We present high resolution MERLIN radio images of multiple relativistic ejections from GRS 1915+105 in 1997 October / November. The observations were made at a time of complex radio behaviour, corresponding to multiple optically-thin outbursts and several days of rapid radio flux oscillations. The radio imaging resolved four major ejection events from the system. As previously reported from earlier VLA observations of the source, we observe apparent superluminal motions resulting from intrinsically relativistic motions of the ejecta. However, our measured proper motions are significantly greater than those observed on larger angular scales with the VLA. Under the assumption of an intrinsically symmetric ejection, we can place an upper limit on the distance to GRS 1915+105 of 11.2 +/- 0.8 kpc. Solutions for the velocities unambiguously require a higher intrinsic speed by about 0.1c than that derived from the earlier VLA observations, whilst the angle to the line-of-sight is not found to be significantly different. At a distance of 11 kpc, we obtain solutions of v = 0.98 (-0.05,+0.02)c and theta = 66 +/- 2 degrees. The jet also appears to be curved on a scale which corresponds to a period of around 7 days. We observe significant evolution of the linear polarisation of the approaching component, with large rotations in position angle and a general decrease in fractional polarisation. The power input into the formation of the jet is very large, >10^38 erg/s at 11 kpc for a pair plasma. If the plasma contains a cold proton for each electron, then the mass outflow rate, >10^18 g/sec is comparable to inflow rates previously derived from X-ray spectral fits.Comment: 14 pages, 7 figures. Accepted for publication in MNRA

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.

We compare high-resolution optical and radio imaging of 12 luminous submillimeter (submm) galaxies at a median z = 2.2 ± 0.2 observed with Hubble Space Telescope (HST) and the MERLIN and VLA radio interferometers at comparable spatial resolution, ∼ 0.3 ′′ (∼ 2 kpc). The radio emission is used as a tracer of the likely far-infrared morphology of these dusty, luminous galaxies. In ∼30% of the sample the radio emission appears unresolved at this spatial scale, suggesting that the power source is compact and may either be an obscured AGN or a compact nuclear starburst. However, in the majority of the galaxies, ∼70% (8/12), we find that the radio emission is resolved by MERLIN/VLA on scales of ∼ 1 ′′ (∼10 kpc). For these galaxies we also find that the radio morphologies are often broadly similar to their restframe UV emission traced by our HST imaging. To assess whether the radio emission may be extended on even larger scales, ≫ 1 ′′ , resolved out by the MERLIN+VLA synthesized images, we compare VLA B-array (5 ′′ beam) to VLA A-array (1.5 ′′ beam) fluxes for a sample of 50 µJy radio sources, including 5 submm galaxies. The submm galaxies have comparable fluxes at these resolutions and we conclude that the typical radio emitting region in these galaxies are unlikely to be much larger than ∼ 1 ′′ (∼10 kpc). We discuss the probable mechanisms for the extended emission in these galaxies and conclude that their luminous radio and submm emission arises from a large, spatially-extended starburst. The median star formation rates for these galaxies are ∼ 1700 M ⊙ yr −1 (M> 0.1M ⊙ ) occuring within regions with typical sizes of ∼40 kpc 2 , giving a star formation density of 45 M ⊙ yr −1 kpc −2 . Such vigorous and extended starburst appear to be uniquely associated with the submm population. A more detailed comparison of the distribution of UV and radio emission in these systems shows that the broad similarities on large scales are not carried through to smaller scales, where there is rarely a one-to-one correspondance between the structures seen in the two wavebands. We interpret these differences as resulting from highly structured internal obscuration within the submm galaxies, suggesting that their vigorous activity is producing wind-blown channels through their obscuring dust clouds. If correct this underlines the difficulty of using UV morphologies to understand structural properties of this population and also may explain the surprising frequency of Lyα emission in the spectra of these very dusty galaxies.