On the hidden nuclear starburst in Arp 220

We construct a starburst model for the hidden starbursts in Arp 220 based o n the new Starburst99 models of Leitherer and coworkers. Comparing these st ellar population synthesis models with observations, we show that the hidde n power source must be due to star formation (as opposed to an active galac tic 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 cur rent star formation rate of 270 M. yr(-1) and an extinction mag for our lin e of A(v) > 30 sight to this hidden starburst.