Understanding Of Active Galactic Nuclei Research Articles

Compact Symmetric Objects. III. Evolution of the High-luminosity Branch and a Possible Connection with Tidal Disruption Events

We use a sample of 54 compact symmetric objects (CSOs) to confirm that there are two unrelated CSO classes: an edge-dimmed, low-luminosity class (CSO 1), and an edge-brightened, high-luminosity class (CSO 2). Using blind tests, we show that CSO 2s consist of three subclasses: CSO 2.0, having prominent hot spots at the leading edges of narrow jets and/or narrow lobes; CSO 2.2, without prominent hot spots and with broad jets and/or lobes; and CSO 2.1, which exhibit mixed properties. Most CSO 2s do not evolve into larger jetted active galactic nuclei (AGN), but spend their whole life cycle as CSOs of size ≲500 pc and age ≲5000 yr. The minimum energies needed to produce the radio luminosity and structure in CSO 2s range from ∼10−4 M ⊙ c 2 to ∼7 M ⊙ c 2. We show that the transient nature of most CSO 2s, and their birth rate, can be explained through ignition in the tidal disruption events of stars. We also consider possibilities of tapping the spin energy of the supermassive black hole, and tapping the energy of the accretion disk. Our results demonstrate that CSOs constitute a large family of AGN in which we have thus far studied only the brightest. More comprehensive CSO studies, with higher sensitivity, resolution, and dynamic range, will revolutionize our understanding of AGN and the central engines that power them.

Completing the Census of AGN in GOODS-S/HUDF: New Ultradeep Radio Imaging and Predictions for JWST

Abstract A global understanding of active galactic nuclei (AGN) and their host galaxies hinges on completing a census of AGN activity without selection biases down to the low-luminosity regime. Toward that goal, we identify AGN within faint radio populations at cosmic noon selected from new ultradeep, high-resolution imaging from the Karl G. Jansky Very Large Array at 6 and 3 GHz. These radio data are spatially coincident with the ultradeep legacy surveys in the GOODS-S/HUDF region, particularly the unparalleled Chandra 7 Ms X-ray imaging. Combined, these data sets provide a unique basis for a thorough census of AGN, allowing simultaneous identification via (1) high X-ray luminosity, (2) hard X-ray spectra, (3) excess X-ray relative to 6 GHz, (4) mid-IR colors, (5) SED fitting, (6) radio excess via the radio–infrared relation, (7) flat radio spectra via multiband radio, and (8) optical spectroscopy. We uncover AGN in fully half our faint radio sample, indicating a source density of one AGN arcmin−2, with a similar number of radio-undetected AGN identified via X-ray over the same area. Our radio-detected AGN are majority radio-quiet, with radio emission consistent with being powered predominantly by star formation. Nevertheless, we find AGN radio signatures in our sample: ∼12% with radio excess indicating radio-loud activity, and ∼16% of radio-quiet AGN candidates with flat or inverted radio spectra. The latter is a lower limit, pending our upcoming deeper 3 GHz survey. Finally, despite these extensive data sets, this work is likely still missing heavily obscured AGN. We discuss in detail this elusive population and the prospects for completing our AGN census with James Webb Space Telescope/MIRI.

Excitation mechanism in the intracluster filaments surrounding the Brightest Cluster Galaxies

AbstractMulti-phase filamentary structures surrounding giant elliptical galaxies at the center of cool-core clusters, the Brightest Cluster Galaxies (BCGs), have been detected from optical to submillimeter wavelengths. The source of the ionisation in the filaments is still debated. Studying the excitation of these structures is key to our understanding of Active Galactic Nuclei (AGN) feedback in general, and more precisely of the impact of environmental and local effects on star formation. One possible contributor to the excitation of the filaments is the thermal radiation from the cooling of the hot plasma surrounding the BCGs, the so-called cooling flow.

AGN Feedback and its Importance to Galaxy Evolution in the Era of the ngVLA

AbstractEnergetic feedback by Active Galactic Nuclei (AGN) plays an important evolutionary role in the regulation of star formation (SF) on galactic scales. However, the effects of this feedback as a function of redshift and galaxy properties such as mass, environment and cold gas content remain poorly understood. The broad frequency coverage (1 to 116 GHz), high collecting area (about ten times higher than the Karl G. Jansky Very Large Array), and superb angular resolution (maximum baselines of at least a few hundred km) of the proposed next-generation Very Large Array (ngVLA) are uniquely poised to revolutionize our understanding of AGN and their role in galaxy evolution.

