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

We present sensitive imaging at 1.4 GHz of the two highest redshift far-infrared (FIR) luminous QSOs SDSS J114816.65+525150.2 (z = 6.42) and SDSS J104845.05+463718.3 (z = 6.2). Radio continuum emission is detected from J1148+5251 with S 1.4 = 55 ± 12µJy, while J1048+4637 is marginally detected with S 1.4 = 26 ± 12µJy. Comparison of the radio and FIR luminosities shows that both sources follow the radio-FIR correlation for star forming galaxies, with implied (massive) star formation rates ∼ 10 3 M ⊙ year −1 , although we cannot rule-out as much as 50% of the FIR luminosity being powered by the AGN.Five bright (> 22 mJy) radio sources are detected within 8 ′ of J1148+5251. This is a factor 30 more than expected for a random field. Two sources have SDSS redshifts, including a z = 1.633 radio loud quasar and a z = 0.05 radio galaxy. However, we do not find evidence for a galaxy cluster in the SDSS data, at least out to z = 0.2. Considering the faint SDSS magnitudes of the remaining radio sources, we conclude that the overdensity of radio sources could either be a statistical fluke, or a very large scale structure (> 8Mpc comoving) at z ≥ 1. We also consider the possibility of gravitational lensing by the closest (in angle) bright galaxy in the SDSS data at z = 0.05, and conclude that the galaxy provides negligible magnification.

We have imaged the H92α and H75α radio recombination line (RRL) emissions from the starburst galaxy NGC 253 with a resolution of ∼4 pc. The peak of the RRL emission at both frequencies coincides with the unresolved radio nucleus. Both lines observed towards the nucleus are extremely wide, with FWHM of ∼200 km s −1 . Modeling the RRL and radio continuum data for the radio nucleus shows that the lines arise in gas whose density is ∼10 4 cm −3 and mass is few thousand M ⊙ , which requires an ionizing flux of 6-20×10 51 photons s −1 . We consider a SNR expanding in a dense medium, a star cluster and also an AGN as potential ionizing sources. Based on dynamical arguments, we rule out an SNR as a viable ionizing source. A star cluster model was considered and the dynamics of the ionized gas in a stellar-wind driven structure was investigated. Such a model is consistent with the properties of the ionized gas only for a cluster younger than ∼10 5 years. The existence of such a young cluster at the nucleus seems improbable. The third model assumes the ionizing source to be an AGN at the nucleus. In this model, it was shown that the observed X-ray flux is too weak to account for the required ionizing photon flux. However, the ionization requirement can be explained if the accretion disk is assumed to have a Big Blue Bump in its spectrum. Hence, we favor an AGN at the nucleus as the source responsible for ionizing the observed RRLs. A hybrid model consisting of a inner ADAF disk and an outer thin disk is suggested, which could explain the radio, UV and the X-ray luminosities of the nucleus.

Abstract.We have imaged the H92α (8.3 GHz), H75α (15 GHz), and H166α (1.4 GHz) Radio Recombination Lines (RRLs) from NGC 253 at resolutions of 4.5 pc (0.4 ) , 2.5 pc (0.2 ) and 53 pc (4.5 ) respectively. The H92α line arises from individual compact sources, most of which possess radio continuum counterparts. The line widths range from ∼200 km s −1 for the sources near the radio nucleus to 70-100 km s −1 for the extranuclear ones. These lines are emitted by gas at a density ∼10 4 cm −3 . The remainder of the cm-wave RRLs arise in lower density gas (∼500 cm −3 ) with a higher area filling factor and with ten times higher mass. A third component of higher density gas (>10 4 cm −3 ) is required to explain the mm-wave RRLs.

We have detected the H92a radio recombination line from two dwarf starburst galaxies, NGC 5253 and He 2-10, using the Very Large Array. Both the line data as well as the radio continuum data are used to model the properties of the ionized gas in the centers of these galaxies. We consider a multidensity model for radio recombination lines and show why previous models, which were based on the assumption of gas at a single density, are valid in many situations. The models show that the ionized gas has a density of D104 cm~3 in both galaxies, with an e †ective size of 2È10 pc and a total mass of D104The derived production rate of Lyman continuum photons is D2.5 ] 1052 s~1 in both the galaxies, M _ . and the corresponding mass of stars (assuming a Salpeter initial mass function) is D105 The M _ . implied stellar density shows that the observed radio recombination lines arise from ionized gas around super star clusters (SSCs) in both galaxies (these SSCs have been recently detected through their radio continuum emission). The existence of D104 of ionized gas within a few parsecs of an SSC places M _ strict constraints on dynamical models. Using simple arguments, the parameter space for a few possible models are derived. The well-known radioÈfar-infrared correlation also holds for NGC 5253, although the radio emission from this galaxy is almost completely thermal. It is shown that NGC 5253 is strong evidence that the component of far-infrared emission from warm dust is correlated separately with the component of radio emission from thermal bremsstrahlung.

Abstract. We present the results of sub-mm, mm (850 µm, 450 µm and 1250 µm) and radio (1.4 and 4.8 GHz) continuum observations of a sample of 27 K-selected Extremely Red Objects, or EROs, (14 of which form a complete sample with K < 20 and I − K > 5) aimed at detecting dusty starbursts, deriving the fraction of UltraLuminous Infrared Galaxies (ULIGs) in ERO samples, and constraining their redshifts using the radio-FIR correlation. One ERO was tentatively detected at 1250 µm and two were detected at 1.4 GHz, one of which has a less secure identification as an ERO counterpart. Limits on their redshifts and their star forming properties are derived and discussed. We stacked the observations of the undetected objects at 850 µm, 1250 µm and 4.8 GHz in order to search for possible statistical emission from the ERO population as a whole, but no significant detections were derived either for the whole sample or as a function of the average NIR colours. These results strongly suggest that the dominant population of EROs with K < 20 is not comprised of ULIGs like HR 10, but is probably made of radio-quiet ellipticals and weaker starburst galaxies with L < 10 12 L and SF R < few hundred M yr −1 .

The next generation radiotelescopes / Les radiotélescopes du futur LOFAR calibration and wide-field imaging Étalonnage de LOFAR et l'imagerie en champ large

Abstract.We have detected the radio recombination lines H91α and H92α with rest frequencies of 8.6 GHz and 8.3 GHz from the starburst nucleus NGC 3256 at an angular resolution of 16.4 × 9.6 using the Australia Telescope Compact Array and at an angular resolution of 12.0 × 2.9 using the VLA. The line was detected at ∼1 mJy beam −1 peak with a width of 160 km s −1 with the ATCA and at ∼0.5 mJy beam −1 peak with a width of 114 km s −1 with the VLA. Modelling the line emitting region as a collection of H II regions, we derive constraints on the required number of H II regions, their temperature, density, and distribution. We find that a collection of 10 to 300 H II regions with temperatures of 5000 K, densities of 1000 cm −3 to 5000 cm −3 and diameters of 15 pc produced good matches to the line and continuum emmission. The Lyman continuum production rate required to maintain the ionization is 2 × 10 52 s −1 to 6 × 10 53 s −1 , which requires 600 to 17 000 O5 stars to be produced in the starburst.