Found 30 talks width keyword active galaxies

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Tuesday October 4, 2011
Dr. Ivan Agudo
Instituto Astrofisica de Andalucia, Spain

Abstract

Relativistic jets in AGN in general, and in blazars in particular, are the most energetic and among the most powerful astrophysical objects known so far. Their relativistic nature provides them the ability to emit profusely in all spectral ranges from radio wavelengths to gamma-rays, as well as abrupt variability in all time scales (from hours to years). Since the birth of gamma-ray astronomy, locating the origin of gamma-ray emission has been a fundamental problem for the knowledge of the emission processes involved. Deep and densely time sampled monitoring programs with the Fermi Gamma-ray Space Telescope and several other facilities at most of the available spectral ranges (including polarization measurements where possible) are starting to shed light for the case of blazars. After a short review of the status of the problem, some of the latest results locating the GeV emission in the jets of some blazars, at >10 parsec from the central AGN engine, will be presented together with their implications about the gamma-ray emission mechanisms involved


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Tuesday May 3, 2011
Miss Josefa Becerra
Instituto de Astrofísica de Canarias, Spain

Abstract

The MAGIC telescopes discovered very high energy (VHE, E>100 GeV) gamma-ray emission coming from the distant Flat Spectrum Radio Quasar (FSRQ) PKS 1222+21 (4C +21.35, z=0.432). It is the second most distant VHE gamma-ray source, with well measured redshift, detected until now. The detection coincides with high energy MeV/GeV gamma-ray activity measured by the Large Area Telescope (LAT) on board the Fermi satellite. The VHE and MeV/GeV spectra, corrected for the absorption by the extragalactic background light, can be described by a single power law with photon index 2.72 ± 0.34 between 3 GeV and 400 GeV, consistent with gamma-ray emission belonging to a single component in the jet. The absence of a spectral cutoff constrains the gamma-ray emission region to lie outside the Broad Line Region, which would otherwise absorb the VHE gamma-rays. On the other hand, the MAGIC measurement of a doubling time of about 9 minutes indicates an extremely compact emission region, in conflict with the "far dissipation" scenario. This result challenges jet emission models in FSRQs and indicates the importance of jet sub-structures.


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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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Friday October 29, 2010
Mr. Hugo Messias
Centro de Astronomia e Astrofísica da Universidade de Lisboa, Portugal

Abstract

In this talk, I will cover our contribution to the study of extremely red galaxy (ERG) populations presenting a multi-wavelength analysis of these objects, selected in the GOODS-South/Chandra Deep Field South field. By using all the photometric (from X-rays to radio) and spectroscopic information available on large deep samples of extremely red objects (EROs, 645 sources), infrared EROs (IEROs, 294 sources) and distant red galaxies (DRGs, 350 sources), we derive redshift distributions, identify AGN powered and star-formation powered galaxies (based on X-ray properties and a new IR AGN diagnostic developed by us), and, using the radio observations of this field, estimate robust (AGN- and dust-unbiased) star formation rate densities (SFRD) for these populations. Applying a redshift separation (1 ≤ z < 2 and 2 ≤ z ≤ 3) we find a significant rise (a factor of 1.5 — 3) of SFRD for EROs and DRGs toward high-z, while none is observed for IEROs. As expected, we find a significant overlap between the ERG populations, and investigate the properties of "pure" (galaxies that conform to only one of the three considered ERG criteria) and "combined" (galaxies conforming to all three criteria) sub-populations. We find ERG sub-populations with no AGN activity and intense star-formation rates. With average values of ~180 M⊙/yr at 2 ≤ z < 3, they reasonably contribute to the global star-formation rate density, reaching a > 20% level. Strong AGN behaviour is not observed in the ERG population, with AGN only increasing the average radio luminosity of ERGs by 10 — 20%. However, AGN are frequently found (in up to 27% of the ERG population), and would increase the SFRD estimate by over 100%. Thus, and while the contribution of SF processes to the radio luminosity in galaxies with AGN remains uncertain, a comprehensive identification of AGN in these populations is necessary to obtain meaningful results. The dust content to each population is also derived by correlating UV and Radio SFRs, giving a higher obscuration for more active SF sources. Also, know to be amongst the most massive galaxies in the high-z universe, I will show that ERGs may constitute up to 60% of the total mass in the universe at 1 ≤ z ≤ 3. Finally, preliminary and promising results are presented on the morphologies of ERGs (CAS and Gini/M20 parameters) based on the v1.9 ACS GOODS-S images.

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Thursday March 11, 2010
Dr. Almudena Prieto
Instituto de Astrofísica de Canarias, Spain

Abstract

Spectral energy distributions (SEDs) of the central few tens of parsec region of some of the nearest active galactic nuclei (AGN) are presented. Peering into the nucleus at these scales, it is found that the intrinsic shape of the spectral energy distribution of an AGN and inferred bolometric luminosity largely depart from those currently on use, mostly extracted from low resolution data. The shape of the SED is different and the AGN luminosities can be overestimated by up to two orders of magnitude if relying on IR satellite data.
Although the shape of these SEDs are currently limited by the availability of high angular resolution data beyond ~20 μ, a prediction from this work is that a major contribution from cold dust below 100 K to these cores is not expected. Over the nine orders of magnitude in frequency covered by these SEDs, the power stored in the IR bump is by far the most energetic fraction of the total energy budget in these cores, accounting for more than 70% of the total.

