We have used a Very Long Baseline Interferometry (VLBI) array at 18cm wavelength to image the nucleus of the luminous IR galaxy Arp 220 at ~1 pc linear resolution, and with very high sensitivity. The resulting map has an rms of 5.5 microJy/beam, and careful image analysis results in 49 confirmed point sources ranging in flux density from 1.2 mJy down to ~60 microJy. Comparison with high sensitivity data from 12 months earlier reveals at least four new sources. The favored interpretation of these sources is that they are radio supernovae, and if all new supernovae are detectable at this sensitivity, a resulting estimate of the supernova rate in the Arp 220 system is 4 +/- 2 per year. The implied star formation rate is sufficient to power the entire observed far-infrared luminosity of the galaxy. The two nuclei of Arp 220 exhibit striking similarities in their radio properties, though the western nucleus is more compact, and appears to be ~3 times more luminous than the eastern nucleus. There are also some puzzling differences, and differential free-free absorption, synchrotron aging and expansion losses may all be playing a role. Comparison with the nearby starburst galaxy M82 supports the hypothesis that the activity in Arp 220 is essentially a scaled-up version of that in M82.Comment: 24 pages, 3 figures, 1 table. Accepted for publication in Ap.

We report the first detection at multiple radio wavelengths (13, 6, and 3.6 cm) of the compact sources within both nuclei of the ultraluminous infrared galaxy Arp 220, presenting radio spectra of the 18 detected sources. In just over half of these, we find that these spectra and other properties are consistent with the standard model of powerful Type IIn supernovae interacting with their preexplosion stellar wind. The rate of appearance of new radio sources identified with these supernova events suggests that an unusually large fraction of core-collapse supernovae in Arp 220 are highly luminous, possibly implying a radically different stellar initial mass function or stellar evolution compared to galactic disks. Another possible explanation invokes very short ($3 ; 10 5 yr) intense ($10 3 M yr À1 ) star formation episodes with a duty cycle of $10%. A second group of our detected sources, consisting of the brightest and longest monitored sources at 18 cm, do not easily fit the radio supernova model. These sources show a range of spectral indexes from À0.2 to À1.9. We propose that these are young supernova remnants that have just begun interacting with a surrounding ISM with a density between 10 4 and 10 5 cm À3 . One source is probably resolved at 3.6 cm wavelength with a diameter 0.9 pc. In the western nucleus we estimate that the ionized component of the ISM gives rise to foreground free-free absorption with opacity at 18 cm of <0.6 along the majority of lines of sight. Other sources may be affected by absorption with opacity in the range 1Y2. These values are consistent with previous models as fitted to the radio recombination lines and the continuum spectrum.

In this paper, we investigate the radio‐mid‐infrared (MIR) correlation at very low flux densities using extremely deep 1.4‐GHz subarcsec angular resolution Multi‐Element Radio‐Linked Interferometer + Very Large Array observations of a field centred upon the Hubble Deep Field‐North, in conjunction with Spitzer 24‐μm data. From these results, the MIR‐radio correlation is extended to the very faint (∼μJy) radio source population. Tentatively, we detect a small deviation from the correlation at the faintest infrared flux densities. We suggest that this small observed change in the gradient of the correlation is the result of a suppression of the MIR emission in faint star‐forming galaxies. This deviation potentially has significant implications for using either the MIR or non‐thermal radio emission as a star formation tracer of very low luminosity galaxies.

Context. A 10-arcmin region around the Hubble Deep Field (North) contains 92 radio sources brighter than 40 µJy which are wellresolved by MERLIN+VLA at 0. 2−2 resolution (average size ∼1 ). 55 of these have Chandra X-ray counterparts in the 2-Ms CDF(N) field including at least 17 with a hard X-ray photon index and high luminosity characteristic of a type-II (obscured) AGN. More than 70% of the radio sources have been classified as starbursts or AGN using radio morphologies, spectral indices and comparisons with optical appearance and rest-frame MIR emission. On this basis, starbursts outnumber radio AGN 3:1. Aims. We investigate the possibility that very luminous radio and X-ray emission originates from different phenomena in the same high-redshift galaxies. Methods. This study extends the Virtual Observatory (VO) methods previously used to identify X-ray-selected obscured type-II AGN, to examine the relationship between radio and X-ray emission. We describe a VO cut-out server for MERLIN+VLA 1.4-GHz radio images in the HDF(N) region. Results. The high-redshift starbursts have typical sizes of 5-10 kpc and star formation rates of ∼1000 M yr −1 , an order of magnitude more extended and intense than in the local universe. There is no obvious correlation between radio and X-ray luminosities nor spectral indices at z > ∼ 1.3. About 70% of both the radio-selected AGN and the starburst samples were detected by Chandra. The X-ray luminosity indicates the presence of an AGN in at least half of the 45 cross-matched radio starbursts. Eleven of these are type-II AGN, of which 7 are at z ≥ 1.5. This distribution overlaps closely with the X-ray detected radio sources which were also detected by SCUBA. In contrast, all but one of the AGN-dominated radio sources are at z < 1.5, including the 4 which are also X-ray selected type-II AGN. The stacked 1.4-GHz emission at the positions of radio-faint X-ray sources is correlated with X-ray hardness. Conclusions. Almost all extended radio starbursts at z > 1.3 host X-ray selected obscured AGN. The radio emission from most of these ultra-luminous objects is dominated by star formation although the highest redshift (z = 4.424) source has a substantial AGN contribution. Star-formation appears to contribute less than 1/3 of their X-ray luminosity. Our results support the inferences from SCUBA and IR data, that at z > ∼ 1.5, star formation is observably more extended and more copious, it is closely linked to AGN activity and it is triggered differently, compared with star formation at lower redshifts.