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Search Results: 1 - 10 of 219572 matches for " C. Mundell "
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Violence in the Hearts of Galaxies - Aberration or Adolescence?
C. G. Mundell
Physics , 2002, DOI: 10.1098/rsta.2002.1058
Abstract: Violent activity in the nuclei of galaxies has long been considered a curiosity in its own right; manifestations of this phenomenon include distant quasars in the early Universe and comparatively nearby Seyfert galaxies, both thought to be powered by the release of gravitational potential energy as material from the host galaxy accretes onto a central supermassive black hole (SMBH). Traditionally, the broader study of the formation, structure and evolution of galaxies has largely excluded active galactic nuclei. Recently however, this situation has changed dramatically, both observationally and theoretically, with the realisation that the growth and influence of the SMBH, the origin and development of galaxies and nuclear activity at different epochs in the Universe may be intimately related. I review the intriguing evidence for causal links between supermassive black holes, nuclear activity and the formation and evolution of galaxies, and describe opportunities for testing these relationships using the next generation of earth-bound and space-borne astronomical facilities. (Abridged)
The Gaseous Environment of Seyfert Galaxies
C. G. Mundell
Physics , 2000,
Abstract: The VLA has been used to image HI in a heterogeneous sample of nine Seyferts as part of a continuing investigation of the role of galactic host gas in the triggering and fuelling of nuclear activity. Where previous single-dish observations indicated complex, poorly understood kinematics, improved angular resolution and sensitivity reveal tidal tails and intragroup gas, distinguish dwarfs from main disks and resolve non-linear gas dynamical features such as shocks in bars. Seyferts may be abnormally gas-rich, but a larger, statistically significant comparison of HI properties of normal and active galaxies is now required.
Cold Molecular Gas in the Inner Two Kiloparsec of NGC4151
G. Dumas,E. Schinnerer,C. G. Mundell
Physics , 2010, DOI: 10.1088/0004-637X/721/1/911
Abstract: We present the first spatially resolved spectroscopic imaging observations of the 12CO(1-0) line emission in the central 2.5 kpc of the Seyfert 1 galaxy NGC4151, obtained with the IRAM Plateau de Bure Interferometer (PdBI). Most of the cold molecular gas is distributed along two curved gas lanes about 1 kpc north and south of the active nucleus, coincident with the circumnuclear dust ring noted by previous authors. These CO arcs lie within the Inner Lindblad Resonance of the large scale oval bar and have kinematics consistent with those derived from neutral hydrogen observations of the disk and bar. Two additional gas clumps are detected that show non-circular motions - one associated with the southern gas lane and one lying ~600 pc north of the nucleus. Closer to the nucleus, no cold molecular gas is detected in the central 300 pc where abundant near-IR H2 line emission arises. This suggests that the H2 line emission is not a good indicator for a cold gas reservoir in NGC4151 and that the H2 is likely photo-excited by the AGN. The upper limit of the CO mass in the central 300 pc is sufficient to support the AGN activity at its current level for 10^7 yrs. The total cold molecular mass detected by PdBI is 4.3 10^7 Msun. Finally, 3 mm continuum emission arising from the location of the AGN is detected with a flux of S~14 mJy and appears to be unresolved at an angular resolution of 2.8" (~180 pc).
Radio-Quiet AGN and the Transient Radio Sky
C. G. Mundell,N. Nagar,P. Ferruit
Physics , 2011,
Abstract: 8.4-GHZ radio imaging study of an optically selected sample of early type Seyfert galaxies; comparison of images taken at two epochs reveals possible variation in the nuclear radio flux density in five of them over a seven year period. It is shown that there is a possible correlation between the presence of nuclear radio variability and the absence of hundred parsec-scale radio emission, analogous with radio-loud AGN. Our results suggest that all Seyferts may exhibit variation in their nuclear radio flux density at 8.4 GHz, but that variability is more easily recognized in compact sources in which emission from the variable nucleus is not diluted by unresolved, constant flux density radio jet emission within the central 50 pc. Taken in combination with other Seyfert properties, these results suggest a paradigm of intermittent periods of quiescence and nuclear outburst across the Seyfert population and demonstrate the importance of investigating the temporal domain at radio wavelengths, which remains completely unexplored for faint radio-quiet AGN. Discovery of intermittent activity and radio flares has important implications for the AGN duty cycles and the origin of Ultra-High Energy Cosmic Rays. New radio facilities, such as the EVLA/VLBA, eMERLIN, LOFAR and eVLBI on the EVN, will revolutionise the study of radio-quiet AGN; in particular, the combination of increased sensitivity and sampling rate with high-angular resolution and automatic data reduction will open up the transient radio sky and bring major future breakthroughs.
