Evidence for a Young Stellar Population in Nearby Type 1 Active Galaxies

To understand the physical origin of the close connection between supermassive black holes (BHs) and their host galaxies, it is vital to investigate star formation properties in active galaxies. Using a large data set of nearby type 1 active galactic nuclei (AGNs) with detailed structural decomposition based on high-resolution optical images obtained with the Hubble Space Telescope, we study the correlation between BH mass and bulge luminosity and the (Kormendy) relation between bulge effective radius and surface brightness. In both relations, the bulges of type 1 AGNs tend to be more luminous than those of inactive galaxies with the same BH mass or the same bulge size. This suggests that the central regions of AGN host galaxies have characteristically lower mass-to-light ratios than inactive galaxies, most likely due to the presence of a younger stellar population in active systems. In addition, the degree of luminosity excess appears to be proportional to the accretion rate of the AGN, revealing a physical connection between stellar growth and BH growth. Adopting a simple toy model for the increase of stellar mass and BH mass, we show that the fraction of young stellar population flattens out toward high accretion rates, possibly reflecting the influence of AGN-driven feedback.