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

Abstract

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 $S_{22\mathrm{GHz}}=48.2\pm 6.4\mu$Jy, to a FWHM diameter $d_{22\mathrm{GHz}}<$ 59\,mas (2.8\,pc). We find no evidence for adjacent emission within a stagnation region of radius $R_{\mathrm{sta}}\sim$ 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 $L_{22\mathrm{GHz}}=5.4\times 10^{17}\mathrm{W}\mathrm{H}{\mathrm{z}}^{-1}$ is about 60 times that of Sagittarius\,A${}^{\star }$. The nucleus' spectral index ${\alpha }_{10\mathrm{GHz}}^{22\mathrm{GHz}}=-1.85\pm 0.18$ ($S_{\nu }\propto {\nu }^{\alpha }$) indicates optically-thin synchrotron emission. The spectral energy distribution of the nucleus peaks near ${\nu }_{\mathrm{peak}}=$ 9\,GHz. Modeling this radio peak as an ADAF implies a black hole mass $M_{\mathrm{ADAF}}=(1.2\pm 0.2)\times 10^9{M}_{\odot }$, 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, ${\dot{M}}_{\mathrm{ADAF}}/{\dot{M}}_{\mathrm{Edd}}=1.2_{-0.6}^{+1.0}\times 10^{-8}$, is about 4-8 times lower than recent estimates for Sagittarius\,A${}^{\star }$.

11 pages, 6 figures, accepted by ApJ