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Search Results: 1 - 10 of 397300 matches for " J. F. Gallimore "
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Importance of relative humidity in the oxidative ageing of organic aerosols: case study of the ozonolysis of maleic acid aerosol
P. J. Gallimore,P. Achakulwisut,F. D. Pope,J. F. Davies
Atmospheric Chemistry and Physics (ACP) & Discussions (ACPD) , 2011, DOI: 10.5194/acp-11-12181-2011
Abstract: Many important atmospheric aerosol processes depend on the chemical composition of the aerosol, e.g. water uptake and particle cloud interactions. Atmospheric ageing processes, such as oxidation reactions, significantly and continuously change the chemical composition of aerosol particles throughout their lifetime. These ageing processes are often poorly understood. In this study we utilize an aerosol flow tube set up and an ultra-high resolution mass spectrometer to explore the effect of relative humidity (RH) in the range of <5–90% on the ozonolysis of maleic acid aerosol which is employed as model organic aerosol system. Due to the slow reaction kinetics relatively high ozone concentrations of 160–200 ppm were used to achieve an appreciable degree of oxidation of maleic acid. The effect of oxidative ageing on the hygroscopicity of maleic acid particles is also investigated using an electrodynamic balance and thermodynamic modelling. RH has a profound effect on the oxidation of maleic acid particles. Very little oxidation is observed at RH < 50% and the only observed reaction products are glyoxylic acid and formic acid. In comparison, when RH > 50% there are about 15 oxidation products identified. This increased oxidation was observed even when the particles were exposed to high humidities long after a low RH ozonolysis reaction. This result might have negative implications for the use of water as an extraction solvent for the analysis of oxidized organic aerosols. These humidity-dependent differences in the composition of the ozonolyzed aerosol demonstrate that water is both a key reactant in the oxidation scheme and a determinant of particle phase and hence diffusivity. The measured chemical composition of the processed aerosol is used to model the hygroscopic growth, which compares favourably with water uptake results from the electrodynamic balance measurements. A reaction mechanism is presented which takes into account the RH dependent observations. This study emphasises the importance of studying the combined effects of several atmospheric parameters such as oxidants and RH to accurately describe the complex oxidation scheme of organic aerosols.
Expansion of the R4 Water Maser Arc Near Cepheus A HW 2
J. F. Gallimore,R. J. Cool,M. D. Thornley,J. McMullin
Physics , 2002, DOI: 10.1086/346266
Abstract: We present new (April 2000) MERLIN observations of the H2O masers located near the protostar Cepheus A HW2. The MERLIN observations detect many of the structures found in earlier (1996) VLBA observations of Torrelles and collaborators, and the changed positions of these structures are compatible with the VLBA proper motions and astrometric uncertainties. The radius of curvature of the R4 structure of maser arcs appears to have grown by a factor of two, and the displacement of the arcs between 1996 and 2000 are compatible with expansion about a common center. In addition, the MERLIN observations detect red-shifted masers not previously found; taken with the newly discovered masers, the R4 structure now resembles patchy emission from an elliptical ring. We demonstrate that a simple bow-shock model cannot simultaneously account for the shape and the velocity gradient of the R4 structure. A model involving a slow, hydromagnetic shock propagating into a rotating, circumstellar disk better describes the maser spot kinematics and luminosities. In this model, the central mass is 3 solar masses, and we demonstrate that the mass of the disk is negligible in comparison. The expansion velocity of the post-shock gas, roughly 5 km / s, is slow compared to the average shock velocity (roughly 13 km / s) suggesting that the post-shock gas is magnetically supported with a characteristic field strength of roughly 30 mG. We speculate that the expanding maser rings R4 and R5 may be generated by periodic, instability-driven winds from young stars that periodically send spherical shocks into the surrounding circumstellar material.
Localised Neutral Hydrogen Absorption Towards the Radio Jet of Markarian 6
J. F. Gallimore,A. J. Holloway,A. Pedlar,C. G. Mundell
Physics , 1997,
Abstract: We present 0.15 arcsec (56 pc) resolution MERLIN observations of neutral hydrogen (HI) 21 cm absorption detected towards the arcsecond-scale radio jet of the Seyfert 1.5 galaxy Markarian 6. Absorption is detected only towards a bright, compact radio feature located, in projection, ~ 380 pc north of the likely location of the optical nucleus. Based on comparison with an archival HST image, we propose a geometry in which the HI absorption arises in a dust lane passing north of, but not covering, the optical nucleus, and the southern lobe of the jet is oriented on the near side of the inclined galaxian disk. We note that this result is contrary to previous models which place the extended narrow-line region on the near side of the disk.
