Cool Ionized Gas in Galaxy Thick Disks

Wetensch. publicati

Clusters of galaxies : observational properties of the diffuse radio emission

Clusters of galaxies, as the largest virialized systems in the Universe, are ideal laboratories to study the formation and evolution of cosmic structures...(abridged)... Most of the detailed knowledge of galaxy clusters has been obtained in recent years from the study of ICM through X-ray Astronomy. At the same time, radio observations have proved that the ICM is mixed with non-thermal components, i.e. highly relativistic particles and large-scale magnetic fields, detected through their synchrotron emission. The knowledge of the properties of these non-thermal ICM components has increased significantly, owing to sensitive radio images and to the development of theoretical models. Diffuse synchrotron radio emission in the central and peripheral cluster regions has been found in many clusters. Moreover large-scale magnetic fields appear to be present in all galaxy clusters, as derived from Rotation Measure (RM) studies. Non-thermal components are linked to the cluster X-ray properties, and to the cluster evolutionary stage, and are crucial for a comprehensive physical description of the intracluster medium. They play an important role in the cluster formation and evolution. We review here the observational properties of diffuse non-thermal sources detected in galaxy clusters: halos, relics and mini-halos. We discuss their classification and properties. We report published results up to date and obtain and discuss statistical properties. We present the properties of large-scale magnetic fields in clusters and in even larger structures: filaments connecting galaxy clusters. We summarize the current models of the origin of these cluster components, and outline the improvements that are expected in this area from future developments thanks to the new generation of radio telescopes.Comment: Accepted for the publication in The Astronomy and Astrophysics Review. 58 pages, 26 figure

The radio properties of the DME flare star Proxima Centauri

We present radio observations of the dM5.5e flare star Proxima Centauri at 20, 13, 6, and 3.5 cm. The star was only detected during an impulsive, highly circularly polarized, and apparently narrow-band flare at 20 cm, similar to those seen on other dMe flare stars. This flare was detected in just ∼1.7 hr of observing time spanning an ∼12 hr period, suggesting that Proxima Cen may be a prolific producer of coherent bursts at 20 cm. On the other hand, despite ∼30 hr of observing time at both 6 and 3.5 cm over a 4 yr period, the star was not detected as either a flaring or a quasi-steady (quiescent) source at these wavelengths. We place an upper limit of ∼2 × 10 11 ergs Hz -1 s -1 on its radio luminosity at 6 and 3.5 cm, the lowest detection threshold yet reached for a star other than the Sun. This upper limit is approximately equal to the radio luminosity of the active Sun (i.e., at or close to the peak on its activity cycle) outside of flares. Our results place important constraints on the filling factor of ∼500-1000 G magnetic loops containing X-ray-emitting plasma on Proxima Cen. Because such loops should be optically thick to gyroresonance emission at centimeter wavelengths, their filling factor can be inferred directly from the measured stellar radio flux density. Our radio results imply that loops at temperatures ∼2 × 107 K, representative of the hot stellar X-ray component, have a filling factor of ≤13%. Loops at temperatures ∼3 × 10 6 K, similar in temperature to the nonflaring solar active region corona, have a filling factor of ≤88%. Our results are compatible with present empirical relationships for the magnetic field parameters of late-type dwarf stars as applied to Proxima Cen. Based on its measured rotation period (P rot ≈ 41 days) and the ratio of its soft X-ray to bolometric luminosity (L x/L bo1 ≈ 2.4 × 10 -4), these relationships predict that Proxima Cen should be about an order of magnitude below the saturation limit in magnetic activity, where the entire surface of stars is thought to be covered by kilogauss X-ray loops. We compare our results with the contrasting case of UV Ceti, a dM5.5e flare star that according to the same empirical relationships should be approximately as magnetically active as Proxima Cen. UV Ceti, however, displays quiescent radio emission with a luminosity that is more than an order of magnitude higher than the upper limit placed on Proxima Cen. Our radio observations place an upper limit of ∼7 × 10 -12 M ⊙ yr -1 on the mass-loss rate by any stellar wind (assumed to have a velocity of 300 km s -1) from Proxima Cen. This upper limit is almost 2 orders of magnitude lower than that inferred by Mullan et al. from millimeter wavelength observations of other dMe flare stars. We show that the high mass-loss rate inferred by Mullan et al. is untenable if our present understanding of the centimeter wavelength radio emission of dMe flare stars is correct. © 1996. The American Astronomical Society. All rights reserved.link_to_subscribed_fulltex

Correlation effects in microwave observations of selected RS CVn-like stars

Sets of dual frequency microwave data on selected chromospherically active stars, from the Australia Telescope Compact Array, have been investigated for their auto and cross-correlation effects. Comparison of cross-correlation peak values with theoretical expectation indicates a high degree of real physical connection between the emission at the pairs of frequencies (4.8 and 8.64 GHz) compared. This fact should help constrain models for the emission mechanism. The timescale of observed time-shifts between the emissions at the two frequencies is consistent, in general, with the underlying energization being propagated by magnetohydrodynamic waves in a compact turbulent medium. © 2002 Elsevier Science B.V. All rights reserved.link_to_subscribed_fulltex

A millisecond pulsar

The radio properties of 4C21.53 have been an enigma for many years. First, the object displays interplanetary scintillations (IPS) at 81 MHz, indicating structure smaller than 1 arc s, despite its low galactic latitude (−0.3°). IPS modulation is rare at low latitudes because of interstellar angular broadening. Second, the source has an extremely steep (~v^(−2)) spectrum at decametric wavelengths. This combination of properties suggested that 4C21.53 was either an undetected pulsar or a member of some new class of objects. This puzzle may be resolved by the discovery and related observations of a fast pulsar, 1937+214, with a period of 1.558 ms in the constellation Vulpecula only a few degrees from the direction to the original pulsar, 1919+21. The existence of such a fast pulsar with no evidence either of a new formation event or of present energy losses raises new questions about the origin and evolution of pulsars

The Emergence of Radio Astronomy in Asia: Opening a New Window on the Universe

Some countries in the greater Asian area, namely Australia, China, India, Japan and New Zealand, played important roles in the early development of radio astronomy from the 1940s through into the 1960s. In this paper—which is based on the Public Lecture that we presented in Pune during the ICOA-9 conference—we trace these early developments. We then finish this review paper by briefly surveying the exciting new radio astronomical developments that are currently occurring throughout the greater Asian region