The origin of optical emission from super-Eddington accreting Active Galactic Nuclei: The case of Ton S 180

Abstract

Self-gravitating accretion discs have only been studied in a few nearby objects using maser spots at the parsec-scale. We find a new spectral window for observing the self-gravitating accretion disc in super-Eddington accreting Active Galactic Nuclei (AGNs). This window is determined by calculating the outermost radius (r sg) of a non self-gravitating disc and the corresponding emission wavelength (λ sg) as a function of various disc parameters. We find that λ sg reaches ∼4000 Å for α = 0.1, when M ≳ 70 (M BH/10 7 M⊙) -1 L Edd/c 2 (where α, M, M BH and L Edd are, respectively, the viscosity parameter, gas accretion rate onto the central black hole (BH), the BH mass and the Eddington luminosity). Moreover, λ sg is as small as ∼1500 Å for α = 0.001, which is the smallest α case in this study. Therefore, the window for observing the self-gravitating part of an AGN accretion disc is from ∼2 μm to λ sg. Incidentally, r sg can be less than the photon trapping radius for M ≳ 10 3.3 L Edd/c 2. Namely, a self-gravitating, optically-thick, advection-dominated accretion disc is expected to appear in the extremely high accretion rate regime. Next, we demonstrate that the Mid-Infrared to X-ray spectrum of a bright, well-studied Narrow-Line Seyfert 1 galaxy, Ton S 180, is indeed well fitted by the spectrum arising from the following three components: an inner slim disc (with a corona), an outer, self-gravitating non-Keplerian disc and a dusty torus. The total mass, BH mass plus the entire disc mass, is found to be about (1.4-8.0) M BH. If the surface density varies with radius r in proportion to r -0.6, the total mass is consistent with the central mass estimated by Hβ and [O III] widths.