The central star cluster of the star-forming dwarf galaxy NGC 5253

We have observed the star-forming dwarf galaxy NGC 5253 at optical, radio, and infrared wavelengths. Our data, combined with other observations from the literature, indicate that star formation in this galaxy has proceeded from the outer parts inward and that the center of NGC 5253 is now the site of an obscured, concentrated, and extremely young burst of star formation. The centimeter-wavelength radio continuum spectrum is very flat, which indicates that the radio emission is almost entirely due to thermal emission from H II regions. There is no evidence for synchrotron emission from supernova remnants, which is most unusual for a starburst galaxy. The observed radio continuum flux is consistent with infrared recombination-line observations, with N-Lyc similar to 3 x 10(52) s(-1). The infrared spectrum resembles II Zw 40 and no other star-forming galaxy in that it has strong [S IV] 10.5 mu m emission. We analyze the infrared spectrum with a grid of model H II regions and conclude that the young star cluster is dominated by stars of 42,000-45,000 K. The star cluster is probably a few Myr, and in any case less than 10 Myr, old, and the stellar density in the starburst is 0.5-1 O stars pc(-3).