Inside the Stagnation Radius of the Nearest Billion-solar-mass Black Hole

We used the NSF Jansky Very Large Array at a frequency ν = 22 GHz to study the nearest billion-solar-mass black hole (BH), in the early-type galaxy NGC 3115 at a distance of 9.7 Mpc. We localize a faint continuum nucleus, with flux density S 22 GHz = 48.2 ± 6.4 μ Jy, to a FWHM diameter d 22 GHz < 59 mas (2.8 pc). We find no evidence for adjacent emission within a stagnation region of radius R sta ∼ 360 mas (17 pc) identified in a recent hydrodynamic simulation tailored to NGC 3115. Within that region, the simulated gas flow developed into an advection-dominated accretion flow (ADAF). The nucleus’ luminosity density L 22 GHz = 5.4 × 10 17 W Hz −1 is about 60 times that of Sagittarius A ⋆ . The nucleus’ spectral index α 10 GHz 22 GHz = − 1.85 ± 0.18 ( S ν ∝ ν α ) indicates optically thin synchrotron emission. The spectral energy distribution of the nucleus peaks near ν peak = 9 GHz. Modeling this radio peak as an ADAF implies a BH mass M ADAF = (1.2 ± 0.2) × 10 9 M ⊙ , consistent with previous estimates of (1–2) × 10 9 M ⊙ from stellar or hot-gas dynamics. Also, the Eddington-scaled accretion rate for NGC 3115, M ̇ ADAF / M ̇ Edd = 1 . 2 − 0.6 + 1.0 × 1 0 − 8 , is about 4–8 times lower than recent estimates for Sagittarius A ⋆ .