2025ApJ...990..191W

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 μJy, 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 × 10¹⁷ W Hz^–1 is about 60 times that of Sagittarius A^⋆. The nucleus' spectral index ${\alpha }_{10\,{\rm{GHz}}}^{22\,{\rm{GHz}}}=-1.85\pm 0.18$ (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) × 10⁹M_⊙, consistent with previous estimates of (1-2) × 10⁹M_⊙ from stellar or hot-gas dynamics. Also, the Eddington-scaled accretion rate for NGC 3115, ${\dot{M}}_{{\rm{ADAF}}}/{\dot{M}}_{{\rm{Edd}}}=1.{2}_{-0.6}^{+1.0}\times 1{0}^{-8}$, is about 4-8 times lower than recent estimates for Sagittarius A^⋆.