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Angular Size (angular + size)
Selected AbstractsPhase imaging using highly coherent X-rays: radiography, tomography, diffraction topographyJOURNAL OF SYNCHROTRON RADIATION, Issue 3 2000José Baruchel Several hard X-rays imaging techniques greatly benefit from the coherence of the beams delivered by the modern synchrotron radiation sources. This is illustrated with examples recorded on the `long' (145,m) ID19 `imaging' beamline of the ESRF. Phase imaging is directly related to the small angular size of the source as seen from one point of the sample (`effective divergence' , microradians). When using the `propagation' technique, phase radiography and tomography are instrumentally very simple. They are often used in the `edge detection' regime, where the jumps of density are clearly observed. The in situ damage assessment of micro-heterogeneous materials is one example of the many applications. Recently a more quantitative approach has been developed, which provides a three-dimensional density mapping of the sample (`holotomography'). The combination of diffraction topography and phase-contrast imaging constitutes a powerful tool. The observation of holes of discrete sizes in quasicrystals, and the investigation of poled ferroelectric materials, result from this combination [source] Seven young star clusters in the inner region of the Small Magellanic CloudMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2008Andrés E. Piatti ABSTRACT We present CCD photometry in the Washington system C and T1 passbands down to T1, 22 in the fields of L35, L45, L49, L50, L62, L63 and L85, seven poorly studied star clusters in the inner region of the Small Magellanic Cloud (SMC). We measured T1 magnitudes and C,T1 colours for a total of 114 826 stars distributed throughout cluster areas of 13.7 × 13.7 arcmin2 each. Cluster radii were estimated from star counts distributed throughout the entire observed fields. The seven clusters are generally characterized by a relatively small angular size and by a high field star contamination. We performed an in-depth analysis of the field star contamination of the colour,magnitude diagrams (CMDs), and statistically cleaned the cluster CMDs. Based on the best fits of isochrones computed by the Padova group to the (T1, C,T1) CMDs, we derive ages for the sample, assuming Z= 0.004, finding ages between 25 Myr and 1.2 Gyr. We then examined different relationships between positions in the SMC, age and metallicity of a larger sample of clusters including our previous work whose ages and metallicities are on the same scale used in this paper. We confirm previous results in the sense that the further a cluster is from the centre of the galaxy, the older and more metal poor it is, with some dispersion; although clusters associated with the Magellanic Bridge clearly do not obey the general trend. The number of clusters within , 2° of the SMC centre appears to have increased substantially after ,2.5 Gyr ago, hinting at a burst. [source] The cluster environments of radio-loud quasars at 0.6 < z < 1.1MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2003J. M. Barr ABSTRACT We have carried out multicolour imaging of a complete sample of radio-loud quasars at 0.6 < z < 1.1 and find groups or clusters of galaxies in the fields of at least eight and possibly 13 of the 21 sources. There is no evidence for an evolution in the richness of the environments of radio-loud quasars from other low-redshift studies to z, 0.9. The quasars associated with groups and clusters in our sample do not necessarily reside in the centre of the galaxy distribution, which rarely displays a spherical geometry. Clustering is preferentially associated with small or asymmetric steep-spectrum radio sources. The quasars with the largest projected angular size are, in nearly all cases, found in non-clustered environments. Radio-based selection (including source size) of high-redshift groups and clusters can be a very efficient method of detecting rich environments at these redshifts. We find that in optical searches for galaxy overdensities above z, 0.6, multiple filters must be used. If the single-filter counting statistics used by groups at lower redshift are applied to our data, uncertainties are too large to make accurate quantifications of cluster richness. This means that genuine clustering of galaxies about quasars will be missed and, in ,10 per cent of cases, putative clusters turn out to be false detections. The statistics are further diluted by the fact that galaxy overdensities are generally not centred on the quasar. [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] 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] | ||||||||||