Hidden starbursts in Seyfert 1 galaxies

We report the detection of the 3.3-mum polycyclic aromatic hydrocarbon (PAH) feature in two Seyfert 1 galaxies (NGC 3227 and Mrk 766) and one QSO (Mrk 478), observed with SpeX at the Infrared Telescope Facility at a spectral resolution not previously attained for these types of objects. Except for NGC 3227, this is the first time that the 3.3-mum PAH emission has been detected in Mrk 766 and 478. The widths of the emission, reported also for the first time, are rather similar, ranging from 450 to 550 Angstrom. The luminosity of the 3.3-mum PAH emission measured in the QSO Mrk 478 places it at a level similar to that found in starburst and infrared-luminous galaxies, and implies that this object is having a vigorous burst of star formation. The spatial resolution of the spectra allows us to constrain the location of the star-forming region to the inner 1 kpc for the QSO and 150 pc for the Seyferts. Our results support the idea that these objects reside in molecular gas-rich galaxies and that their observed infrared excess is primarily due to star formation, as previously indicated by CO and H-2 observations. We also report, for Mrk 1239, the presence of a broad emission feature centred at 3.43 mum, not previously detected in an extragalactic object and the origin of which is not yet clear.