The Nuclear Regions of the Seyfert Galaxy NGC 4151 - Parsec-scale HI Absorption and a Remarkable Radio Jet
C. G. Mundell,J. M. Wrobel,A. Pedlar,J. F. Gallimore
Physics , 2002, DOI: 10.1086/345356
Abstract: Sensitive high angular and linear resolution radio images of the 240-pc radio jet in NGC4151, imaged at linear resolutions of 0.3 to 2.6 pc using the VLBA and phased VLA at 21 cm, are presented and reveal for the first time a faint, highly collimated jet (diameter <1.4 pc) underlying discrete components, seen in lower resolution MERLIN and VLA images, that appear to be shock-like features associated with changes in direction as the jet interacts with small gas clouds within the central 100 pc of the galaxy. In addition, 21-cm spectral line imaging of the neutral hydrogen in the nuclear region reveals the spatial location, distribution and kinematics of the neutral gas detected previously in a lower resolution MERLIN study. Neutral hydrogen absorption is detected against component C4W (E+F) as predicted by Mundell et al, but the absorption, extending over 3 pc, is spatially and kinematically complex on sub-parsec scales, suggesting the presence of small, dense gas clouds with a wide range of velocities and column densities. The main absorption component matches that detected in the MERLIN study and is consistent with absorption through a clumpy neutral gas layer in the putative obscuring torus, with higher velocity blue- and red-shifted systems with narrow linewidths also detected across E+F. The spatial location and distribution of the absorbing gas across component E+F rules out component E as the location of the AGN (as suggested by Ulvestad et al.) and, in combination with the well-collimated continuum structures seen in component D, suggests that component D is the most likely location for the AGN. We suggest that components C and E are shocks produced in the jet as the plasma encounters, and is deviated by, dense clouds with diameters smaller than ~1.4 pc. (Abridged)
Gas Dynamics in the Luminous Merger NGC 6240
L. J. Tacconi,R. Genzel,M. Tecza,J. F. Gallimore,D. Downes,N. Z. Scoville
Physics , 1999, DOI: 10.1086/307839
Abstract: We report 0.5"x0.9" resolution, interferometric observations of the 1.3 mm CO J=2-1 line in the infrared luminous galactic merger NGC 6240. About half of the CO flux is concentrated in a rotating but highly turbulent, thick disk structure centered between the two radio and near-infrared nuclei. A number of gas features connect this ~500 pc diameter central disk to larger scales. Throughout this region the molecular gas has local velocity widths which exceed 300 km/s FWHM and even reach FWZP line widths of 1000 km/s in a number of directions. The mass of the central gas concentration constitutes a significant fraction of the dynamical mass, M_gas(R<470 pc) ~ 2-4x10^9 M_o ~ 0.3-0.7 M_dyn. We conclude that NGC 6240 is in an earlier merging stage than the prototypical ultraluminous galaxy, Arp 220. The interstellar gas in NGC 6240 is in the process of settling between the two progenitor stellar nuclei, is dissipating rapidly and will likely form a central thin disk. In the next merger stage, NGC 6240 may well experience a major starburst like that observed in Arp 220.
Subparsec-scale HI in the nucleus of NGC 4151
C. G. Mundell,J. M. Wrobel,A. Pedlar,J. F. Gallimore
Physics , 2000,
Abstract: We present sensitive, high-resolution 21-cm VLBA+VLA observations of the radio jet and nuclear HI absorption in NGC 4151. The 25-mas (1.6-pc) resolution continuum image reveals a highly collimated radio jet, underlying the discrete components seen previously with MERLIN and the VLA. Spatially and kinematically complex HI absorption is detected against the whole 3-pc extent of the continuum component predicted by Ulvestad et al. to contain the AGN. Instead, we suggest the component against which the absorption is detected is part of the eastern counterjet, ruling it out as the location for the AGN.
Probing Radio Emission in Seyfert Galaxies on Parsec- and Kiloparsec-scales
P. Kharb,V. Singh,J. F. Gallimore,C. H. Ishwara-Chandra
Physics , 2014,
Abstract: Seyfert galaxies have traditionally been classified as radio-quiet active galactic nuclei. A proper consideration of the nuclear optical emission however proves that a majority of Seyferts are radio-loud. Kpc-scale radio lobes/bubbles are in fact revealed in sensitive observations at low radio frequencies of several Seyferts. Through the use of very long baseline interferometry, we have been able to determine the direction of the parsec-scale jets in some of these Seyfert galaxies. The misalignment between the parsec-scale jets and the kpc-scale lobes that is typically observed, is either suggestive of no connection between the two, or the presence of curved jets that power the radio lobes. In this context, we briefly discuss our new low radio frequency GMRT observations of two Seyfert galaxies with lobes.
