Infrared 3–4 μm Spectroscopy of Nearby PG QSOs and AGN–Nuclear Starburst Connections in High-Luminosity AGN Populations

Abstract

Abstract We present the results of infrared $L$-band (3–4$\ \mu$m) slit spectroscopy of 30 PG QSOs at z$\lt$ 0.17, a representative sample of local high-luminosity, optically selected AGNs. The 3.3$\ \mu$m polycyclic aromatic hydrocarbon (PAH) emission feature is used to probe nuclear ($\lt$ a few kpc) starburst activity and to investigate the connections between AGNs and nuclear starbursts in PG QSOs. The 3.3$\ \mu$m PAH emission is detected in the individual spectra of 5/30 of the observed PG QSOs. We constructed a composite spectrum of PAH-undetected PG QSOs and discerned the presence of the 3.3$\ \mu$m PAH emission therein. We estimated the nuclear-starburst and AGN luminosities from the observed 3.3$\ \mu$m PAH emission and 3.35$\ \mu$m continuum luminosities, respectively, and found that the nuclear-starburst-to-AGN luminosity ratios in PG QSOs are similar to those of previously studied AGN populations with lower luminosities, suggesting that AGN–nuclear starburst connections are valid over a wide luminosity range of AGNs in the local universe. The observed nuclear-starburst-to-AGN luminosity ratios in PG QSOs with available supermassive black-hole masses are comparable to a theoretical prediction based on the assumption that the growth of a supermassive black hole is controlled by starburst-induced turbulence.