The continuum radio structure of the nucleus of M82

Abstract

VLA radio maps of M82 show a complex of hotspots and jetlike features within the 700 pc nuclear region, many of which are still unresolved at 1 arcsec resolution. Accurate positional comparison of compact optical knots and the new 5 GHz radio map shows that the stellar knots visible at about 1 micron lie close to the outer, sharp gradient of the radio complex. Most of the dense optical knots do not themselves have detectable radio emission, and, in particular, two very bright knots (about 10 to the 8th solar luminosities within approximately 15 pc) give less than about 1 mJy of radio emission - equal to that of Cassiopeia A at the distance of M82. The centimeter wavelength intensity of the brightest radio hotspot, 41.9 + 58, is found to be decreasing with an e-folding time of about 12 years. Various mechanisms for this interesting source are discussed. The high local total gas pressure, indicated by the cosmic ray electron energy density of the radio emitting knots and filaments (greater than or equal to 3 x 10 to the -9th erg/cu cm), will cause the present radio complex to disperse within less than approximately 10 to the 7th years. Assuming that the radio emission comes from cosmic ray electrons originating in supernova remnants, it is argued that the current star formation burst comes from a population having an excess of massive stars. This burst of star formation may have been triggered by the high ambient gas pressure, which in turn may have been originally caused by infall of material.