Home About us Contact
|
Home About us Contact | ||
|
|
||
Nuclear Activity (nuclear + activity)
Selected AbstractsThe U.S. Policy and Strategy toward DPRK: Comparison and Evalution of the Clinton and Bush AdministrationsPACIFIC FOCUS, Issue 2 2002Hun Kyung Lee This article focuses on studying and evaluating the Clinton and Bush administrations' policies and strategies toward North Korea. The Clinton administration's policy toward North Korea was a continuation of the abandonment of containment and confrontation strategies of the Cold War era. That policy was based on a strategic transfer of power for the purpose of preventing a war, through a combination of aid and deterrence in the Korean peninsula by its engagement policy. The Administration believed that additional food aid and easing of economic sanctions would make a contribution to North Korean survival, and hence, a reduction in its bellicose disposition. Providing that this policy continued, it would be possible not merely to lead North Korea's change, but also to help it enter into international society by breaking down its self-imposed isolation. To the contrary, the Bush administration points out that the Clinton administration's engagement policy did not lead to North Korea's change, and even left the wrong precedent in nuclear and missile negotiations. Focusing on nonproliferation of weapons of mass destruction with an emphasis of transparency, monitoring, and verification, the Bush administration has claimed a broad agenda. This includes an improved implementation of the Agreed Framework relating to North Korea's nuclear activities, verifiable control over North Korea's missile programs and a ban on its missile exports, and a less threatening conventional military posture. With the different views of these two administrations as a background, this article explores the U.S. efforts for achieving such policy goals as freezing North Korea's nuclear weapons program and halting its missile development and sales, together with looking at North Korea's response. American efforts for supporting the necessities for life, easing of some economic sanctions toward DPRK are also described. At the same time, the U.S. policy toward DPRK is evaluated on the whole in considering U.S. policy limits for nonproliferation of weapons of mass destruction, the lack of effort by North Korea for peacemaking and survival, and inconsistency on U.S. assistance. Lastly, this article suggests a way for an alternative solution by thinking about some dilemmas for the U.S. and the DPRK. [source] The ionization of the emission-line gas in young radio galaxiesMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2009J. Holt ABSTRACT This paper is the second in a series in which we present intermediate-resolution, wide-wavelength coverage spectra for a complete sample of 14 compact radio sources, taken with the aim of investigating the impact of the nuclear activity on the circumnuclear interstellar medium (ISM) in the early stages of radio source evolution. In the first paper (Holt, Tadhunter & Morganti), we presented the kinematic results from the nuclear emission-line modelling and reported fast outflows in the circumnuclear gas. In this paper, we use the line fluxes to investigate the physical conditions and dominant ionization mechanisms of the emission-line gas. We find evidence for large electron densities and high reddening in the nuclear regions, particularly in the broader, blueshifted components. These results are consistent with the idea that the young, recently triggered radio sources still reside in dense and dusty cocoons deposited by the recent activity triggering event (merger/interaction). In addition, we find that the quiescent nuclear and extended narrow components are consistent with active galactic nucleus (AGN) photoionization, split between simple-slab AGN photoionization and mixed-medium photoionization models. For the nuclear broader and shifted components, the results are less clear. Whilst there are suggestions that the broader components may be closer to shock plus precursor models on the diagnostic diagrams, and that the electron temperatures and densities are high, we are unable to unambiguously distinguish the dominant ionization mechanism using the optical emission-line ratios. This is surprising given the strong evidence for jet,cloud interactions (broad emission lines, large outflow velocities and strong radio-optical alignments), which favours the idea that the warm gas has been accelerated in shocks driven by the radio lobes expanding through a dense cocoon of gas deposited during the triggering event. [source] Are galaxies with active galactic nuclei a transition population?MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007P. B. Westoby ABSTRACT We present the results of an analysis of a well-selected sample of galaxies with active and inactive galactic nuclei from the Sloan Digital Sky Survey, in the range 0.01 < z < 0.16. The SDSS galaxy catalogue was split into two classes of active galaxies, Type 2 active galactic nuclei (AGN) and composites, and one set of inactive, star-forming/passive galaxies. For each active galaxy, two inactive control galaxies were selected by matching redshift, absolute magnitude, inclination, and radius. The sample of inactive galaxies naturally divides into a red and a blue sequence, while the vast majority of AGN hosts occur along the red sequence. In terms of H, equivalent width (EW), the population of composite galaxies peaks in the valley between the two modes, suggesting a transition population. However, this effect is not observed in other properties such as the colour,magnitude space or colour,concentration plane. Active galaxies are seen to be generally bulge-dominated systems, but with enhanced H, emission compared to inactive red-sequence galaxies. AGN and composites also occur in less dense environments than inactive red-sequence galaxies, implying that the fuelling of AGN is more restricted in high-density environments. These results are therefore inconsistent with theories in which AGN host galaxies are a ,transition' population. We also introduce a systematic 3D spectroscopic imaging survey, to quantify and compare the gaseous and stellar kinematics of a well-selected, distance-limited