What Fraction of the Young Clusters in the Antennae Galaxies Are “Missing”?

A reexamination of the correspondence between 6 cm radio continuum sources and young star clusters in the Antennae galaxies indicates that ∼85% of the strong thermal sources have optical counterparts, once the optical image is shifted 1.″2 to the southwest. A sample of 37 radio-optical matches are studied in detail, showing correlations between radio properties (i.e., total flux and spectral index) and a variety of optical characteristics (i.e., intrinsic cluster brightness, Hα flux and equivalent width, extinction, and cluster ages). There is a strong correlation between the radio flux and the intrinsic optical brightness. In particular, the brightest radio source is also the intrinsically brightest optical cluster (WS 80). It is also the most extincted cluster in the sample, the strongest CO source, and the strongest 15 micron source. Furthermore, the 10 brightest radio sources are all among the youngest clusters, with ages in the range 0–4 Myr and extinctions from AV = 0.5 to 7.6 mag (with a median value of 2.6 mag). A weak correlation between age and AV suggests that ∼6 Myr are typically required to remove enough dust to reach AV = 1 mag. The radio-bright phase lasts only about 10 Myr in these clusters, consistent with the interpretation that most of the radio emission originates from hot gas. Many of the nondetections are probably individual supernova remnants since they have relatively steep radio indices typical of nonthermal sources. Only a few of the very red clusters originally discovered by Whitmore and Schweizer are radio sources, contrary to earlier suggestions. Finally, a new hybrid method of determining cluster ages has been developed using both UBVI colors and Hα equivalent widths to break the age-reddening degeneracy. We find that the Bruzual and Charlot models, which use the Padova spectral evolution tracks, fit the data reasonably well, while the Leitherer & Heckman models, using the Geneva tracks, have a large red loop for ages 8–13 Myr that does not agree with the data.