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Register a new userLow radiative efficiency accretion at work in active galactic nuclei: the nuclear spectral energy distribution of NGC4565
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We derive the spectral energy distribution (SED) of the nucleus of the Seyfert galaxy NGC4565. Despite its classification as a Seyfert2, the nuclear source is substantially unabsorbed. The absorption we find from Chandra data (N_H=2.5 X 10^21 cm^-2) is consistent with that produced by material in the galactic disk of the host galaxy. HST images show a nuclear unresolved source in all of the available observations, from the near-IR H band to the optical U band. The SED is completely different from that of Seyfert galaxies and QSO, as it appears basically ``flat in the IR-optical region, with a small drop-off in the U-band. The location of the object in diagnostic planes for low luminosity AGNs excludes a jet origin for the optical nucleus, and its extremely low Eddington ratio L_o/L_Edd indicates that the radiation we observe is most likely produced in a radiatively inefficient accretion flow (RIAF). This would make NGC4565 the first AGN in which an ADAF-like process is identified in the optical. We find that the relatively high [OIII] flux observed from the ground cannot be all produced in the nucleus. Therefore, an extended NLR must exist in this object. This may be interpreted in the framework of two different scenarios: i) the radiation from ADAFs is sufficient to give rise to high ionization emission-line regions through photoionization, or ii) the nuclear source has recently ``turned-off, switching from a high-efficiency accretion regime to the present low-efficiency state.
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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.
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.
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.
The discovery of hard, power-law X-ray emission from a sample of six nearby elliptical galaxies, including the dominant galaxies of the Virgo,Fornax and Centaurus clusters (M87, NGC 1399 and NGC 4696, respectively), and NGC 4472, 4636 and 4649 in the Virgo cluster, has important implications for the study of quiescent supermassive black holes. We describe how the broad band spectral energy distributions of these galaxies, which accrete from their hot gaseous halos at rates comparable to their Bondi rates, can be explained by low-radiative efficiency accretion flows in which a significant fraction of the mass, angular momentum and energy is removed from the flows by winds. The observed suppression of the synchrotron component in the radio band and the systematically hard X-ray spectra, which are interpreted as thermal bremsstrahlung emission, support the conjecture that significant mass outflow is a natural consequence of systems accreting at low-radiative efficiencies. We briefly discuss an alternative model for the X-ray emission, namely that it is due to nonthermal synchrotron-self Compton processes in the accretion flow, or wind. This appears to require implausibly weak magnetic fields. Emission from a collimated jet viewed off axis should be distinguishable from the bremsstrahlung model by variability and thermal line emission studies. We argue that the difference in radiative efficiency between the nuclei of spiral and elliptical galaxies arises from the different manner in which interstellar gas is fed into the nuclei. In ellipticals, matter fed from the hot (slowly cooling) ISM is likely to be highly magnetized and with low specific angular momentum, both of which favor low-radiative efficiency accretion solutions and possibly the formation of the observed jets.
The accretion efficiency for individual black holes is very difficult to determine accurately. There are many factors that can influence each step of the calculation, such as the dust and host galaxy contribution to the observed luminosity, the black hole mass and more importantly, the uncertainties on the bolometric luminosity measurement. Ideally, we would measure the AGN emission at every wavelength, remove the host galaxy and dust, reconstruct the AGN spectral energy distribution and integrate to determine the intrinsic emission and the accretion rate. In reality, this is not possible due to observational limitations and our own galaxy line of sight obscuration. We have then to infer the bolometric luminosity from spectral measurements made in discontinuous wavebands and at different epochs. In this paper we tackle this issue by exploring different methods to determine the bolometric luminosity. We first explore the trend of accretion efficiency with black hole mass (efficiency proportional to M^{sim 0.5}) found in recent work by Davis & Laor and discuss why this is most likely an artefact of the parameter space covered by their PG quasar sample. We then target small samples of AGN at different redshifts, luminosities and black hole masses to investigate the possible methods to calculate the accretion efficiency. For these sources we are able to determine the mass accretion rate and, with some assumptions, the accretion efficiency distributions. Even though we select the sources for which we are able to determine the parameters more accurately, there are still factors affecting the measurements that are hard to constrain. We suggest methods to overcome these problems based on contemporaneous multi-wavelength data measurements and specifically targeted observations for AGN in different black hole mass ranges.
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