Gas Dynamics in the Barred Seyfert Galaxy NGC4151 - I. HI Streaming Shocks and Inflow Along the Bar
C. G. Mundell,D. L. Shone
Physics , 1998, DOI: 10.1046/j.1365-8711.1999.02330.x
Abstract: We present sensitive, high resolution observations of neutral hydrogen (HI) in the unusually gas-rich, oval distortion of the Seyfert galaxy NGC4151. The gas dynamics of the oval are found to be consistent with those of a kinematically weak bar, fully confirming previous suggestions for the presence of a fat bar, and, for the first time, individual gaseous features in the bar are spatially resolved. In particular, the two bright regions close to the leading edges of the bar in NGC4151 exhibit kinematics strikingly similar to the signature of bar shocks seen in gas-dynamical simulations, and demonstrate how strong the gaseous response may be even in such a weak bar potential. The residual velocity field, showing deviations from circular motion, is largely consistent with streaming in a bar potential, and, in addition, clearly shows that inflow is concentrated in narrow regions originating in the shocks. This inflow may represent an early stage in the fuelling process of the AGN. The presence and properties of the shocks in NGC4151 indicate that, in addition to the x_1 orbits, the family of x_2 orbits exist and are of significant extent in the bar of NGC4151, with gas streaming from the shocks making the transition between the two families. We therefore suggest that the circumnuclear ellipse, identified optically by previous authors and associated with gas flowing in x_2 orbits, has formed as a natural consequence of the gas flows in the bar without the requirement for a second, inner bar. Observations of HI were previously thought to be ill-suited to the study of bar shocks due to limitations in angular resolution and sensitivity. However, our observations show that, following recent instrumental enhancements, such measurements are now feasible, albeit at the limits of instrumental capability.
Nuclear gas dynamics in Arp 220 - sub-kiloparsec scale atomic hydrogen disks
C. G. Mundell,P Ferruit,A Pedlar
Physics , 2001, DOI: 10.1086/322508
Abstract: We present new, high angular resolution (~0.22") MERLIN observations of neutral hydrogen (HI) absorption and 21-cm radio continuum emission across the central ~900 parsecs of the ultraluminous infrared galaxy, Arp220. Spatially resolved HI absorption is detected against the morphologically complex and extended 21-cm radio continuum emission, consistent with two counterrotating disks of neutral hydrogen, with a small bridge of gas connecting the two. We propose a merger model in which the two nuclei represent the galaxy cores which have survived the initial encounter and are now in the final stages of merging, similar to conclusions drawn from previous CO studies (Sakamoto, Scoville & Yun 1999). However, we suggest that instead of being coplanar with the main CO disk (in which the eastern nucleus is embedded), the western nucleus lies above it and, as suggested by bridge of HI connecting the two nuclei, will soon complete its final merger with the main disk. We suggest that the collection of radio supernovae (RSN) detected in VLBA studies in the more compact western nucleus represent the second burst of star formation associated with this final merger stage and that free-free absorption due to ionised gas in the bulge-like component can account for the observed RSN distribution. (Abridged)
Gamma-Ray Bursts in the Era of Rapid Followup
C. G. Mundell,C. Guidorzi,I. A. Steele
Advances in Astronomy , 2010, DOI: 10.1155/2010/718468
Abstract: We present a status report on the study of gamma-ray bursts (GRBs) in the era of rapid followup using the world's largest robotic optical telescopes—the 2?m Liverpool and Faulkes telescopes. Within the context of key unsolved issues in GRB physics, we describe (1) our innovative software that allows real-time automatic analysis and interpretation of GRB light curves, (2) the novel instrumentation that allows unique types of observations (in particular, early time polarisation measurements), and (3) the key science questions and discoveries to which robotic observations are ideally suited, concluding with a summary of current understanding of GRB physics provided by combining rapid optical observations with simultaneous observations at other wavelengths. 1. Introduction Gamma-Ray Bursts (GRBs) are the most powerful explosions in the Universe and, arguably, represent the most significant new astrophysical phenomenon since the discovery of quasars and pulsars. As their name suggests, GRBs are detected as brief, intense and totally unpredictable flashes of high-energy gamma rays, thought to be produced during the core collapse of massive stars (long-soft bursts, seconds) or the merger of two compact objects such as two neutron stars or a neutron star and a stellar-mass black hole (short-hard bursts, seconds). Although discovered through their -ray emission [1], they are now known to emit nonthermal radiation detectable across the electromagnetic spectrum [2–4]. However, despite their enormous luminosity, their unpredictability and short duration limit rapid, accurate localisation and observability with traditional telescopes. Consequently, new ground and space-based facilities have