Active Galactic Nuclei at the Crossroads of Astrophysics

Lobanov, A. P.; Zensus, J. A.

doi:10.1007/978-3-540-39756-4_43

Cited by 10 publications

(11 citation statements)

References 193 publications

(256 reference statements)

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“…We have so far considered star formation and AGN activity as causes of the X-ray emission observed, but a third possibility is an extended hot gas halo as seen around many nearby early-type galaxies in the same mass-range as our sample galaxies (Mulchaey & Jeltema 2010) and predicted/observed around similar spirals (Toft et al 2002;Rasmussen et al 2009;Anderson & Bregman 2011;Dai et al 2012). AGN X-ray emission is expected to come from a very small, R < 1 pc, accretion disk surrounding the central black hole of the host galaxy (Lobanov & Zensus 2007), whereas very extended star formation in the galaxy or a hot gas halo surrounding it would lead to more extended emission. We investigate the possibilities for these latter cases by comparing radial surface brightness profiles of the 51 individually X-ray detected galaxies out to a radius of 8 ′′ in both the stacked observed image and a correspondingly stacked PSF image.…”

Section: Contribution From Hot Gas Halosmentioning

confidence: 99%

EVIDENCE FOR WIDESPREAD ACTIVE GALACTIC NUCLEUS ACTIVITY AMONG MASSIVE QUIESCENT GALAXIES ATz∼ 2

Olsen

Rasmussen

Toft

et al. 2013

ApJ

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We quantify the presence of active galactic nuclei (AGNs) in a mass-complete (M * > 5 × 10 10 M ⊙ ) sample of 123 star-forming and quiescent galaxies at 1.5 ≤ z ≤ 2.5, using X-ray data from the 4 Ms Chandra Deep Field-South (CDF-S) survey. 41% ± 7% of the galaxies are detected directly in X-rays, 22% ± 5% with rest-frame 0.5-8 keV luminosities consistent with hosting luminous AGNs (L 0.5-8 keV > 3 × 10 42 erg s −1 ). The latter fraction is similar for star-forming and quiescent galaxies, and does not depend on galaxy stellar mass, suggesting that perhaps luminous AGNs are triggered by external effects such as mergers. We detect significant mean X-ray signals in stacked images for both the individually non-detected star-forming and quiescent galaxies, with spectra consistent with star formation only and/or a low-luminosity AGN in both cases. Comparing star formation rates inferred from the 2-10 keV luminosities to those from rest-frame IR+UV emission, we find evidence for an X-ray excess indicative of low-luminosity AGNs. Among the quiescent galaxies, the excess suggests that as many as 70% − 100% of these contain low-or high-luminosity AGNs, while the corresponding fraction is lower among star-forming galaxies (43% − 65%). Our discovery of the ubiquity of AGNs in massive, quiescent z ∼ 2 galaxies provides observational support for the importance of AGNs in impeding star formation during galaxy evolution.

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Section: Contribution From Hot Gas Halosmentioning

confidence: 99%

EVIDENCE FOR WIDESPREAD ACTIVE GALACTIC NUCLEUS ACTIVITY AMONG MASSIVE QUIESCENT GALAXIES ATz∼ 2

Olsen

Rasmussen

Toft

et al. 2013

ApJ

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“…Extragalactic relativistic jets are formed in the immediate vicinity of the central black holes in galaxies, at distances on the order of 100 gravitational radii, and they become visible in the radio at distances of about 1000 gravitational radii (Lobanov & Zensus 2007). This apparent origin of the radio jets is commonly called the "core".…”

Section: Introductionmentioning

confidence: 99%

Opacity in compact extragalactic radio sources and its effect on astrophysical and astrometric studies

Kovalev

Lobanov

Pushkarev

et al. 2008

A&A

211

267

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Context. The apparent position of the "core" in a parsec-scale radio jet (a compact, bright emitting region at the narrow end of the jet) depends on the observing frequency, owing to synchrotron self-absorption and external absorption. While providing a tool probing physical conditions in the vicinity of the core, this dependency poses problems for astrometric studies using compact radio sources. Aims. We investigated the frequency-dependent shift in the positions of the cores (core shift) observed with very long baseline interferometry (VLBI) in parsec-scale jets. We discuss related physics, as well as its effect on radio astrometry and the connection between radio and optical positions of astrometric reference objects. Methods. We searched for the core shift in a sample of 277 radio sources imaged at 2.3 GHz (13 cm) and 8.6 GHz (4 cm) frequency bands using VLBI observations made in 2002 and 2003. The core shift was measured by referencing the core position to optically thin jet features whose positions are not expected to change with frequency. Results. We present here results for 29 selected active galactic nuclei (AGN) with bright distinct VLBI jet features that can be used in differential measurements and that allow robust measurements of the shift to be made. In these AGN, the magnitude of the measured core shift between 2.3 and 8.6 GHz reaches 1.4 mas, with a median value for the sample of 0.44 mas. Nuclear flares result in temporal variability of the shift.Conclusions. An average shift between the radio (4 cm) and optical (6000 Å) bands is estimated to be approximately 0.1 mas, and it should be taken into account in order to provide the required accuracy of the radio-optical reference frame connection. This can be accomplished with multi-frequency VLBI measurements yielding estimates of the core shift in the sources used for the radio reference frame and radio-optical position alignment.

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“…Substantial progress achieved during the past decade in studies of active galactic nuclei (see [51] for a review of recent results) has brought an increasingly wider recognition of the ubiquity of relativistic outflows (jets) in galactic nuclei [17,84], turning them into an effective probe of nuclear regions in galaxies [46]. Emission properties, dynamics, and evolution of an extragalactic jet are intimately connected to the characteristics of the supermassive black hole, accretion disk, and broad-line region (BLR) in the nucleus of the host galaxy [51]. The jet continuum emission is dominated by non-thermal synchrotron and inverse-Compton radiation [78].…”

Section: Introductionmentioning

confidence: 99%

Compact radio jets and nuclear regions in active galaxies

Lobanov¹

2007

Proceedings of 8th European VLBI Network Symposium — PoS(8thEVN)

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VLBI observations of relativistic outflows (jets) in galactic nuclei, complemented with detailed studies made in other spectral domains, have become an effective tool for investigating the physics of nuclear regions in galaxies. High-resolution radio observations access directly the regions where the jets are formed, and trace their evolution and interaction with the nuclear environment. The emission properties, dynamics, and evolution of jets in AGN are intimately connected to the characteristics of the central supermassive black hole, accretion disk, and broad-line region (BLR) in active galaxies. Large VLBI surveys (15 GHz VLBA survey, MOJAVE) and dedicated monitoring programmes follow systematically the evolution of several hundreds of relativistic jets. These observations, combined with optical and X-ray studies, yield arguably the most detailed picture of the galactic nuclei. Recent results from studies of the nuclear regions in several active galaxies with prominent outflows are reviewed in this contribution.

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Active Galactic Nuclei at the Crossroads of Astrophysics

Cited by 10 publications

References 193 publications

EVIDENCE FOR WIDESPREAD ACTIVE GALACTIC NUCLEUS ACTIVITY AMONG MASSIVE QUIESCENT GALAXIES ATz∼ 2

EVIDENCE FOR WIDESPREAD ACTIVE GALACTIC NUCLEUS ACTIVITY AMONG MASSIVE QUIESCENT GALAXIES ATz∼ 2

Opacity in compact extragalactic radio sources and its effect on astrophysical and astrometric studies

Compact radio jets and nuclear regions in active galaxies

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