Observations of the starburst galaxy, M82, have been made with the VLA in its Aconfiguration at 15 GHz and MERLIN at 5 GHz enabling a spectral analysis of the compact radio structure on a scale of < 0.1 ′′ (1.6 pc). Crucial to these observations was the inclusion of the Pie Town VLBA antenna, which increased the resolution of the VLA observations by a factor of ∼2. A number of the weaker sources are shown to have thermal spectra and are identified as Hii regions with emission measures ∼10 7 cm −6 pc. Some of the sources appear to be optically thick at 5 GHz implying even higher emission measures of ∼10 8 cm −6 pc. The number of compact radio sources in M82 whose origin has been determined is now 46, of which 30 are supernova related and the remaining 16 are Hii regions. An additional 15 sources are noted, but have yet to be identified, meaning that the total number of compact sources in M82 is at least 61. Also, it is shown that the distribution of Hii regions is correlated with the large-scale ionised gas distribution, but is different from the distribution of supernova remnants. In addition, the brightest Hii region at (B1950) 09 h 51 m 42.21 s +69• 54′ 59.2 ′′ shows a spectral index gradient across its resolved structure which we attribute to the source becoming optically thick towards its centre.

A B S T R A C TObservations of the starburst galaxy, M82, have been made with a 20-station global very long-baseline interferometry (VLBI) array at l 18 cm. Maps are presented of the brightest young supernova remnants (SNR) in M82 and the wide-field mapping techniques used in making images over a field of view of ,1 arcmin with 3-milliarcsecond resolution are discussed. A limit has been placed on the power-law deceleration of the young supernova remnant (SNR) 43.311592 with an index greater than 0X73^0X11 from observations with the European VLBI Network. Using the global array we have resolved compact knots of radio emission in the source which, with future global observations, will enable better constraints to be placed on the expansion parameters of this SNR.The latest global observations have also provided high-resolution images of the most compact radio source in M82, 41.951575. We determine an upper limit to the radial expansion rate along the major axis of 2000 km s 21 . However, the new images also show structure resembling that of collimated ejection which brings into question the previous explanation of the source as being a result of the confinement of a supernova by a highdensity circumstellar medium.It is apparent that we are now able to image the brightest supernova remnants in M82 with a linear scale which allows direct comparison with galactic SNR such as Cassiopeia A.

We present the results of a second epoch of 18‐cm global very long baseline interferometry (VLBI) observations, taken on 2001 February 23, of the central kiloparsec of the nearby starburst galaxy Messier 82. These observations further investigate the structural and flux evolution of the most compact radio sources in the central region of M82. The two most compact radio objects in M82 have been investigated (41.95+575 and 43.31+592). Using this recent epoch of data in comparison with our previous global VLBI observations and two earlier epochs of European VLBI network observations, we measure expansion velocities in the range of 1500–2000 km s−1 for 41.95+575 and 9000–11 000 km s−1 for 43.31+592 using various independent methods. In each case, the measured remnant expansion velocities are significantly larger than the canonical expansion velocity (500 km s−1) of supernova remnants (SNRs) within M82 predicted from theoretical models. In this paper, we discuss the implications of these measured expansion velocities with respect to the high‐density environment that the SNRs are expected to reside in within the centre of the M82 starburst.