  We construct a starburst model for the hidden starbursts in Arp 220 based on
the new Starburst99 models of Leitherer et al. 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 AGN) at the 50% level or
more in order to avoid an ionizing photon excess problem, and this starburst
must be young ($< 7 \times 10^7$ yr). We derive a current star formation rate
of $270 M_{\odot}$ yr$^{-1}$, and an extinction $A_V > 30$ mag for our line of
sight to this hidden starburst.
