Testing the agn unified model using spectropolarimetry



Дата24.04.2016
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Testing the AGN unified model using spectropolarimetry

Cristina Ramos Almeida (1,2), María Jesús Martínez González (1,2), Andrés Asensio Ramos (1,2), José Antonio Acosta-Pulido (1,2) and Rafael Manso Sainz (1,2)

1 : Instituto de Astrofísica de Canarias, Calle Vía Láctea, s/n, E-38205, La Laguna, Tenerife, Spain. cra@iac.es

2 : Departamento de Astrofísica, Universidad de La Laguna, E-38205, La Laguna, Tenerife, Spain

The origin of the unification scheme for Seyfert galaxies (Antonucci & Miller 1985) was the detection of polarized broad lines in the nucleus of the Sy2 galaxy NGC 1068. Although very successful and widely used, this simplistic geometrical model is unable to explain several observational properties of Seyfert galaxies. For example, only 30-50% of nearby Sy2s show a Sy1-type polarized spectrum (Tran 2001). This might imply that either 1) not all Seyferts harbour a hidden BLR (e.g. Tran 2001, 2003; Gu & Huang 2002) or 2) the distribution of dust within the torus and its inclination are not as simple as predicted by the unified model.




Figure 1: Rest-frame infrared SED of the Seyfert 2 galaxy NGC 3081 (dots). Solid and dashed lines are the ``best fit'' to the data (MAP) and the model described by the median of the posteriors respectively. The shaded region indicates the range of models compatible with the observations at the 2 sigma level.

Concerning the physical properties of the central parsecs of Seyfert galaxies, we recently have reproduced high spatial resolution spectral energy distributions (SEDs) of a sample of nearby Seyfert galaxies with clumpy torus model templates (see Figure 1) using the in-house developed Bayesian tool BayesClumpy to perform the fits (Asensio Ramos & Ramos Almeida 2009). The clumpy torus models propose that the dust is distributed in clumps, instead of homogeneously filling the torus volume. From our fitting, we find that the torus width and the number of clumps in Sy1 and Sy2 tori are different, and the values of the torus inclination angle are typically intermediate between 0 and 90 degrees for the two Seyfert types, and not 0 for Sy1s and 90 for Sy2s, in clear contradiction with the simplest unification model. Thus, what it can be happening is that not all AGN have the same torus: its properties might vary with luminosity, type, etc., and thus, AGN with denser and broader tori will be more likely observed as Type-2 AGN, whereas those with narrower tori and containing less clumps will probably be observed as Type-1 AGN. This finding could explain the lack of polarized broad lines in 50-70% of nearby Seyfert 2 galaxies: the torus has to have a determined orientation (very close to 90 degrees) and enough clumps to make the broad lines visible in polarized light.



Thus, considering all the previous evidence, we plan to obtain a homogeneous dataset of polarized optical spectra with ISIS/WHT for a representative flux-limited sample of Seyfert galaxies for which we have constraints on their torus properties. Such constraints were obtained from the high angular resolution SED modelling with clumpy torus models presented in Ramos Almeida et al. (2009, 2011) and Alonso-Herrero et al. (2011). The observations are scheduled in January 2013 and the sample contains similar numbers of Seyfert 1, 1.2, 1.5, 1.9 and 2 types, to study how the polarization signal varies between types.
References: Alonso-Herrero, A. et al. 2011, ApJ, 736, 82 // Antonucci, R. R. J. & Miller, J. S. 1985, ApJ 297, 621 // Asensio Ramos, A. & Ramos Almeida, C. 2009, ApJ, 696, 2075 // Gu, Q. & Huang, J. 2002, ApJ, 579 205 // Ramos Almeida, C. et al. 2009, ApJ, 702, 1127 // __2011, ApJ, 731, 92 // Tran, H. D. 2001, ApJ, 554, L19 // __2003, ApJ, 583, 632

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