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Search Results: 1 - 10 of 461791 matches for " A. Pedlar "
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Jets and the emission-line spiral structure in IRAS 04210+0400
W. Steffen,A. J. Holloway,A. Pedlar
Physics , 1996, DOI: 10.1093/mnras/282.1.130
Abstract: We examine models in which jets are responsible for the formation of the emission-line spiral structure in IRAS 04210+0400. The kiloparsec-scale radio lobes in this active galaxy appear to be related to its extended emission-line spiral structure. The radio structure consists mainly of extended symmetrically bent, FR\,I-type lobes, which follow the emission-line spiral structure at their inner edge. In the central region of the galaxy a double radio source is observed with a separation of approximately 1\,arcsec between its components, which are extremely well aligned with the hotspot from which the southern lobe expands outwards. Hill et al (1988) suggested a model for the emission-line spiral structure invoking compressed interstellar matter, which is dragged away from the original jet path by the rotating ambient medium. From consideration of the propagation speed of the jets and the transverse ram pressure exerted by the rotating environment, we exclude this scenario as a possible origin of the spiral structure. We favour a model in which the jets themselves are bent by the rotating interstellar medium and possibly follows the emission-line spiral arms. We present fits of the model to the observed optical spiral structure. High sensitivity radio observations will be required to decide on the nature of the peculiar spiral structure in IRAS 04210+0400.
IRAS04210+0400: Modeling the optical spectra from flaring large scale jets
W. Steffen,A. J. Holloway,A. Pedlar
Physics , 1996, DOI: 10.1093/mnras/282.4.1203
Abstract: The emission-lines in the active galaxy IRAS\,0421+0400 show a dramatic ($\sim$\,900\kms) increase in the velocity spread at the position of radio hot-spots which are located at the beginning of extended radio lobes. We study a simple geometric model of an opening outflow which reproduces the structure found in the long-slit emission-line spectrum of the hot-spot regions. The predicted bifurcations in the optical image structure of these regions is confirmed by deep \oiii\,line-imaging. We propose that this phenomenon is the result of a jet emerging from the galaxy through the boundary between the interstellar and intergalactic medium. A similar model has previously been suggested as an explanation for wide angle tail radio sources (WAT's). If our model proves to be correct in more detailed future observations, then IRAS\,0421+0400 provides a unique possibility to study the flaring jet phenomenon at optical wavelengths.
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)
The orientation of the Seyfert nucleus in Markarian 348
S. Anton,A. Thean,A. Pedlar,I. W. A. Browne
Physics , 2002, DOI: 10.1046/j.1365-8711.2002.05763.x
Abstract: Multi-frequency observations of Mrk 348 are presented consisting of 2 epochs of MERLIN data at 5 GHz, ISOPHOT data at 170, 90, 60 and 25 micron, NOT images at U, B, R and I bands and data at 1350 micron from the archive for SCUBA. The new optical images reveal a disrupted arm that ends towards the eastern companion of Mrk 348, consistent with the hypothesis that Mrk 348 and its companion form an interacting system. 5 GHz MERLIN imaging shows that only one of the radio components of Mrk 348 is polarized (%P=5). The broadband spectrum of Mrk 348 is flat between the radio and millimetre bands, suggesting that synchrotron emission extends to high frequencies. Mrk 348 has many of the characteristics of a radio-loud object. We discuss the orientation of the radio axis of Mrk 348 with respect to the line of sight. We conclude that the evidence is conflicting, and the geometry in Mrk 348 is not well-described by a simple edge-on or face-on model.
IRAS 0421+0400: jets crossing an ISM/IGM interface?
W. Steffen,A. J. Holloway,A. Pedlar,D. J. Axon
Physics , 1995,
Abstract: The emission lines in the active galaxy IRAS 0421+0400 show a dramatic (~ 900 km/s) increase in the velocity spread at the position of radio hotspots which are located at the beginning of extended radio lobes. We study a model which explains this phenomenon as the result of a jet emerging through the boundary between the interstellar and intergalactic media. A similar scenario has previously been suggested as an explanation for wide angle tail radio sources (WATs). Based on our model, we simulate the longslit spectra of these regions and compare the results with the observations and find that it can explain most of the details in the observed longslit spectra.
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.
