18 days of MERLIN data and 42 h of A‐array VLA data at 1.4 GHz have been combined to image a 10‐arcmin field centred on the Hubble Deep Field (HDF). This area also includes the Hubble Flanking Fields (HFF). A complete sample of 92 radio sources with S1.4 > 40 μJy was detected using the VLA data alone and then imaged with the MERLIN+VLA combination. The combined images offer (i) higher angular resolution (synthesized beams of diameter 0.2–0.5 arcsec), (ii) improved astrometric accuracy, and (iii) improved sensitivity compared with VLA‐only data. The images are amongst the most sensitive yet made at 1.4 GHz, with rms noise levels of 3.3 μJy beam−1 in the 0.2‐arcsec images. Virtually all the sources are resolved, with angular sizes in the range 0.2 to 3 arcsec. The central 3‐arcmin square was imaged separately to search for sources down to 27 μJy. No additional sources were detected, indicating that sources fainter than 40 μJy are heavily resolved with MERLIN and must have typical angular sizes > 0.5 arcsec. Radio sources associated with compact galaxies have been used to align the HDF, the HFF and a larger CFHT optical field to the radio‐based International Celestial Reference Frame. The HST optical fields have been registered to <50 mas in the HDF itself, and to ≤150 mas in the outer parts of the HFF. We find a statistical association of very faint (≥2 μJy) radio sources with optically bright HDF galaxies down to ∼23 mag. Of the 92 radio sources above 40 μJy, ∼85 per cent are identified with galaxies brighter than I= 25 mag; the remaining 15 per cent are associated with optically faint systems close to or beyond the HFF (or even the HDF) limit. The high astrometric accuracy and the ability of radio waves to penetrate obscuring dust have led to the correct identification of several very red, optically faint systems, including the the strongest submillimetre source in the HDF, HDF 850.1. On the basis of their radio structures and spectra, 72 per cent (66 sources) can be classified as starburst or active galactic nucleus‐type systems; the remainder are unclassified. The proportion of starburst systems increases with decreasing flux density; below 100 μJy > 70 per cent of the sources are starburst‐type systems associated with major disc galaxies in the redshift range 0.3–1.3. Chandra detections are associated with 55 of the 92 radio sources, but their X‐ray flux densities do not appear to be correlated with the radio flux densities or morphologies. The most recent submillimetre results on the HDF and HFF do not provide any unambiguous identifications with these latest radio data, except for HDF 850.1, but suggest at least three strong candidates.

Nova outbursts 1 take place in binary star systems comprising a white dwarf and either a low-mass Sun-like star or, as in the case of the recurrent nova RS Ophiuchi 2 , a red giant.Although the cause of these outbursts is known to be thermonuclear explosion of matter transferred from the companion onto the surface of the white dwarf 3 , models of the previous (1985) outburst of RS Ophiuchi failed to adequately fit the X-ray evolution 4 and there was controversy over a single-epoch high-resolution radio image, which suggested that the remnant was bipolar 5,6 rather than spherical as modelled. Here we report the detection of spatially resolved structure in RS Ophiuchi from two weeks after its 12 February 2006 outburst. We track an expanding shock wave as it sweeps through the red giant wind, producing a remnant similar to that of a type II supernova but evolving over months rather than millennia 7 . As in supernova remnants, the radio emission is non-thermal (synchrotron emission), but asymmetries and multiple emission components clearly demonstrate that contrary to the assumptions of spherical symmetry in models of the 1985 explosion, the ejection is jet-like, collimated by the central binary whose orientation on the sky can be determined from these observations. During the previous outburst of RS Ophiuchi (RS Oph) in 1985 a campaign was organized incorporating observations ranging from radio to X-ray wavelengths. The results included the detection of bright, evolving X-ray emission from hot gas suggested to arise from the expanding shock wave 8 . This time we have monitored RS Oph from much earlier in the outburst, both in X-rays [9][10][11][12][13] , and at radio wavelengths with the Multi-Element RadioLinked Interferometer Network (MERLIN), the Very Large Array (VLA), the Very Long Baseline Array (VLBA) and the European VLBI Network (EVN) [14][15][16] .

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We present polarization observations of 28 compact steep‐spectrum sources made with the upgraded MERLIN at 5 GHz. With an angular resolution of 60 milliarcsec and rms noise levels of about 0.1 mJy beam−1, the total intensity images reveal new details in many of these sources. A few of the more extended lobes and jets are quite highly polarized, but more than half of the components are completely unpolarized. Comparison with published data implies that this is due to Faraday depolarization, probably occurring in the surrounding medium with nB ∼ 1 cm−3μG. The high resolution of the present observations implies that the variations in Faraday rotation, probably due to magnetic field tangling, occur on scales of less than about 100 pc.

SN 2000ft is the first radio supernova detected in the circumnuclear starburst of a luminous infrared Seyfert 1 galaxy. It is located at a distance of 600 pc from the QSO-like nucleus of NGC 7469. We report the temporal evolution of SN 2000ft during the 3 years after its discovery. Although SN 2000ft has exploded in the dusty and very dense environment that exists in the nuclear regions of luminous infrared galaxies, it shows the radio evolution properties characteristic of radio supernovae identified as Type II supernovae, aside from some foreground free-free absorption. The peak luminosity and circumstellar matter opacity of SN 2000ft are similar to other compact radio sources detected in luminous infrared galaxies such as NGC 6240, Arp 299, and Arp 220 and identified as Type II supernovae.

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We report on radio observations of the 1999 September event of the X-ray transient V4641 Sgr (\XTE J1819[254\SAX J1819.3[2525). This event was extremely rapid in its rise and decay across radio, optical, and X-ray wavelengths ; the X-rays rose to 12 crab within 8 hr and faded to below 0.1 crab in less than 2 hr. Radio observations were made with seven telescopes during the Ðrst day following the onset of the strong X-ray event, revealing a strong radio source that was detected for 3 further weeks by the more sensitive telescopes. The radio source was resolved even in the Ðrst Very Large Array (VLA) images (September 16.027 UT), being long with an axis ratio of at least 10 : 1. The total Ñux D0A .25 density decayed by a factor of D4 over the Ðrst day, and by September 17.94 UT the radio emission was conÐned to a slowly decaying, marginally resolved remnant located at one side of the early elongated emission. The H I absorption spectrum gives a minimum kinematic distance of about 400 pc ; various other arguments suggest that the true distance is not much greater than this. The inferred proper motions for the early extended emission day~1) correspond to v/c D 1.0È (0A .4È1A .1 3.2 (d/0.5 kpc), and this together with the radio morphology argues that this is a relativistic jet source like GRS 1915]105 and GRO J1655[40. The proper motion of the late-time remnant is at least 100 times smaller. One simple interpretation posits the ejection of a single short-lived jet segment, followed by a more slowly decaying, optically thin jet segment ejection. These two components can explain both the multifrequency radio light curves and the radio images. The most likely parameters for the fast-jet system with net-averaged proper motion of day~1, assuming d \ 0.5 kpc, are v D 0.85c and D0A .4 i D 63¡, where i is the inclination to the line of sight. The corresponding apparent velocities are 1.4c and 0.6c for the approaching and receding jets, making V4641 Sgr the closest superluminal jet source known.

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