Found 4 talks width keyword Seyfert galaxies

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Tuesday July 8, 2014
Dr. Omaira Gonzalez Martin
IAC

Abstract

The active galactic nuclei is conformed by a number of classes. Optically they are defined using diagnostics based on optical emission lines. At X-rays they are classified by the power of the AGN continuum and the shape of the X-ray spectra. Therefore, optical and X-ray classes are independent classifications. However, optical and X-ray classes show many discrepancies not fully understood yet. Some AGN at X-rays do not show any AGN signature at optical wavelengths (called optical elusive). Classical obscured AGN are ’sometimes’ not obscured at X-rays.

We have studied the ‘synapses’ between them using artificial neural networks (Gonzalez-Martin+14). To do so, we used flux-calibrated X-ray spectra of a sample of 90 emission line nuclei (ELN) observed with XMM-Newton. It includes starbursts (SB), transition objects (T2), LINERs (L1.8 and L2), and Seyferts (S1, S1.8, and S2).

The ELN can be classified into six classes, based on the shape of their X-ray spectra. These classes are associated with most of the optical classes. The key parameters to explain them at X-rays are three. The first parameter is an AGN-like component, which is present in all of them (even non-AGN at optical wavelengths!). The second one is obscuration, which almost certainly drives the Type-1/Type-2 dichotomy, but may also explain why L1.8 are more similar to S1s while L2/T2 are more similar to S1.8s. The third component is star-forming activity happening at the host galaxy and contributing at X-rays. The AGN strength, relative to the host-galaxy component, determines the average X-ray spectrum for these classes as follows: S1 -> S1.8 -> L1.8/S2 -> L2/T2/ -> SB.


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Tuesday April 26, 2011
Dr. Cristina Ramos Almeida
University of Sheffield, UK

Abstract

I will present new mid-infrared imaging data for a sample of ~20 nearby Seyfert galaxies obtained with T-ReCS and MICHELLE on the Gemini Telescopes at subarcsecond resolution. Our aim is to compare the properties of Type-1 and Type-2 Seyfert tori using clumpy torus models and a Bayesian approach to fit the infrared nuclear spectral energy distributions (SEDs). These dusty tori have physical sizes smaller than 10 pc radius, as derived from our fits. Unification schemes of AGN account for a variety of observational differences in terms of viewing geometry. However, we find evidence that strong unification may not hold, and that the immediate dusty surroundings of Type-1 and Type-2 Seyfert nuclei are intrinsically different. The Type-2 tori studied here are broader, have more clumps, and these clumps have lower optical depths than those of Type-1 tori. The larger the covering factor of the torus, the smaller the probability of having direct view of the AGN, and vice-versa. In our sample, Seyfert 2 tori have larger covering factors and smaller escape probabilities than those of Seyfert 1. Thus, on the basis of the results presented here, the classification of a Seyfert galaxy as a Type-1 or Type-2 depends more on the intrinsic properties of the torus rather than on its mere inclination, in contradiction with the simplest unification model.


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Thursday October 9, 2008
Dr. Juan Antonio Fernández Ontiveros
Instituto de Astrofísica de Canarias, Spain

Abstract

Starbursts and AGNs are frequently coupled in the central kiloparsecs of Seyfert galaxies, where molecular gas plays a critical role in fueling nuclear starburst activity and feeding the central black hole. Unveiling the dusty nuclear regions with high-spatial resolution techniques in the near-infrared (NIR) permits us to disentangle the AGN and the stellar clusters, characterizing both sources separately. In this context, a small sample of nearby galaxies have been observed with VLT/NaCo adaptive optics in the NIR. These observations were completed with similar high-spatial resolution data in the mid-infrared (VLT/VISIR), optical (HST) and radio wavelengths (VLA). A new alignment for the starburst galaxy NGC 253 was found based on NIR and radio data, due to the high-spatial resolution in both spectral regions, finding NIR counterparts for 8 known radio sources. It is remarkable the lack of any optical or IR counterpart for the radio core, proposed as a low luminosity AGN, which presents an IR-to-radio emission ratio similar (or even lower) than Sgr. A*. Using the high-spatial resolution aligned dataset from optical-IR to radio wavelengths we derived a representative spectral energy distribution (SED) based on 37 young dust embedded clusters resolved in the inner 0.4 kpc. The template is characterized by a maximum at 20 μ and a gentle bump in the 1-2 μ range. These features, absent in lower spatial resolution templates, can be well reproduced by considering an important contribution of very young stellar objects to the IR, and are thus associated with hot dust surrounding the protostars. The average SED was then compared with the nuclear star forming regions found in the Seyfert 2/starburst galaxy NGC 7582.

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Thursday May 22, 2008
Dr. Ruymán Azzollini, Dr. Cristina Ramos Almeida
Instituto de Estructura de la Materia, CSIC, Spain
University of Sheffield, UK

Abstract

1) In this brief seminar (<~25 minutes+questions) I will present recent results on the study of "deep", high resolution, surface brightness profiles of a sample of ~500 late-type galaxies in the redshift range 0.1—1.1, making use of publicly available HST/ACS imaging of the GOODS-South field. We have classified and parameterized, according to usual prescriptions in this kind of analysis, these profiles, with special emphasis put on the so called ``truncated'' disks. This is the case in which, beyond a certain radius, termed as ``Break Radius'', the exponentially decaying surface brightness profile along the stellar disk gives way to an even more abrupt exponential decay. This radius can be taken as an spatial "scale" for the disk, as observed in a given band (in our case, the rest-frame B-band). Comparing with analogous analysis for galaxies in the Local Universe, as we have done, it is possible to extract valuable information on the evolution of several photometric properties of the stellar disks of galaxies, related to the stellar populations distributions. I will also present results on the analysis of the color profiles of this sample of galaxies, which have yield an interesting result which is, perhaps, the main reason that justifies calling this talk a "breaking news" seminar, as I will show. Summarizing, an overview of the results we have obtained will be given, and our conclusions on them, explaining how they can be understood in the frame of Galaxy Evolution (2) Based on high quality near-infrared spectroscopy (obtained with WHT/LIRIS) we reveal that the nucleus of Mrk 573 is an obscured Narrow-Line Seyfert 1 and not an archetypal Seyfert 2 as it has been classified until now. Currently only four AGNs have been classified into this category. We have detected permitted OI and FeII transitions, which indicates the existence of a high density region similar to the BLRs detected in type 1 AGN.


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