2007ApJ...661L..25L

We report the first detection of the 6.2 and 7.7 µm 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∼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 (5.4x10¹² L_☉, corrected for lensing). The Cloverleaf's period of dominant QSO activity (L_Bol∼7x10¹³ L_☉) is coincident with an intense (star formation rate ∼1000 M_☉/yr) and short (gas exhaustion time ∼3x10⁷ yr) star-forming event.