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Extragalactic Radio Sources (extragalactic + radio_source)
Selected AbstractsLimits on the detectability of the CMB B-mode polarization imposed by foregroundsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2005M. Tucci ABSTRACT We investigate which practical constraints are imposed by foregrounds on the detection of the B-mode polarization generated by gravitational waves, in the case of experiments of the type currently being planned. As the B-mode signal is probably dominated by foregrounds at all frequencies, the detection of the cosmological component depends drastically on our ability to remove foregrounds. We provide an analytical expression with which to estimate the level of the residual polarization for Galactic foregrounds, according to the method employed for their subtraction. We interpret this result in terms of the lower limit of the tensor-to-scalar ratio r that allows us to disentangle the cosmological B-mode polarization from the foreground contribution. Polarized emission from extragalactic radio sources and gravitational lensing is also taken into account. As a first approach, we consider the ideal limit of an instrumental noise-free experiment: for full-sky coverage and a resolution of 1°, we obtain a limit of r, 10,4. This value can be improved by high-resolution experiments and, in principle, there is no clear fundamental limit on the detectability of the polarization of gravitational waves. Our analysis is also applied to planned or hypothetical future polarization experiments, taking into account expected noise levels. [source] The depolarization properties of powerful radio sources: breaking the radio power versus redshift degeneracyMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2004J. A. Goodlet ABSTRACT We define three samples of extragalactic radio sources of Fanaroff,Riley type II, containing 26 objects in total. The control sample consists of 6C and 7C sources with radio powers of around 1027 W Hz,1 at 151 MHz and redshifts of z, 1. The other samples contain 3CRR sources either with comparable redshifts but radio powers about a decade larger or with comparable radio powers but redshifts around z, 0.4. We use these samples to investigate the possible evolution of their depolarization and rotation measure properties with redshift and radio power independently. We used VLA data for all sources at ,4800 MHz and two frequencies within the 1400-MHz band, either from our own observations or from the archive. We present maps of the total intensity flux, polarized flux, depolarization, spectral index, rotation measure and magnetic field direction where not previously published. Radio cores were detected in 12 of the 26 radio sources. Of the sources, 14 show a strong Laing,Garrington effect, but almost all of the sources show some depolarization asymmetry. All sources show evidence for an external Faraday screen being responsible for the observed depolarization. We find that sources at higher redshift are more strongly depolarized. Rotation measure shows no trend with either redshift or radio power. However, variations in the rotation measure across individual sources increase with the redshift of the sources but do not depend on their radio power. [source] On the evolution of young radio-loud AGNMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2000I. A. G. Snellen This paper describes an investigation of the early evolution of extragalactic radio sources using samples of faint and bright gigahertz peaked spectrum (GPS) and compact steep spectrum (CSS) radio galaxies. Correlations found between their peak frequency, peak flux density and angular size provide strong evidence that synchrotron self-absorption is the cause of the spectral turnovers, and indicate that young radio sources evolve in a self-similar way. In addition, the data seem to suggest that the sources are in equipartition while they evolve. If GPS sources evolve to large size radio sources, their redshift dependent birth-functions should be the same. Therefore, since the lifetimes of radio sources are thought to be short compared to the Hubble time, the observed difference in redshift distribution between GPS and large size sources must be due to a difference in slope of their luminosity functions. We argue that this slope is strongly affected by the luminosity evolution of the individual sources. A scenario for the luminosity evolution is proposed in which GPS sources increase in luminosity and large-scale radio sources decrease in luminosity with time. This evolution scenario is expected for a ram-pressure confined radio source in a surrounding medium with a King profile density. In the inner parts of the King profile, the density of the medium is constant and the radio source builds up its luminosity, but after it grows large enough the density of the surrounding medium declines and the luminosity of the radio source decreases. A comparison of the local luminosity function (LLF) of GPS galaxies with that of extended sources is a good test for this evolution scenario. Unfortunately, only a handful of GPS sources are known at low redshift, and an LLF can only be derived, assuming that their cosmological number density evolution is similar to that of steep spectrum sources. The LLF derived in this way is shown to be in good agreement with the proposed evolution scenario. However, the uncertainties are large, and larger, homogeneously selected samples of GPS sources are needed. [source] Transverse motions in CSOs?ASTRONOMISCHE NACHRICHTEN, Issue 2-3 2009C. Stanghellini Abstract The measurement of proper motions in CSOs is a powerful tool to determine the dynamical evolution of the newly born extragalactic radio sources. We observed 3 CSOs with the VLBA in 2004 and in 2006 to monitor changes in their structure and measure the separation velocity of the hot spots. It is important to increase the size of the samples of CSOs with measured expansion velocity to test the existence of frustrated objects, and put stringent constraints on the current models. We found for all the three objects observed a transverse motion of the hot-spots, and we suggest as the more likely explanation a precession in the jet axis. This behaviour likely inhibits or at least slows down the radio source growth because the head of the hot-spot continuously hits new regions of the ISM. Therefore these radio sources may represent an old population of GPS/CSOs (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Further evidence for synchrotron self-absorption from the CORALZ sample of young radio-loud AGNASTRONOMISCHE NACHRICHTEN, Issue 2-3 2009N. de Vries Abstract Young radio-loud active galactic nuclei form an important tool to investigate the evolution of extragalactic radio sources. In this paper we present a summary of our recent work on the CORALZ sample of young radio sources at low redshift. We have found strong evidence that the radio spectral turnovers in GPS and CSS sources are caused by synchrotron selfabsorption (SSA): the CORALZ sources follow the well established relation between radio spectral peak frequency and largest angular size, but with significantly smaller sizes at any particular peak frequency, compared to more powerful and more distant GPS/CSS sources, as expected from SSA theory combined with simple self-similar evolution models. Current models that invoke free-free absorption (FFA) to explain the spectral turnovers in GPS and CSS sources can not easily explain the relatively small angular sizes found for the CORALZ sources. We therefore conclude that, although FFA may play a role in some sources, the radio spectral turnovers in GPS and CSS radio sources are generally caused by SSA (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] | ||||||||||