been developed over the past decade; dedicated satellites optimised for GRB detection and followup, such as Swift [5], are revolutionizing GRB studies by locating 100 bursts per year with -ray positions accurate to and X-ray positions accurate to within seconds or minutes of the burst. Here we describe the automatic ground-based followup of GRBs with the world's largest robotic optical telescopes that use intelligent software and innovative instruments. The Era of Rapid Followup: Predictions and Outcomes Before the launch of current satellites such as Swift, Integral, and Fermi, significant progress in understanding GRBs had been made since their discovery, in particular the general and X-ray properties. The first crucial step in disseminating real-time GRB positions to ground observers was triggered by BATSE on the CGRO [6] through the GRB Coordinates Network (GCN) [7] via
Radio Variability in Seyfert Nuclei
C. G. Mundell,P. Ferruit,N. Nagar,A. S. Wilson
Physics , 2009, DOI: 10.1088/0004-637X/703/1/802
Abstract: Comparison of 8.4-GHz radio images of a sample of 11 early-type Seyfert galaxies with previous observations reveals possible variation in the nuclear radio flux density in 5 of them over a 7-yr period. We find no correlation between radio variability and nuclear radio luminosity or Seyfert nuclear type, although the sample is small and dominated by type 2 Seyferts. Instead, a possible correlation between the presence of nuclear radio variability and the absence of ~100-pc-scale radio emission is seen. NGC2110 is the only source with significant extended radio structure and strong nuclear variability (>38% nuclear decline over seven years). Our results suggest that all Seyferts may exhibit variation in their nuclear radio flux density at 8.4 GHz, but that variability is more easily recognised in compact sources in which emission from the variable nucleus is not diluted by unresolved, constant flux density radio-jet emission within the central ~50 pc. If flares in radio light curves correspond to ejection of new relativistic components or emergence of shocks in the underlying flow, we suggest that radio jets may be intrinsically non-relativistic during quiescence, but that Seyferts, as black-hole driven AGN, have the capacity to accelerate relativistic jets during radio flares. Taken together with the increased detection rate of flat spectrum radio nuclei in Seyferts imaged at VLBI resolutions and the detection of variable water megamaser emission, our results support the paradigm of intermittent periods of quiescence and nuclear outburst across the Seyfert population. (Abridged).
Gas Dynamics in the Barred Seyfert Galaxy NGC4151 - II. High Resolution HI Study
C. G. Mundell,A. Pedlar,D. L. Shone,A. Robinson
Physics , 1998, DOI: 10.1046/j.1365-8711.1999.02331.x
Abstract: We present sensitive, high angular resolution (6" x 5") 21-cm observations of the neutral hydrogen in the nearby barred Seyfert galaxy, NGC4151. These HI observations, obtained using the VLA in B-configuration, are the highest resolution to date of this galaxy, and reveal hitherto unprecedented detail in the distribution and kinematics of the HI on sub-kiloparsec scales. A complete analysis and discussion of the HI data are presented and the global properties of the galaxy are related to the bar dynamics presented in Paper I.
Chandra Imaging of the X-ray Core of the Virgo Cluster
A. J. Young,A. S. Wilson,C. G. Mundell
Physics , 2002, DOI: 10.1086/342918
Abstract: We report sub-arcsecond X-ray imaging spectroscopy of M87 and the core of the Virgo cluster with the Chandra X-ray Observatory. The X-ray morphology shows structure on arcsecond (~100 pc) to ten arcminute (~50 kpc) scales, the most prominent feature being an "arc" running from the east, across the central region of M87 and off to the southwest. A ridge in the radio map, ending in an "ear"-shaped structure, follows the arc to the east. Depressions in the X-ray surface brightness correspond to the inner radio lobes and there is no evidence of shock-heated gas surrounding them. There are also at least two approximately circular (centered near the nucleus) "edges" in the X-ray brightness distribution, the radii of which are slightly larger than the nuclear distances of the inner radio lobes and intermediate radio ridges, respectively. We speculate that these discontinuities may be spherical pulses or "fronts" driven by the same jet activity as is responsible for the radio structure; such pulses are found in recent numerical simulations. All these results provide good evidence that the nuclear activity affects the intra-cluster medium. We present a temperature map of the intra-cluster medium, and obtain the temperature, pressure and cooling time as a function of nuclear distance for the arcs and the ambient intra-cluster medium. We show that the gas in the arcs is cooler than, and probably over-pressured with respect to, the ambient intra-cluster medium. The metal abundances of the cooler gas in the arc are somewhat enhanced relative to the ambient intra-cluster medium, favoring a ``buoyant plume'' origin for the X-ray arc, in which ambient gas near the nucleus is entrained by buoyant radio plasma and carried to larger nuclear distances. (Abstract truncated).
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