VLA Observations of the Nearby Merger NGC 4038/4039: H [CSC]ii[/CSC] Regions and Supernova Remnants in the “Antennae”

Abstract

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 ~ 1'' and 26, or ~ 100 and 260 pc. We detect numerous compact radio sources embedded in more diffuse radio emission; the compact radio sources produce ~ 12% and ~ 25% of the 6 and 4 cm radio emission in the system, respectively. The strongest radio continuum emission occurs between the galaxies, at an optically unremarkable location near but not coincident with 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 sources and 63 individual 4 cm sources in the system, to a limiting luminosity of ~ 2 × 1018 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 observations. 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 spectra dominated by nonthermal emission from supernova remnants. For the compact sources dominated by thermal emission, we derive typical masses of 104 to 105 M⊙ in massive young stars. Using reasonable assumptions about source sizes, we estimate associated ionized gas masses of 103 to 104 M⊙. A comparison with the ~ (2–5) × 103 M⊙ typical optical clusters detected 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 105–106 M⊙ Whitmore 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 μm 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 from their birth cloud and are clearing away the surrounding material. Estimates of stellar ages and sound speeds indicate that we do not see star formation propagating in this system; rather, the star formation sequence is probably related to more global properties of the interaction. From the steep-spectrum 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 observed 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 106 yr in duration and about 3–4 Myr ago.