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

A reexamination of the correspondence between 6 cm radio continuum sourcesand young star clusters in the Antennae galaxies indicates that 85 % of thestrong thermal sources have optical counterparts, once the optical image isshifted 1.2 arcsec to the southwest. A sample of 37 radio-optical matches arestudied in detail showing correlations between radio properties and a varietyof optical characteristics. There is a strong correlation between the radioflux and the intrinsic optical brightness. In particular, the brightest radiosource is also the intrinsically brightest optical cluster (WS80). It is alsothe most extincted cluster in the sample, the strongest CO source and thestrongest 15 micron source . Furthermore, the brightest ten radio sources areall amongst the youngest clusters with ages in the range 0 - 4 Myr andextinctions from A_V = 0.5 to 7.6 mag (with a median value of 2.6 mag). Only afew of the very red clusters originally discovered by Whitmore & Schweizer areradio sources, contrary to earlier suggestions. Finally, a new hybrid method ofdetermining cluster ages has been developed using both UBVI colors and H_alphaequivalent widths to break the age-reddening degeneracy.Comment: 51 pages, 13 postscript figures, LaTex. To appear in the Astronomical Journal, 124, 2002, Septembe