The Many Routes to AGN Feedback

The energy released by Active Galactic Nuclei (AGN) in the form of radiation, winds, or radio plasma jets, is known to impact on the surrounding interstellar medium. The result of these processes, known as AGN (negative) feedback, is suggested to prevent gas, in and around galaxies, from cooling, and to remove, or at least redistribute, gas by driving massive and fast outflows, hence playing a key role in galaxy evolution. Given its importance, a large effort is devoted by the astronomical community to trace the effects of AGN on the surrounding gaseous medium and to quantify their impact for different types of AGN. This review briefly summarizes some of the recent observational results obtained in different wavebands, tracing different phases of the gas. I also summarize the new insights they have brought, and the constraints they provide to numerical simulations of galaxy formation and evolution. The recent addition of deep observations of cold gas and, in particular, of cold molecular gas, has brought some interesting surprises and has expanded our understanding of AGN and AGN feedback.

Polarimetric imaging of NGC 1068 at high angular resolution in the near infrared

Aims. One of the main observational challenges for investigating the central regions of active galactic nuclei (AGN) at short wavelengths, using high angular resolution, and high contrast observations, is to directly detect the circumnuclear optically thick material hiding the central core emission when viewed edge-on. The lack of direct evidence is limiting our understanding of AGN, and several scenarios have been proposed to cope for the diverse observed aspects of activity in a unified approach.Methods. Observations in the near-infrared spectral range have shown themselves to be powerful for providing essential hints to the characterisation of the unified model ingredients because of the reduced optical depth of the obscuring material. Moreover, it is possible to trace this material through light scattered from the central engine’s closest environment, so that polarimetric observations are the ideal tool for distinguishing it from purely thermal and stellar emissions. Results. Here we show strong evidence that there is an extended nuclear torus at the center of NGC 1068 thanks to new adaptive-optics-assisted polarimetric observations in the near-infrared. The orientation of the polarization vectors proves that there is a structured hourglass-shaped bicone and a compact elongated (20 × 60 pc) nuclear structure perpendicular to the bicone axis. The linearly polarized emission in the bicone is dominated by a centro-symmetric pattern, but the central compact region shows a clear deviation from the latter with linear polarization aligned perpendicular to the bicone axis.

Active galactic nuclei and their role in galaxy evolution: The infrared perspective

The remarkable progress made in infrared (IR) astronomical instruments over the last 10–15 years has radically changed our vision of the extragalactic IR sky, and overall understanding of galaxy evolution. In particular, this has been the case for the study of active galactic nuclei (AGN), for which IR observations provide a wealth of complementary information that cannot be derived from data in other wavelength regimes. In this review, I summarize the unique contribution that IR astronomy has recently made to our understanding of AGN and their role in galaxy evolution, including both physical studies of AGN at IR wavelengths, and the search for AGN among IR galaxies in general. Finally, I identify and discuss key open issues that should be possible to address with forthcoming IR telescopes.

XMM-NEWTON SCIENTIFIC HIGHLIGHTS: X-RAY SPECTROSCOPIC POPULATION STUDIES OF AGN

In this paper I review the contribution that the XMM-Newton ESA X-ray mission has given to our understanding of Active Galactic Nuclei, together with other operational, and complementary, X-ray facilities. I will focus on answering three basic questions: a) to which extent do AGN share the same engine?; b) to which extent are AGN “relativistic machines”?; c) to which extent do AGN affect their immediate environment?

Conference Summary

AbstractThis conference on “Multi-wavelength AGN Surveys and Studies” has provided a detailed look at the explosive growth over the past decade, of available astronomical data from a growing list of large scale sky surveys, from radio-to-gamma rays. We are entering an era were multi-epoch (months to weeks) surveys of the entire sky, and near-instantaneous follow-up observations of variable sources, are elevating time-domain astronomy to where it is becoming a major contributor to our understanding of Active Galactic Nuclei (AGN). While we can marvel at the range of extragalactic phenomena dispayed by sources discovered in the original “Markarian Survey” – the first large-scale objective prism survey of the Northern Sky carried out at the Byurakan Astronomical Observtory almost a half-century ago – it is clear from the talks and posters presented at this meeting that the data to be be obtained over the next decade will be needed if we are to finally understand which phase of galaxy evolution each Markarian Galaxy represents.

