VLA observations of the nearby merger NGC 4038/4039: HII regions and supernova remnants in the "antennae"

We present 6 and 4 cm radio images of the inner disks of the nearby merger pair NGC 4038 and NGC 4039 at resolutions between similar to 1 " and 2."6, or similar to 100 and 260 pc. We detect numerous compact radio sources embe dded in more diffuse radio emission; the compact radio sources produce simi lar to 12% and similar to 25% of the 6 and 4 cm radio emission in the syste m, respectively. The strongest radio continuum emission occurs between the galaxies, at an optically unremarkable location near but not coincident wit h an extremely red cluster detected by Whitmore & Schweizer. The radio peak is at a location of intense star formation (as detected in the mid-IR) and of dense molecular gas. We identify 115 individual compact 6 cm radio sour ces and 63 individual 4 cm sources in the system, to a limiting luminosity of similar to 2 x 10(18) W Hz(-1), or,4 times the luminosity of Cas A; 90% of the 6 cm sources and 67% of the 4 cm sources are unresolved in these obs ervations. Of the strongest sources, for which the flux densities are large enough to measure spectral indices with reasonable errors, one-third have nominally flat radio spectra, indicating that they are dominated by thermal radio emission from H II regions, and two-thirds have nominally steep spec tra dominated by nonthermal emission from supernova remnants. For the compa ct sources dominated by thermal emission, we derive typical masses of 10(4) to 10(5) M. in massive young stars. Using reasonable assumptions about sou rce sizes, we estimate associated ionized gas masses of 10(3) to 10(4) M.. A comparison with the similar to(2-5) x 10(3) M. typical optical clusters d etected recently by Whitmore et al. suggests that the thermal radio sources detected here may be composed of tens of these typical young star clusters or else that they are more like the individual massive 10(5)-10(6) M. Whit more et al. clusters. The strongest thermal radio source is associated with none of the Whitmore et al. clusters but is coincident with strong CO and 15 mu m emission, implying that massive star clusters are still enshrouded by dust in this region. The second strongest thermal source is near several optical clusters, suggesting that these clusters are just now emerging fro m their birth cloud and are clearing away the surrounding material. Estimat es of stellar ages and sound speeds indicate that we do not see star format ion propagating in this system; rather, the star formation sequence is prob ably related to more global properties of the interaction. From the steep-s pectrum sources, we derive a total system supernova rate of 0.2-0.3 yr(-1), about an order of magnitude larger than that expected from the currently o bserved O star population. This suggests that a large number of O stars may have formed in the system in a short-lived burst, less than 10(6) yr in du ration and about 3-4 Myr ago.