Infrared emission from the radio supernebula in NGC 5253. A proto-globular cluster?

Hidden from optical view in the starburst region of the dwarf galaxy NGC 5253 lies an intense radio source with an unusual spectrum, which could be interpreted variously as nebular gas ionized by a young stellar cluster or nonthermal emission from a radio supernova or an active galactic nucleus. We have obtained 11.7 and 18.7 mum images of this region at the Keck telescope and find that it is an extremely strong mid-infrared emitter. The infrared-to-radio flux ratio rules out a supernova and is consistent with an H II region excited by a dense cluster of young stars. This "supernebula" provides at least 15% of the total bolometric luminosity of the galaxy. Its excitation requires 10(5)-10(6) stars, giving it the total mass and size (1-2 pc diameter) of a globular cluster. However, its high obscuration, small size, and high gas density all argue that it is very young, no more than a few hundred thousand years old. This may be the youngest globular cluster yet observed.