Radio imaging of core-dominated high redshift quasars

VLA imaging at kiloparsec-scale resolution of sixteen core-dominated radio-loud QSOs is presented. Many objects appear to display variable radio emission and their radio morphologies are significantly smaller than those of steep-spectrum quasars, consistent with these objects being observed at sight lines close to their (relativistic, $\gamma \approx$ 4-7) jet axes. The usefulness of the radio source orientation indicator R_V, being defined as ratio of radio core and rest frame optical V-band luminosity, is confirmed.Comment: 11 pages, 11 postscript figures, uses aa.cls 4.03 for LaTeX2e To appear in Astronomy and Astrophysic

Resolution of the Compact Radio Continuum Sources in Arp220

We present 2 cm and 3.6 cm wavelength very long baseline interferometry images of the compact radio continuum sources in the nearby ultra-luminous infrared galaxy Arp220. Based on their radio spectra and variability properties, we confirm these sources to be a mixture of supernovae (SNe) and supernova remnants (SNRs). Of the 17 detected sources we resolve 7 at both wavelengths. The SNe generally only have upper size limits. In contrast all the SNRs are resolved with diameters {\geq} 0.27 pc. This size limit is consistent with them having just entered their Sedov phase while embedded in an interstellar medium (ISM) of density 10^4 cm^{-3} . These objects lie on the diameter-luminosity correlation for SNRs (and so also on the diameter-surface brightness relation) and extend these correlations to very small sources. The data are consistent with the relation L {\propto} D^{-9/4}. Revised equipartition arguments adjusted to a magnetic field to relativistic particle energy density ratio of 1% combined with a reasonable synchrotron-emitting volume filling factor of 10% give estimated magnetic field strengths in the SNR shells of ~ 15-50 mG. The SNR shell magnetic fields are unlikely to come from compression of ambient ISM fields and must instead be internally generated. We set an upper limit of 7 mG for the ISM magnetic field. The estimated energy in relativistic particles, 2%-20% of the explosion kinetic energy, is consistent with estimates from models that fit the IR-radio correlation in compact starburst galaxies.Comment: 16 pages, 5 figure

Evidence for Super-Alfvenic oscillations in sources of Solar type III radio bursts

At the site of their origin, solar meterwave radio bursts contain pristine information about the local coronal magnetic field and plasma parameters. On its way through the turbulent corona, this radiation gets substantially modified due to the propagation effects. Effectively disentangling the intrinsic variations in emission from propagation effects has remained a challenge. We demonstrate a way to achieve this, using snapshot spectroscopic imaging study of weak type III bursts using data from the Murchison Widefield Array (MWA). We use this study to present the first observational evidence for second-scale Quasi-Periodic Oscillations (QPOs) in burst source sizes and orientation with simultaneous QPOs in intensity. The observed oscillations in source sizes are so fast and so large that, they would require two orders of magnitude larger Alfven speed than the typical 0.5 Mm/s, if interpreted within a MHD framework. These observations imply the presence of a quasi-periodic regulation mechanism operating at the particle injection site, modulating the geometry of energetic electron beams that generate type III bursts. In addition, we introduce a method to characterize plasma turbulence in mid coronal ranges, using such frequent weak bursts. We also detect evidence for a systematic drift in the location of the burst sources superposed on the random jitter induced by scattering. We interpret this as the motion of the open flux tube within which the energetic electron beams travel

Continuum and spectral line observations of the OH Megamaser galaxy Arp 220

We present MERLIN observations of the continuum (both 1.6 and 5 GHz) and OH maser emission towards Arp220. the correct spatial configuration of the various componnents of the galaxy is revealed. In the eastern component the masers are shown to be generally coincident with the larger scale continuum emission; in the west, the masers and continuum do not generally arise from the same location. A velocity gradient (0.32+/-0.03km/s/pc) is found in the eastern nuclear region in MERLIN scales; this gradient is three times smaller than seen in OH and implies that the OH gas lies inside the HI. A re-analysis of previously presented global VLBI data (Lonsdale et al. 1998) reveals a very high velocity gradient (18.67+/-0.12km/s/pc) in one component, possibly the site of a heavily obscured AGN.Comment: 10 pages, 11 figures, accepted by MNRA

Multiwavelength radio observations of the compact starburst in Arp 220

We report the first detection at multiple radio wavelengths (13, 6, and 3.6 cm) of 18 compact sources within both nuclei of the Ultra Luminous Infra-Red Galaxy (ULIRG) Arp 220. In just over half of the sources we find that the observed spectra are consistent with the standard model of powerful Type IIn supernovae interacting with their pre-explosion stellar wind. The rate of appearance of new radio sources ascribed to these supernova events suggests that a large fraction of core-collapse supernovae in Arp 220 are highly luminous, possibly implying a radically different stellar initial mass function (IMF) or stellar evolution compared to galactic disks. A second group of sources, consisting of the brightest and longest monitored sources at 18 cm, do not easily fit the radio supernova model. We propose that these are young supernova remnants that have just begun interacting with their surrounding dense ISM