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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A B S T R A C TWe report the first detection of CO in the bulge of M31. The 12 CO (1±0) and (2±1) lines are both detected in the dust complex D395A/393/384, at 1.3 arcmin (,0.35 kpc) from the centre. From these data and from visual extinction data, we derive a CO luminosity to reddening ratio (and a CO luminosity to H 2 column density ratio) quite similar to that observed in the local Galactic clouds. The (2±1) to (1±0) line intensity ratio points to a CO rotational temperature and a gas kinetic temperature of .10 K. The molecular mass of the complex, inside a 25-arcsec (100 pc) region, is 1X5 Â 10 4 M ( X

Using the Very Large Array (VLA) with an angular resolution of 3A, we have detected the hydrogen recombination line H92a from the galaxies Arp 220, M83, and NGC 2146. The line emission arises from the nuclear regions with a line-to-continuum ratio of 1% or less.In order to Ðt both the observed H92a line and continuum data in the nuclear regions, we have considered two types of models. First, we utilize a model with a collection of H II regions. A large number of compact H II regions are required in this model. With electron temperatures in the range 5 ] 103È 1 ] 104 K and a range of electron densities, this model can account for both the line intensity and the continuum spectrum. In most cases, the H92a line is dominated by internal stimulated emission due to free-free continuum arising within the H II regions. In a low-density case cm~3) for Arp 220, (n e \ 50 about half the line emission comes from external stimulated emission due to the background nonthermal source. Typical rates of ionizing photons predicted from these models are D5 ] 1052 s~1 for M83, D4 ] 1053 s~1 for NGC 2146, and 5 ] 1054 s~1 for Arp 220. We infer that 105 O5 stars are required in Arp 220, which is an order of magnitude greater than in NGC 2146 and 2 orders of magnitude greater than in M83. Alternatively, several uniform slab models with and in the range of 50È T e º 5 ] 103 n e 1 ] 104 cm~3 appear to Ðt both the H92a line and continuum data of Arp 220 and M83. In the lowdensity models, stimulated emission by the background nonthermal radiation appears to be dominant at low frequencies, and the lines at higher frequencies arise primarily from spontaneous emission. The uniform slab model requires a higher ionizing photon rate than the H II region model. No slab models with reasonable can Ðt the data observed in M83 and NGC 2146. T e Combining previous published data with these new observations, a sample of 13 galaxies has been observed for radio recombination lines (RRLs) with the VLA. Nine out of the 13 galaxies have been detected in the H92a line. While the H92a line luminosity appears to be correlated with the Bra line luminosity, we Ðnd that nearly all the RRL galaxies show a signiÐcant excess in H92a line compared to the expected LTE value. The excess in the H92a line Ñux suggests that non-LTE e †ects are important for the H92a line in these starburst nuclei. A strong correlation between H92a and the molecular lines of HCN/HCO`is also found, indicating that the RRL emitters may be spatially associated with the dense molecular cores. The inferred high electron density also suggests an intimate relation between the RRLs and the dense molecular medium in these galaxies.

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Using the Very Large Array (VLA), we have observed the H92a line in the nuclear regions of NGC 1 3628 and Arp 299 (IC 694 ] NGC 3690) with an angular resolution of 1A. The radio recombination lines could provide quite important constraints to the physical conditions for the ionized gas in the nuclear regions. For the central nuclear components in NGC 3628, IC 694, and NGC 3690, we Ðnd that the electron density of the ionized gas is in the range 5 ] 102 to 5 ] 104 cm~3, with corresponding geometrical sizes ranging from 25 to 0.01 pc. For the regions with a large line-to-continuum ratio, such as the anomalous kinematic components in IC 694 and NGC 3628, the electron density of the H II regions is well constrained. The derived parameters suggest the existence of D100 high-density, compact H II regions cm~3 and l D 1È2 pc) in these two components. The large number of high-density, (n e D 104 compact H II regions deduced for these components can be regarded as evidence for enhanced massive star formation in anomalous kinematic structures that are often produced in interacting systems.Our new H92a line observations reveal the presence of rotating circumnuclear ionized gas disks in both NGC 3628 and IC 694. From the analysis of the nuclear kinematics, we Ðnd that a total dynamical mass of 3 ] 108 exists within a radius R \ 120 pc in NGC 3628 and 7 ] 108 within R \ 200 pc M _ M _ in IC 694. The kinematics of the ionized gas and the models with a collection of H II regions infer that the ratio of H II to dynamical mass in the starburst nuclei varies from 1 ] 10~4 (for the high-density, compact H II region model) to 1 ] 10~2 (for the low-density, large H II region model).