Publish in OALib Journal

ISSN: 2333-9721

APC: Only $99


Any time

2020 ( 83 )

2019 ( 827 )

2018 ( 926 )

2017 ( 893 )

Custom range...

Search Results: 1 - 10 of 507856 matches for " M. E. Sadler "
All listed articles are free for downloading (OA Articles)
Page 1 /507856
Display every page Item
The evolution of extragalactic radio sources
Q. Luo,E. M. Sadler
Physics , 2010, DOI: 10.1088/0004-637X/713/1/398
Abstract: A model for the evolution of low-luminosity radio galaxies is presented. In the model, the lobes inflated by low-power jets are assumed to expand in near pressure-balance against the external medium. Both cases of constant external pressure and decreasing external pressure are considered. Evolution of an individual source is described by the power-size track. The source appears as its lobe is inflated and radio luminosity increases to above the detection level; the source then moves along the track and eventually disappears as its luminosity drops below the detection limit. The power-size tracks are calculated including the combined energy losses due to synchrotron radiation, adiabatic expansion, and inverse Compton scattering. It is shown that in general, the constant-pressure model predicts an excess number of luminous, small-size sources while underpredicting large-size sources in the power-size diagram. The predicted spectra are steep for most sources, which is inconsistent with observations. By comparison, the pressure-limiting model fits observations better. In this model, low-luminosity sources undergo substantial expansion losses in the initial phase and as a result, it predicts fewer luminous, small-size sources. The resultant spectra are flat for most sources except for the oldest ones, which seems consistent with observations. The power-size tracks, in contrast to that of high-luminosity radio galaxies, are characterized by a slow increase in luminosity for most of the source's life, followed by a rapid decline when the synchrotron or inverse Compton scattering losses set in.
HI gas disks in elliptical galaxies
E. M. Sadler,T. Oosterloo,R. Morganti
Physics , 2000,
Abstract: We discuss a class of low-luminosity E/S0 galaxies which have both HI disks and (in contrast to more luminous E/S0s with HI) ongoing star formation. We suggest that such objects are common, but that only a few are known at present because optical magnitude-limited galaxy catalogues are biased against them. The HI Parkes All-Sky Survey (HIPASS) should eventually detect many more. We suggest that `boxy' and `disky' ellipticals are distinct not only in their structure and kinematics, but in their star-formation history.
HI in Early-Type Galaxies
T. Oosterloo,R. Morganti,E. M. Sadler
Physics , 1999, DOI: 10.1071/AS99028
Abstract: We summarize the HI properties of early-type galaxies, in particular the differences in HI morphologies observed in early-type galaxies of different luminosities. We find that in low-luminosity early-type galaxies the HI is almost always in a disk-like structure, with central surface densities high enough for star formation to occur. In a few luminous early-type galaxies the HI is also in a disk or in a ring-like structure, but in most luminous early-type galaxies the HI has a relatively irregular morphology. The surface densities in the HI disks in luminous early-type galaxies are lower than in the HI disks in low-luminosity early-type galaxies and no large-scale star formation should occur in these disks. We discuss these different HI characteristics in the context of other properties of early-type galaxies that correlate with luminosity.
HI in Early-Type Galaxies
T. Oosterloo,R. Morganti,E. M. Sadler
Physics , 2000,
Abstract: We briefly discuss the main differences between the HI properties of luminous and low-luminosity early-type galaxies. In luminous early-type galaxies the HI is often irregularly distributed, but in a few cases regular HI disks, of low surface density, are seen. In low-luminosity galaxies, the HI is more often in a disk with high central surface densities. This suggests a different evolution of the gas in these two groups of galaxies.
K Giants in Baade's Window. I. Velocity and Line-strength Measurements
D. M. Terndrup,E. M. Sadler,R. M. Rich
Physics , 1995, DOI: 10.1086/117649
Abstract: This is the first in a series of papers in which we analyze medium--resolution spectra of over 400 K and M giants in Baade's Window. Our sample was selected from the proper motion study of Spaenhauer et al. [AJ, 103, 297 (1992)]. We have measured radial velocities for most of the sample, as well as line--strength indices on the system of Faber et al. [ApJS, 57, 711 (1985)]. We analyze the random and systematic errors in velocities and line strengths, and show that the bright (V < 16.0) stars in our sample are predominantly foreground disk stars along the line--of--sight toward Baade's Window. We find that most of the bulge K giants have stronger Mg absorption at a given color than do stars in the solar neighborhood. If the K giants in our sample are moderately old, we suggest that on average they may have [Mg/Fe] ~ +0.3, consistent with the results of recent high--resolution spectroscopy in Baade's Window.
Application of a Bayesian Method to Absorption Spectral-Line Finding in Simulated ASKAP Data
J. R. Allison,E. M. Sadler,M. T. Whiting
Physics , 2011, DOI: 10.1071/AS11040
Abstract: The large spectral bandwidth and wide field of view of the Australian SKA Pathfinder radio telescope will open up a completely new parameter space for large extragalactic HI surveys. Here we focus on identifying and parametrising HI absorption lines which occur in the line of sight towards strong radio continuum sources. We have developed a method for simultaneously finding and fitting HI absorption lines in radio data by using multi-nested sampling, a Bayesian Monte Carlo algorithm. The method is tested on a simulated ASKAP data cube, and is shown to be reliable at detecting absorption lines in low signal-to-noise data without the need to smooth or alter the data. Estimation of the local Bayesian evidence statistic provides a quantitative criterion for assigning significance to a detection and selecting between competing analytical line-profile models.
