Inside the Stagnation Radius of the Nearest Billion-Solar-Mass Black Hole

We used the NSF Jansky Very Large Array at a frequency $\nu =$ 22,GHz to study the nearest billion-solar-mass black hole, in the early-type galaxy NGC,3115 at a distance of 9.7,Mpc. We localize a faint continuum nucleus, with flux density $S_{22\mathrm{G}\mathrm{H}\mathrm{z}}=48.2\pm 6.4\mu$Jy, to a FWHM diameter $d_{22\mathrm{G}\mathrm{H}\mathrm{z}}<$ 59,mas (2.8,pc). We find no evidence for adjacent emission within a stagnation region of radius $R_{\mathrm{s}\mathrm{t}\mathrm{a}}\sim$ 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 $L_{22\mathrm{G}\mathrm{H}\mathrm{z}}=5.4\times {10}^{17}\mathrm{W}\mathrm{H}{\mathrm{z}}^{-1}$ is about 60 times that of Sagittarius,A${}^{\star }$. The nucleus’ spectral index ${\alpha }_{10\mathrm{G}\mathrm{H}\mathrm{z}}^{22\mathrm{G}\mathrm{H}\mathrm{z}}=-1.85\pm 0.18$ ($S_{\nu }\propto {\nu }^{\alpha }$) indicates optically-thin synchrotron emission. The spectral energy distribution of the nucleus peaks near ${\nu }_{\mathrm{p}\mathrm{e}\mathrm{a}\mathrm{k}}=$ 9,GHz. Modeling this radio peak as an ADAF implies a black hole mass $M_{\mathrm{A}\mathrm{D}\mathrm{A}\mathrm{F}}=(1.2\pm 0.2)\times {10}^9{M}_{\odot }$, consistent with previous estimates of $(1-2)\times {10}^9{M}_{\odot }$ from stellar or hot-gas dynamics. Also, the Eddington-scaled accretion rate for NGC,3115, ${\dot{M}}_{\mathrm{A}\mathrm{D}\mathrm{A}\mathrm{F}}\mathrm{/}{\dot{M}}_{\mathrm{E}\mathrm{d}\mathrm{d}}=1.2_{-0.6}^{+1.0}\times {10}^{-8}$, is about 4-8 times lower than recent estimates for Sagittarius,A${}^{\star }$.