A parsec-scale study of the 5/15-GHz spectral indices of the compact radio sources in M82

Abstract

Observations of the starburst galaxy, M82, have been made with the Very Large Array (VLA) in its A configuration at 15 GHz and the Multi-Element Radio-Linked Interferometer Network (MERLIN) at 5 GHz, enabling a spectral analysis of the compact radio structure on a scale of <0.1 arcsec (1.6 pc). Crucial to these observations was the inclusion of the Pie Town Very Long Baseline Array (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 H II 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 ionized gas distribution, but is different from the distribution of supernova remnants. In addition, the brightest HII region at (B1950) 09 h 51 m 42 s .21 + 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.