Does AGN fraction depend on redshift or luminosity? An extinction-free test by 18-band near- to mid-infrared SED fitting in the AKARI NEP wide field

Abstract

Abstract Revealing what fraction of galaxies harbor active galactic nuclei (AGNs) is central to understanding the black hole accretion history of the universe. However, optical and soft X-ray surveys miss the most highly obscured AGNs. Infrared (IR), instead, is more robust against absorption. Previous IR photometric surveys, however, only had four or five filters in mid-IR. Our AKARI North Ecliptic Pole wide field sample has 18 filters in mid-IR (nine from AKARI, four from WISE, and five from Spitzer), for the first time allowing a sophisticated mid-IR spectral energy distribution (SED) fitting diagnosis for a statistical number of sources (89178 over 5.4 deg2). By using an SED fitting technique, we investigated the evolution of AGN fraction as a function of redshift and IR (8–1000 μm) luminosity in an extinction-free way. We found that the AGN fraction (FAGN) shows no sign of strong redshift evolution. Instead, FAGN increases with increasing IR luminosity in all redshift bins (0 < $z$ < 2).