Prospect for future MeV gamma-ray active galactic nuclei population studies

While the X-ray, GeV gamma-ray, and TeV gamma-ray skies have been extensively studied, the MeV gamma-ray sky has not yet been well investigated since the Imaging Compton Telescope (COMPTEL) scanned the sky about two decades ago. In this paper, we investigate the prospects for active galactic nuclei population studies with future MeV gamma-ray missions using recent spectral models and luminosity functions of Seyferts and flat-spectrum radio quasars (FSRQs). Both of them are plausible candidates for the origins of the cosmic MeV gamma-ray background. If the cosmic MeV gamma-ray background radiation is dominated by non-thermal emission from Seyferts, a sensitivity of 10−12 erg cm−2 s−1 is required to detect several hundred Seyferts in the entire sky. If FSRQs make up the cosmic MeV gamma-ray background, a sensitivity of ∼ 4 × 10−12 erg cm−2 s−1 is required to detect several hundred FSRQs following the recent FSRQ X-ray luminosity function. However, based on the latest FSRQ gamma-ray luminosity function, with which FSRQs can explain up to ∼ 30% of the MeV background, we can expect several hundred FSRQs even with a sensitivity of 10−11 erg cm−2 s−1, which is almost the same as the sensitivity goal of the next-generation MeV telescopes.