New method for exploring super-Eddington active galactic nuclei by near-infrared observations




We propose a new method to explore the candidate super-Eddington active galactic nuclei (AGNs). We examine the properties of infrared (IR) emission from the inner edge of the dusty torus in AGNs, which are powered by super- or sub-Eddington accretion flows around black holes, by considering the dependence of the polar angle on the radiation flux of accretion flows. We find that for super-Eddington AGNs, of which the mass accretion rate is more than 102 times larger than the Eddington rate, the ratio of the AGN IR luminosity and the disc bolometric luminosity is less than 10−2, unless the half opening angle of the torus (θtorus) is small (θtorus < 65°). This is due to the self-occultation effect, whereby the self-absorption at the outer region of the super-Eddington flow dilutes the illumination of the torus. Such a small luminosity ratio is not observed in sub-Eddington AGNs, whose mass accretion rate is comparable to or no more than 10 times larger than the Eddington mass accretion rate, except for extremely thin tori (θtorus > 85°). We also consider the properties of the near-IR (NIR) emission radiated from hot dust >1000 K. We find that super-Eddington AGNs have a ratio of the NIR luminosity to the bolometric luminosity, LNIR, AGN/Lbol, disc, at least one order of magnitude smaller than for sub-Eddington AGNs for a wide range of half opening angle (θtorus > 65°), for various types of dusty torus model. Thus, a relatively low LNIR, AGN/Lbol, disc is a property that allows identification of candidate super-Eddington AGNs. Lastly, we discuss the possibility that NIR-faint quasars at redshift z∼ 6 discovered by a recent deep Sloan Digital Sky Survey may be young quasars whose black holes grow via super-Eddington accretion.