The connection between the jet and accretion disk power for blazars

Disk-jet symbiosis is a fundamental problem in our understanding of active galactic nuclei (AGN). Using a sample of 47 blazars and each with an observed synchrotron peak luminosity (νLν)Syn,obspeak, Doppler factor δ and total luminosity of the broad-line region LBLR from the literature, we show that a significant correlation exists between the jet power indicator (νLν)Syn,obspeak and the disk power indicator LBLR even after the Doppler enhancement of the synchrotron emission is removed. This result suggests a close link between the jet powering process and the disk accreting process. Furthermore, our statistical result is qualitatively and quantitatively consistent with the theoretical expectations and therefore provides an independent evidence for the disk-jet connection.

VERITAS UPPER LIMIT ON THE VERY HIGH ENERGY EMISSION FROM THE RADIO GALAXY NGC 1275

The recent detection by the Fermi gamma-ray space telescope of high-energy gamma-rays from the radio galaxy NGC 1275 makes the observation of the very high energy (VHE: E > 100 GeV) part of its broadband spectrum particularly interesting, especially for the understanding of active galactic nuclei (AGN) with misaligned multi-structured jets. The radio galaxy NGC 1275 was recently observed by VERITAS at energies above 100 GeV for about 8 hours. No VHE gamma-ray emission was detected by VERITAS from NGC 1275. A 99% confidence level upper limit of 2.1% of the Crab Nebula flux level is obtained at the decorrelation energy of approximately 340 GeV, corresponding to 19% of the power-law extrapolation of the Fermi Large Area Telescope (LAT) result.

Adaptive optics: Observations and prospects for studies of active Galactic Nuclei

These lectures take a look at how observations with adaptive optics (AO) are beginning to influence our understanding of active galactic nuclei (AGN). By focussing on a few specific topics, the aim is to highlight the different ways in which enhanced spatial resolution from AO can aid the scientific analysis of AGN data. After presenting some background about how AO works, I will describe a few recent observations made with AO of QSO host galaxies, the Galactic Center, and nearby AGN, and show how they have contributed to our knowledge of these enigmatic objects.

The Granada workshop on High Redshift Radio Galaxies: An overview

AbstractThe Granada workshop on High Redshift Radio Galaxies (HzRGs) gave an excellent overview of the progress that has been made in this field during the last 3 years. Here we briefly review some of the results, with an emphasis on what studies of HzRGs can teach us about the formation and evolution of massive galaxies, clusters and active galactic nuclei (AGN). Of great relevance for this workshop are scenarios that describe certain aspects of the evolution of radio galaxies, including (i) the sequence of events after merging of galaxies that ultimately lead to extended powerful radio sources and (ii) the mass assembly and virialization of the hosting massive galaxies and their associated (proto‐)clusters. Furthermore, I briefly discuss two projects that are important for a further understanding of AGN and high redshift radio galaxies. First, using the MIDI instrument mounted on the VLT Interferometer, the dusty tori of nearby AGN can be studied in the range of 8–13 micron at high angular resolution. The first result on the nearby AGN NGC 1068 as presented by Jaffe et al. (2004) indicated the presence of a hot (T > 800 K), compact (≲1 pc) component, possible identified with the base of the jet and a warm (270 K), well‐resolved (3 × 4 pc) component associated with the alleged torus. Second, LOFAR is a new low frequency radio telescope that is currently being build in the Netherlands and is expected to be operational in 2008. With 50 stations spread over an area of 100 km in diameter, its resolution and sensitivity will be unprecedented in the frequency range 10–240 MHz. LOFAR will be a unique instrument that will impact a broad range of astrophysical topics varying from the epoch of reionisation, to gamma ray bursts and cosmic rays. Surveys with LOFAR will be of paramount importance for studies of HzRGs: It will enable (i) defining samples of radio galaxies with redshifts higher than 6, (ii) observations of starbursting galaxies in proto‐clusters, and (iii) mapping out the low‐frequency radio emission of virtually all northern radio‐loud AGN in revolutionary detail. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Implications of VERITAS on AGN astrophysics

The EGRET detector and existing ground-based γ-ray telescopes have greatly impacted our understanding of blazar-like active galactic nuclei (AGN), but they have also left many questions unanswered. Among these are the nature of the particles which produce the γ-rays, and the mechanisms for accelerating the particles and producing the γ-rays. VERITAS will help answer these questions through precise measures of short time scale phenomena, accurate energy spectra spanning 50 GeV-50 TeV, and the detection of more sources. In this paper, we discuss how VERITAS will contribute to our understanding of AGN.