcting the observed SED for reddening within the host galaxy and 4 can be fit by adding a disc wind to the model. In four of these 8 sources, Milky Way-type extinction, with the strong 2175AA feature, provides the best reddening correction. The distribution in spin parameter covers the entire range, from -1 to 0.998, and the most massive BHs have spin parameters greater than 0.7. This is consistent with the spin-up model of BH evolution. Altogether, these results indicate that thin ADs are indeed the main power houses of AGN, and earlier claims to the contrary are likely affected by variability and a limited observed wavelength range.
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