The 8-13 micron spectrum of ARP 299C

Arp 299C is a $5 imes 10^{10}$ Lsun infrared source in the merging galaxy system Arp 299. It is the most luminous object known that is not obviously associated with a galaxy nucleus. Our data show that the 8--13 mum spectrum of Arp 299C has a strong NeII emission line and emission features like those of polycyclic aromatic hydrocarbon molecules. These emission features are characteristic of HII regions associated with a burst of star formation. We did not detect any high-excitation ionic lines characteristic of an active galactic nucleus, nor a deep silicate absorption feature that might indicate a hidden compact nucleus. We deduce that Arp 299C is powered by an intense burst of star formation.