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. 8th European VLBI Network Symposium

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The central 1150 pc region of the starburst galaxy NGC 253 is shown to have a distinct gaseous kinematic subsystem, exhibiting rotation in a plane perpendicular to the galactic disk, and an interior region with possible counterrotation in the plane of the disk. In addition, solid-body rotation in the same sense as the galactic disk is observed in the outer parts of the central region. We suggest that this kinematic subsystem in NGC 253 may be indicative of a secondary bar inside the known primary bar. Alternatively, it may be a signature of a merger or an accretion event during the history of the galaxy. The dynamical mass within a radius of 5Љ is 13 ϫ 10 8 M J . These results are based on a two-dimensional image of the velocity field of the H92␣ recombination line in the central 9Љ ϫ 4Љ region, with an angular resolution of 1"8 ϫ 1"0, made using observations in the B, C, and D configurations of the VLA.

We present observations of low-frequency recombination lines of carbon toward Cas A near 34.5 MHz (n D 575) using the Gauribidanur radio telescope and near 560 MHz (n D 225) and 770 MHz (n D 205) using the NRAO 140 foot (43 m) telescope in Greenbank. We also present high angular resolution (1@) observations of the C270a line near 332 MHz using the Very Large Array in B-conÐguration. A high signal-to-noise ratio spectrum is obtained at 34.5 MHz, which clearly shows a Voigt proÐle with distinct Lorentzian wings, resulting from signiÐcant pressure and radiation broadening at such high quantum numbers. The emission lines detected near 332, 550, and 770 MHz, on the other hand, are narrow and essentially Doppler-broadened. The measured Lorentzian width at 34.5 MHz constrains the allowed combinations of radiation temperature, electron density, and electron temperature in the line-forming region. Radiation broadening at 34.5 MHz places a lower limit of 115 pc on the separation between Cas A and the line-forming clouds. Modeling the variation in the integrated line-to-continuum ratio with frequency indicates that the region is likely to be associated with the cold atomic hydrogen component of the interstellar medium, and the physical properties of this region are likely to be K,