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

Abstract 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 &lt; 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mi>α</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>10</mml:mn> <mml:mspace width="0.25em"/> <mml:mi mathvariant="normal">GHz</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>22</mml:mn> <mml:mspace width="0.25em"/> <mml:mi mathvariant="normal">GHz</mml:mi> </mml:mrow> </mml:msubsup> <mml:mo>=</mml:mo> <mml:mo>−</mml:mo> <mml:mn>1.85</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.18</mml:mn> </mml:math> ( 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, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mover accent="true"> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>̇</mml:mo> </mml:mrow> </mml:mover> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">ADAF</mml:mi> </mml:mrow> </mml:msub> <mml:mo>/</mml:mo> <mml:msub> <mml:mrow> <mml:mover accent="true"> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>̇</mml:mo> </mml:mrow> </mml:mover> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">Edd</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>1</mml:mn> <mml:mo>.</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.6</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>1.0</mml:mn> </mml:mrow> </mml:msubsup> <mml:mo>×</mml:mo> <mml:mn>1</mml:mn> <mml:msup> <mml:mrow> <mml:mn>0</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>8</mml:mn> </mml:mrow> </mml:msup> </mml:math> , is about 4–8 times lower than recent estimates for Sagittarius A ⋆ .