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 .