Neutral Hydrogen (21 cm) Absorption in Seyfert Galaxies: Evidence for Free-free Absorption and Sub-kiloparsec Gaseous Disks
J. F. Gallimore,S. A. Baum,C. P. O'Dea,A. Pedlar,E. Brinks
Physics , 1999, DOI: 10.1086/307853
Abstract: Active galaxies are thought to be both fueled and obscured by neutral gas removed from the host galaxy and funneled into a central accretion disk. We performed a VLA imaging survey of 21 cm absorption in Seyfert and starburst nuclei to study the neutral gas in the near-nuclear environment. With the exception of NGC 4151, the absorbing gas traces 100 pc scale, rotating disks aligned with the outer galaxy disk. These disks appear to be rich in atomic gas relative to nuclear disks in non-active spirals. We find no strong evidence for rapid in-fall or out-flow of neutral hydrogen, but our limits on the mass infall rates are compatible with that required to feed a Seyfert nucleus. Among the galaxies surveyed here, neutral hydrogen absorption traces parsec-scale gas only in NGC 4151. Based on the kinematics of the absorption line, the disk symmetry axis appears to align with the radio jet axis rather than the outer galaxy axis. The most surprising result is that we detect no 21 cm absorption towards the central radio sources of the hidden Seyfert 1 nuclei Mrk 3, Mrk 348, and NGC 1068. Moreover, 21 cm absorption is commonly observed towards extended radio jet structure but appears to avoid central, compact radio sources in Seyfert nuclei. To explain these results, we propose that 21 cm absorption towards the nucleus is suppressed by either free-free absorption, excitation effects (i.e., enhanced spin temperature), or rapid motion in the obscuring gas. Ironically, the implications of these effects is that the obscuring disks must be small, typically not larger than a few tens of parsecs.
Integral Field Near-IR Spectroscopy of a Sample of Seyfert and LINER Galaxies I: The Data
R. M. Sosa-Brito,L. E. Tacconi-Garman,M. D. Lehnert,J. F. Gallimore
Physics , 2001, DOI: 10.1086/321796
Abstract: We present near-IR integral field spectroscopy of a sample of 31 Seyfert and LINER galaxies which were selected both to span a wide range of nuclear magnitudes and to possess roughly equal numbers of Seyfert type 1 and 2 nuclei. Moderate resolution (R~1000; R~2000 for 3 cases) integral field K-band spectra were obtained for all 31 galaxies in our sample and for 18 galaxies (R~1000; R~2000 for 4 cases) H-band integral field spectra were also obtained. In each case, we present nuclear, larger aperture, and difference spectra with corresponding information about emission line wavelengths, fluxes, and widths. Line-free H- and K-band continuum images as well as [FeII] 1.644 micron, Brackett-gamma, and H_2 1-0 S(1) emission lines are also presented. In addition, we provide extensive information about each galaxy obtained from the literature that will be useful subsequently for characterizing the sample and for comparison with our near-IR data.
Large-Scale Outflows in Edge-On Seyfert Galaxies. II. Kiloparsec-scale Radio Continuum Emission
E. J. M. Colbert,S. A. Baum,J. F. Gallimore,C. P. O'Dea,J. A. Christensen
Physics , 1996, DOI: 10.1086/177633
Abstract: We present deep images of the kpc-scale radio continuum emission in 14 edge-on galaxies (ten Seyfert and four starburst galaxies). Observations were taken with the VLA at 4.9~GHz (6~cm). The Seyfert galaxies were selected from a distance-limited sample of 22 objects (defined in paper~I). The starburst galaxies were selected to be well-matched to the Seyferts in radio power, recessional velocity and inclination angle. All four starburst galaxies have a very bright disk component and one (NGC~3044) has a radio halo that extends several kpc out of the galaxy plane. Six of the ten Seyferts observed have large-scale (radial extent $\gapprox$1 kpc) radio structures extending outward from the nuclear region, indicating that large-scale outflows are quite common in Seyferts. Large-scale radio sources in Seyferts are similar in radio power and radial extent to radio halos in edge-on starburst galaxies, but their morphologies do not resemble spherical halos observed in starburst galaxies. The sources have diffuse morphologies, but, in general, they are oriented at skewed angles with respect to the galaxy minor axes. This result is most easily understood if the outflows are AGN-driven jets that are somehow diverted away from the galaxy disk on scales $\gapprox$1 kpc. Starburst-driven winds, however, cannot be ruled out. More observational work is needed to determine whether massive star formation is present at high enough rates to drive galactic winds out to kpc scales in Seyfert galaxies.
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