Context. The intra-cluster medium is characterized by thermal emission, and by the presence of large scale magnetic fields. In some clusters of galaxies, a diffuse non-thermal emission is also present, located at the cluster center and called radio halo. These sources indicate the existence of relativistic particles and magnetic fields in the cluster volume. Aims. In this paper we collect data on all known nearby cluster radio halos (z < 0.4), to discuss their statistical properties and to investigate their origin. Methods. We searched for published data on radio halos and reduced new and archive VLA data to increase the number of known radio halos.Results. We present data on 31 radio halos, 1 new relic source, and 1 giant filament. We note the discovery of a small size diffuse radio emission in a cluster (A1213) with very low X-ray luminosity. Among the statistical results, we confirm the correlation between the average halo radio spectral index and the cluster temperature. We also discuss the high percentage of clusters where both a relic and a radio halo is present. Conclusions. The sample of radio halos discussed here represents the population of radio halos observable with current radio telescopes. A new telescope generation is necessary for a more detailed multifrequency study, and to investigate the possible existence of a population of radio halos with different properties.

The European Space Agency's Planck satellite, launched on 14 May 2009, is the third-generation space experiment in the field of cosmic microwave background (CMB) research. It will image the anisotropies of the CMB over the whole sky, with unprecedented sensitivity ( ΔT T ∼ 2 × 10 −6 ) and angular resolution (∼5 arcmin). Planck will provide a major source of information relevant to many fundamental cosmological problems and will test current theories of the early evolution of the Universe and the origin of structure. It will also address a wide range of areas of astrophysical research related to the Milky Way as well as external galaxies and clusters of galaxies. The ability of Planck to measure polarization across a wide frequency range (30−350 GHz), with high precision and accuracy, and over the whole sky, will provide unique insight, not only into specific cosmological questions, but also into the properties of the interstellar medium. This paper is part of a series which describes the technical capabilities of the Planck scientific payload. It is based on the knowledge gathered during the on-ground calibration campaigns of the major subsystems, principally its telescope and its two scientific instruments, and of tests at fully integrated satellite level. It represents the best estimate before launch of the technical performance that the satellite and its payload will achieve in flight. In this paper, we summarise the main elements of the payload performance, which is described in detail in the accompanying papers. In addition, we describe the satellite performance elements which are most relevant for science, and provide an overview of the plans for scientific operations and data analysis.

We present an extended morphometric system to automatically classify galaxies from astronomical images. The new system includes the original and modified versions of the CASGM coefficients (Concentration C 1 , Asymmetry A 3 , and Smoothness S 3 ), and the new parameters entropy, H, and spirality σ ψ . The new parameters A 3 , S 3 and H are better to discriminate galaxy classes than A 1 , S 1 and G, respectively. The new parameter σ ψ captures the amount of non-radial pattern on the image and is almost linearly dependent on T-type. Using a sample of spiral and elliptical galaxies from the Galaxy Zoo project as a training set, we employed the Linear Discriminant Analysis (LDA) technique to classify Baillard et al. (2011, 4478 galaxies), Nair & Abraham (2010, 14123 galaxies) and SDSS Legacy (779,235 galaxies) samples. The cross-validation test shows that we can achieve an accuracy of more than 90% with our classification scheme. Therefore, we are able to define a plane in the morphometric parameter space that separates the elliptical and spiral classes with a mismatch between classes smaller than 10%. We use the distance to this plane as a morphometric index (M i ) and we show that it follows the human based T-type index very closely. We calculate morphometric index M i for ∼780k galaxies from SDSS Legacy Survey -DR7. We discuss how M i correlates with stellar population parameters obtained using the spectra available from SDSS-DR7.