Star Formation and AGN Activity in Galaxies Classified Using the 1.6 μm Bump and PAH Features at z = 0.4–2

Abstract

Abstract We studied the star-formation and AGN activity of massive galaxies in the redshift range $ z$$ =$ 0.4–2, which were detected in a deep survey field using the AKARI InfraRed (IR) astronomical satellite and Subaru telescope toward the North Ecliptic Pole (NEP). The AKARI/IRC Mid-InfraRed (MIR) multiband photometry was used to trace the star-forming activities with Polycyclic Aromatic Hydrocarbon (PAH) emission, which is effective not only to distinguish between star-forming and AGN galaxies, but also to estimate the Star Formation Rate (SFR) with converting its flux to the total emitting IR (TIR) luminosity. In combination with the analyses of the stellar components, we studied the MIR SED features of star-forming and AGN-harboring galaxies, which we summarize below: (1) The rest-frame 7.7-$ \mu $ m and 5-$ \mu $ m luminosities are good tracers of star-forming and AGN activities from their PAH and dusty tori emissions, respectively. (2) For dusty star-forming galaxies without AGN, their SFR shows a correlation that is nearly proportional to their stellar mass, and their specific SFR (sSFR) per unit stellar mass increases with redshift. Extinctions estimated from their TIR luminosities are larger than those from their optical SED fittings, which may be caused by geometric variations of dust in them. (3) Even for dusty star-forming galaxies with AGN, SFRs can be derived from their TIR luminosities with subtraction of the obscured AGN contribution, which indicates that their SFRs were possibly quenched around $ z$$ \simeq$ 0.8 compared with those without AGN. (4) The AGN activity from their rest-frame 5-$ \mu $ m luminosity suggests that their Super Massive Black Holes (SMBHs) could already have grown to $ \simeq$ 3 $ \times$ 10$ ^{8}$$ M_{\odot}$ in most massive galaxies with 10$ ^{11}$$ M_{\odot}$ at $ z$$ \lt$ 1.2, and the mass relation between SMBHs and their host galaxies has already become established by $ z$$ \simeq$ 1–2.