Parsec-scale radio structures in the nuclei of four Seyfert galaxies
M. J. Kukula,T. Ghosh,A. Pedlar,R. T. Schilizzi
Physics , 1999, DOI: 10.1086/307254
Abstract: We present 18-cm radio maps of four Seyfert nuclei, Mrk 1, Mrk 3, Mrk 231 and Mrk 463E, made with the European VLBI Network (EVN). Linear radio structures are present in three out of four sources on scales of ~100 pc to ~1 kpc, and the 20-mas beam of the EVN enables us to resolve details within the radio structures on scales of <10 pc. Mrk 3 was also imaged using MERLIN and the data combined with the EVN data to improve the sensitivity to extended emission. We find an unresolved flat-spectrum core in Mrk 3, which we identify with the hidden Seyfert 1 nucleus in this object, and we also see marked differences between the two highly-collimated radio jets emanating from the core. The western jet terminates in a bright hotspot and resembles an FRII radio structure, whilst the eastern jet has more in common with an FRI source. In Mrk 463E, we use the radio and optical structure of the source to argue that the true nucleus lies approximately 1 arcsec south of the position of the radio and optical brightness peaks, which probably represent a hotspot at the working surface of a radio jet. The EVN data also provide new evidence for a 100-pc radio jet powering the radio source in the Type 1 nucleus of Mrk 231. However, the Seyfert 2 galaxy Mrk 1 shows no evidence for radio jets down to the limits of resolution (~10 pc). We discuss the range of radio source size and morphology which can occur in the nuclei of Seyfert galaxies and the implications for Seyfert unification schemes and for radio surveys of large samples of objects.
Radio observations of the circumnuclear ring in NGC6951
D. J. Saikia,B. Phookun,A. Pedlar,K. Kohno
Physics , 2002, DOI: 10.1051/0004-6361:20011710
Abstract: We present sensitive, high-resolution radio observations of the circumnuclear region of the barred spiral galaxy NGC6951. These observations reveal a ring of radio emission with many discrete components and a marginally resolved nuclear component. We compare the radio ring with observations at other wavelengths, and discuss the nature of the compact radio components.
Ionized Gas in the SgrA Complex - VLA Observations of H168alpha and H270alpha Recombination Lines
K. R. Anantharamaiah,A. Pedlar,W. M. Goss
Physics , 1999,
Abstract: Radio recombination lines at a wavelength of 20 cm reveals the presence of an extended component of ionized gas of lower density in the Sgr A complex. This component extends well beyond the thermal `mini-spiral' SgrA-West, which seem to be embedded in it. The low-density component is present over the entire extent of SgrA East and possibly beyond and it is responsible for the turnover in the spectrum of SgrA East and the halo observed by Pedlar et al (1989) at 90 cm. The radial velocity of the extended ionized gas range from +50 km/s to -200 km/s with minima of emission near -50 and -150 km/s. The total mass of the low-density component is estimated to be 10^4 solar masses. A possible extended feature at a high velocity of 470 km/s is also detected, but needs further observations to confirm it.
A Parsec-Scale Study of the 5/15 GHz Spectral Indices of the Compact Radio Sources in M82
A. R. McDonald,T. W. B. Muxlow,K. A. Wills,A. Pedlar,R. J. Beswick
Physics , 2002, DOI: 10.1046/j.1365-8711.2002.05580.x
Abstract: Observations of the starburst galaxy, M82, have been made with the VLA in its A-configuration at 15 GHz and MERLIN at 5 GHz enabling a spectral analysis of the compact radio structure on a scale of < 0.1'' (1.6 pc). Crucial to these observations was the inclusion of the Pie Town VLBA antenna, which increased the resolution of the VLA observations by a factor of ~2. A number of the weaker sources are shown to have thermal spectra and are identified as HII regions with emission measures ~10^7 cm^-6 pc. Some of the sources appear to be optically thick at 5 GHz implying even higher emission measures of ~10^8 cm^-6 pc. The number of compact radio sources in M82 whose origin has been determined is now 46, of which 30 are supernova related and the remaining 16 are HII regions. An additional 15 sources are noted, but have yet to be identified, meaning that the total number of compact sources in M82 is at least 61. Also, it is shown that the distribution of HII regions is correlated with the large-scale ionised gas distribution, but is different from the distribution of supernova remnants. In addition, the brightest HII region at (B1950) 09h 51m 42.21s +69 54' 59.2'' shows a spectral index gradient across its resolved structure which we attribute to the source becoming optically thick towards its centre.
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