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Spectral Turnovers (spectral + turnover)
Selected AbstractsDoes equipartition hold in HFPs?ASTRONOMISCHE NACHRICHTEN, Issue 2-3 2009D. Dallacasa Abstract Equipartition of energy between relativistic particles and magnetic field is known to be consistent with observations in most of the large size (hundreds of kpc) radio galaxies. Here we discuss whether such a property holds in the smallest and youngest radio sources, the High Frequency Peakers (HFPs). A few sources have small components with a turnover frequency occurring at a few GHz and therefore a direct measure of the magnetic field intensity can be derived from the spectral peak assuming the spectral turnover is originated by synchrotron self-absorption. The field strengths computed in this way are generally in excellent agreement with the equipartition fields. However, a few exceptions have been found, in sources with an extra deficit of photons in the optically thick part of the spectrum as a consequence of free-free absorption (FFA) (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [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] High frequency GPS sources in the AT20G surveyASTRONOMISCHE NACHRICHTEN, Issue 2-3 2009P.J. Hancock Abstract The Australia Telescope 20 GHz (AT20G) survey was used to select a complete sample of 656 Gigahertz Peaked Spectrum (GPS) sources with spectral turnovers above 5 GHz. The AT20G has near simultaneous observations at 4.8, 8.6 and 20 GHz, which makes it possible to exclude flat spectrum variability as a cause of a source's peaked spectrum. Optical identification of the sample results in 361 QSOs and 104 galaxies and 191 blank fields. Redshifts are known for 104 of the GPS sources. The GPS sources from the AT20G are discussed and compared to previously known samples. The new sample of high frequency peaking GPS sources is found at a lower redshift than previous samples and to also have a lower 5 GHz radio power. Evidence is found to support the idea that the origin of the GPS spectral shape are intrinsically different for galaxies and QSOs. This paper is an elaboration and extension of the talk given at the 4th CSS/GPS conference in Riccione in May this year (© 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] Future investigations of GPS and CSS radio sources with LOFARASTRONOMISCHE NACHRICHTEN, Issue 2-3 2009I.A.G. Snellen Abstract In the next few years, the Low Frequency Array (LOFAR) will open up one of the last astronomically unexplored wavelength regimes. While the LOFAR core is currently being erected in the Netherlands, its outer stations will cover a large part of Europe, resulting in an unprecedented angular resolution at > meter wavelengths. Next to many other exciting scientific endeavours, LOFAR will be the first instrument to probe the low frequency spectra of Gigahertz Peaked Spectrum (GPS) and Compact Steep Spectrum (CSS) radio sources. It will give new insights into their absorption processes, and probe associated extended emission (possibly linked to earlier epochs of activity) in these enigmatic class of young active galactic nuclei. Furthermore, LOFAR will be sensitive to possibly the most distant GPS and CSS sources, of which their spectral turnovers have redshifted down to the lowest observable radio frequencies (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] | ||||||||||