sample of up to 20 nearby Seyfert galaxies, and 20 inactive control galaxies with well-matched optical properties. The survey aims to search for dynamical triggers of nuclear activity and address outstanding controversies in optical/infrared imaging surveys. [source] Central kiloparsec of Seyfert and inactive host galaxies: a comparison of two-dimensional stellar and gaseous kinematicsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007Gaëlle Dumas ABSTRACT We investigate the properties of the two-dimensional distribution and kinematics of ionized gas and stars in the central kiloparsec of a matched sample of nearby active (Seyfert) and inactive galaxies, using the SAURON integral field unit on the William Herschel Telescope. The ionized gas distributions show a range of low-excitation regions, such as star formation rings in Seyfert and inactive galaxies, and high-excitation regions related to photoionization by the active galactic nucleus (AGN). The stellar kinematics of all galaxies in the sample show regular rotation patterns typical of disc-like systems, with kinematic axes that are well aligned with those derived from the outer photometry and provide a reliable representation of the galactic line of nodes. After removal of the non-gravitational components due to, for example, AGN-driven outflows, the ionized gas kinematics in both the Seyfert and inactive galaxies are also dominated by rotation with global alignment between stars and gas in most galaxies. This result is consistent with previous findings from photometric studies that the large-scale light distribution of Seyfert hosts is similar to that of inactive hosts. However, by fully exploiting the two-dimensional nature of our spectroscopic data, deviations from axisymmetric rotation in the gaseous velocity fields are identified, which suggest that the gaseous kinematics are more disturbed at small radii in the Seyfert galaxies compared with the inactive galaxies. This provides a tentative link between nuclear gaseous streaming and nuclear activity. [source] The clustering of narrow-line AGN in the local UniverseMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2006Cheng Li ABSTRACT We have analysed the clustering of ,90 000 narrow-line active galactic nuclei (AGN) drawn from the Data Release 4 (DR4) of the Sloan Digital Sky Survey. Our analysis addresses the following questions. (i) How do the locations of galaxies within the large-scale distribution of dark matter influence ongoing accretion on to their central black holes? (ii) Is AGN activity triggered by interactions or mergers between galaxies? We compute the cross-correlation between AGN and a reference sample of galaxies drawn from the DR4. We compare this to results for control samples of inactive galaxies matched simultaneously in redshift, stellar mass, concentration, velocity dispersion and mean stellar age, as measured by the 4000-Å break strength. We also compare near-neighbour counts around AGN and around the control galaxies. On scales larger than a few Mpc, AGN have almost the same clustering amplitude as the control sample. This demonstrates that AGN host galaxies and inactive control galaxies populate dark matter haloes of similar mass. On scales between 100 kpc and 1 Mpc, AGN are clustered more weakly than the control galaxies. We use mock catalogues constructed from high-resolution N -body simulations to interpret this antibias, showing that the observed effect is easily understood if AGN are preferentially located at the centres of their dark matter haloes. On scales less than 70 kpc, AGN cluster marginally more strongly than the control sample, but the effect is weak. When compared to the control sample, we find that only one in 100 AGN has an extra neighbour within a radius of 70 kpc. This excess increases as a function of the accretion rate on to the black hole, but it does not rise above the few per cent level. Although interactions between galaxies may be responsible for triggering nuclear activity in a minority of nearby AGN, some other mechanism is required to explain the activity seen in the majority of the objects in our sample. [source] Gas and stars in compact (young) radio sourcesASTRONOMISCHE NACHRICHTEN, Issue 2-3 2009R. Morganti Abstract Gas can be used to trace the formation and evolution of galaxies as well as the impact that the nuclear activity has on the surrounding medium. For nearby compact radio sources, we have used observations of neutral hydrogen , that we detected in emission distributed over very large scales , combined with the study of the stellar population and deep optical images to investigate the history of the formation of their host galaxy and the triggering of the activity. For more distant and more powerful compact radio sources, we have used optical spectra and H I , in absorption , to investigate the presence of fast outflows that support the idea that compact radio sources are young radio loud AGN observed during the early stages of their evolution and currently shredding their natal cocoons through extreme circumnuclear outflows. We will review the most recent results obtained from these projects (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Optical spectra of 15 Low Luminosity Compact Sources and the formation of jetsASTRONOMISCHE NACHRICHTEN, Issue 2-3 2009S. Buttiglione Abstract We present the results of optical spectroscopy of 15 compact radio sources, selected to be among the lowest luminosity objects and expected to evolve into FR I radio galaxies. The aim of the work is to exploit the emission line characteristics as indicators of the nuclear activity , in terms of accretion properties , during the early stages of the production of jets. The emission line ratios identify most Low Luminosity Compact Sources (LLCSs) as low excitation galaxies, similar to evolved FR I. Also in terms of their optical , radio properties they appear to be indistinguishable from FR I. These findings support the view that jets could be produced by nuclei accreting both at high and low rates (and/or radiatively efficient and inefficient discs). They also argue against the idea that FRI represent a late stage after a powerful FR II phase (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] | ||||||||||