PAH emission and star formation in the host of the z∼2.56 Cloverleaf QSO

We report the first detection of the 6.2 and 7.7 mu m infrared polycyclic aromatic hydrocarbon ( PAH) emission features in the spectrum of a high-redshift QSO, from the Spitzer IRS spectrum of the Cloverleaf lensed QSO ( H1413 + 117, z similar to 2.56). The ratio of PAH features and rest-frame far-infrared emission is the same as in lower luminosity star-forming ULIRGs and in local PG QSOs, supporting a predominantly starburst nature of the Cloverleaf's huge far-infrared luminosity (5.4 x 10(12) L-circle dot, corrected for lensing). The Cloverleaf's period of dominant QSO activity (L-Bol similar to 7 x 10(13) L-circle dot) is coincident with an intense (star formation rate similar to 1000 M-circle dot yr(-1)) and short (gas exhaustion time similar to 3 x 10(7) yr) star-forming event.