On the hidden nuclear starburst in Arp 220

We construct a starburst model for the hidden starbursts in Arp 220 based on the new Starburst99 models of Leitherer and coworkers. Comparing these stellar population synthesis models with observations, we show that the hidden power source must be due to star formation (as opposed to an active galactic nucleus) at the 50% level or more in order to avoid an ionizing photon excess problem and this starburst must be young (10(7) yr). We derive a current star formation rate of 270 M. yr(-1) and an extinction mag for our line of A(v) > 30 sight to this hidden starburst.