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

TL;DR

This study uses high-resolution radio observations to analyze the accretion flow around the nearest billion-solar-mass black hole in NGC 3115, confirming the ADAF model and estimating the black hole's mass and accretion rate.

Contribution

It provides the first high-resolution radio imaging of NGC 3115's black hole, constrains the stagnation radius, and models the accretion flow as an ADAF, aligning with previous mass estimates.

Findings

Detected a faint radio nucleus with a flux density of 48.2 μJy.

No evidence of emission within the stagnation radius of 17 pc.

Estimated black hole mass of approximately 1.2 billion solar masses.

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 $S_{\rm 22\,GHz} = 48.2\pm6.4\,\mu$Jy, to a FWHM diameter $d_{\rm 22\,GHz} <$ 59\,mas (2.8\,pc). We find no evidence for adjacent emission within a stagnation region of radius $R_{\rm sta} \sim$ 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 $L_{\rm 22\,GHz} = 5.4 \times 10^{17}\,\rm W\,Hz^{-1}$ is about 60 times that of Sagittarius\,A$^\star$. The nucleus' spectral index $\alpha_{\rm 10\,GHz}^{\rm 22\,GHz} = -1.85\pm0.18$ ($S_\nu \propto \nu^\alpha$) indicates optically-thin synchrotron emission. The spectral energy distribution of the nucleus peaks near $\nu_{\rm peak} =$ 9\,GHz. Modeling this radio peak as an ADAF implies a black hole mass $M_{\rm ADAF} = (1.2\pm0.2) \times 10^9\,M_\odot$, 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, $\dot{M}_{\rm ADAF}/\dot{M}_{\rm Edd} = 1.2^{+1.0}_{-0.6} \times 10^{-8}$, is about 4-8 times lower than recent estimates for Sagittarius\,A$^\star$.