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Monday March 1, 2010
Dr. Masa Imanishi
National Astronomical Observatory of Japan

Abstract

We present the results of our systematic search for optically elusive, but intrinsically luminous buried AGNs in >100 nearby (z < 0.3) luminous infrared galaxies with L(IR) > 1011 L⊙, classified optically as non-Seyferts. To disentangle AGNs and stars, we have performed (1) infrared 2.5-35 μ low-resolution (R ~ 100) spectroscopy using Subaru, AKARI, and Spitzer, to estimate the strengths of PAH (polycyclic aromatic hydrocarbon) emission and dust absorption features, (2) high-spatial-resolution infrared 20 micron imaging observations using Subaru and Gemini, to constrain the emission surface brightnesses of energy sources, and (3) millimeter interferometric measurements of molecular gas flux ratios, which reflect the physical and chemical effects from AGNs and stars. Overall, all methods provided consistent pictures. We found that the energetic importance of buried AGNs is relatively higher in galaxies with higher infrared luminosities (where more stars will be formed), suggesting that AGN-starburst connections are luminosity dependent. Our results might be related to the AGN feedback scenario as the possible origin of the galaxy down-sizing phenomenon.

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Monday February 8, 2010
Dr. Roberto Cid Fernandes
Universidade Federal de Santa Catarina, Florianopolis, Brazil

Abstract

This talk is divided into two related parts. First, we will call your attention to a basic, but often overlooked worrying fact, and presents ways of dealing with it. The fact is: an enormous number of galaxies in surveys like the SDSS have emission lines which are too weak (low S/N) to be classified by usual schemes (ie, diagnostic diagrams). It turns out that most of these are AGN-like, so ignoring them on the basis of low S/N (which most people do) leaves as much as 2/3 of these emission line galaxies unaccounted for. The solution: We present a number of alternative methods to rescue this numerous population from the classification limbo. We find that about 1/3 of these weak-line galaxies are massive, metal rich star-forming systems, while the remaining 2/3 are more like LINERs. In the second part, we revisit the old idea by Binette et al (1994) that post-AGB stars can account for the emission line properties of some galaxies. A "retired galaxy" model is presented and compared to data in the SDSS. We find that about 1/4 of the galaxies classified as LINERs in the SDSS are consistent with this model, where all ionizing radiation is of stellar origin. More dramatically, nearly 100% of weak-line LINERs are perfectly consistent with being just retired galaxies, with no active nucleus. If these ideas are correct, contrary to current practice, relatively few LINERs should be counted as bona fide AGN.

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Thursday November 5, 2009
Prof. Rony Keppens
Centre for Plasma-Astrophysics, K. U. Leuven, Belgium

Abstract

I will present grid-adaptive computational studies of both magnetized and unmagnetized jet flows, with significantly relativistic bulk speeds, as appropriate for AGN jets. Our relativistic jet studies shed light on the observationally established classification of Fanaroff-Riley galaxies, where the appearance in radio maps distinguishes two types of jet morphologies. We investigate how density changes in the external medium can induce one-sided jet decelerations, explaining the existence of hybrid morphology radio sources. Our simulations explore under which conditions highly energetic FR II jets may suddenly decelerate and continue with FR I characteristics. In a related investigation, we explore the role of dynamically important, organized magnetic fields in the collimation of the relativistic jet flows. In that study, we concentrate on morphological features of the bow shock and the jet beam, for various jet Lorentz factors and magnetic field helicities. We show that the helicity of the magnetic field is effectively transported down the beam, with compression zones in between diagonal internal cross-shocks showing stronger toroidal field regions. For the high speed jets considered, significant jet deceleration only occurs beyond distances exceeding hundred jet radii, as the axial flow can reaccelerate downstream to internal cross-shocks. This reacceleration is magnetically aided, due to field compression across the internal shocks which pinch the flow.

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Thursday July 16, 2009
Prof. David Koo
University of California Observatories, Lick Observatory, USA

Abstract

AEGIS (All-wavelength Extended Groth strip International Survey: aegis.ucolick.org) is on-going survey that opens up new views of the development of galaxies and AGN's at redshifts z about 1. AEGIS is panchromatic like GOODS, with coverage ranging from X-ray to radio, and nearly as deep but more panoramic by covering a 4x larger region. Its backbone is the most Northern (accessible to the GTC) of the four fields of the DEEP2 Keck spectroscopic survey, which provides not only precision redshifts that yield reliable pairs, groups, and environments, but also internal kinematics and chemical abundances. After an overview of the DEEP and AEGIS surveys, I will share some recent highlights, including using a new kinematic measure for distant galaxies to track Tully-Fisher-like evolution; discovering metal poor, massive, luminous galaxies; finding ubiquitous galactic gas outflows among distant star forming galaxies; and exploring the nature of distant x-ray AGNs.

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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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