A search for HI absorption in nearby radio galaxies using HIPASS
J. R. Allison,E. M. Sadler,A. M. Meekin
Physics , 2014, DOI: 10.1093/mnras/stu289
Abstract: Using archival data from the HI Parkes All Sky Survey (HIPASS) we have searched for 21 cm line absorption in 204 nearby radio and star-forming galaxies with continuum flux densities greater than $S_{1.4} \approx 250$ mJy within the redshift range $0 < cz < 12000$ km s$^{-1}$. By applying a detection method based on Bayesian model comparison, we successfully detect and model absorption against the radio-loud nuclei of four galaxies, of which the Seyfert 2 galaxy 2MASX J130804201-2422581 was previously unknown. All four detections were achieved against compact radio sources, which include three active galactic nuclei (AGNs) and a nuclear starburst, exhibiting high dust and molecular gas content. Our results are consistent with the detection rate achieved by the recent ALFALFA (Arecibo Legacy Fast Arecibo L-band Feed Array) HI absorption pilot survey by Darling et al. and we predict that the full ALFALFA survey should yield more than three to four times as many detections as we have achieved here. Furthermore, we predict that future all-sky surveys on the Square Kilometre Array precursor telescopes will be able to detect such strong absorption systems associated with type 2 AGNs at much higher redshifts, providing potential targets for detection of H$_{2}$O megamaser emission at cosmological redshifts.
Extended Emission Line Gas in Radio Galaxies - PKS0349-27
B. I. Grimberg,E. M. Sadler,S. M. Simkin
Physics , 1999, DOI: 10.1086/307547
Abstract: PKS0349-27 is a classical FRII radio galaxy with an AGN host which has a spectacular, spiral-like structure in its extended emission line gas (EELG). We have measured the velocity field in this gas and find that it splits into 2 cloud groups separated by radial velocities which at some points approach 400 km/s Measurements of the diagnostic emission line ratios [OIII]5007/H-beta, [SII]6716+6731/H-alpha, and [NII]6583/H-alpha in these clouds show no evidence for the type of HII region emission associated with starburst activity in either velocity system. The measured emission line ratios are similar to those found in the nuclei of narrow-line radio galaxies, but the extended ionization/excitation cannot be produced by continuum emission from the active nucleus alone. We present arguments which suggest that the velocity disturbances seen in the EELG are most likely the result of a galaxy-galaxy collision or merger but cannot completely rule out the possibility that the gas has been disrupted by the passage of a radio jet.
ATLBS: the Australia Telescope Low-brightness Survey
R. Subrahmanyan,R. D. Ekers,L. Saripalli,E. M. Sadler
Physics , 2009, DOI: 10.1111/j.1365-2966.2009.16105.x
Abstract: We present a radio survey carried out with the Australia Telescope Compact Array. A motivation for the survey was to make a complete inventory of the diffuse emission components as a step towards a study of the cosmic evolution in radio source structure and the contribution from radio-mode feedback on galaxy evolution. The Australia Telescope low-brightness survey (ATLBS) at 1388 MHz covers 8.42 sq deg of the sky in an observing mode designed to yield images with exceptional surface brightness sensitivity and low confusion. The ATLBS radio images, made with 0.08 mJy/beam rms noise and 50" beam, detect a total of 1094 sources with peak flux exceeding 0.4 mJy/beam. The ATLBS source counts were corrected for blending, noise bias, resolution, and primary beam attenuation; the normalized differential source counts are consistent with no upturn down to 0.6 mJy. The percentage integrated polarization Pi_0 was computed after corrections for the polarization bias in integrated polarized intensity; Pi_0 shows an increasing trend with decreasing flux density. Simultaneous visibility measurements made with longer baselines yielded images, with 5" beam, of compact components in sources detected in the survey. The observations provide a measurement of the complexity and diffuse emission associated with mJy and sub-mJy radio sources. 10% of the ATLBS sources have more than half of their flux density in extended emission and the fractional flux in diffuse components does not appear to vary with flux density, although the percentage of sources that have complex structure increases with flux density. The observations are consistent with a transition in the nature of extended radio sources from FR-II radio source morphology, which dominates the mJy population, to FR-I structure at sub-mJy flux density. (Abridged)
ATPMN: accurate positions and flux densities at 5 and 8 GHz for 8,385 sources from the PMN survey
D. McConnell,E. M. Sadler,T. Murphy,R. D. Ekers
Physics , 2012, DOI: 10.1111/j.1365-2966.2012.20726.x
Abstract: We present a source catalogue of 9,040 radio sources resulting from high-resolution observations of 8,385 PMN sources with the Australia Telescope Compact Array. The catalogue lists flux density and structural measurements at 4.8 and 8.6 GHz, derived from observations of all PMN sources in the declination range -87 deg < delta < -38.5 deg (exclusive of galactic latitudes |b| < 2 deg) with flux density S4850 > 70 mJy (50 mJy south of delta = -73 deg). We assess the quality of the data, which was gathered in 1992-1994, describe the population of catalogued sources, and compare it to samples from complementary catalogues. In particular we find 127 radio sources with probable association with gamma-ray sources observed by the orbiting Fermi Large Area Telescope.
Page 1 /507856
Display every page Item

Copyright © 2008-2017 Open Access Library. All rights reserved.