The Black Hole Mass of NGC 4151. II. Stellar Dynamical Measurement from Near-infrared Integral Field Spectroscopy

We present a revised measurement of the mass of the central black hole (M _BH ) in the Seyfert 1 galaxy NGC 4151. The new stellar dynamical mass measurement is derived by applying an axisymmetric orbit-superposition code to near-infrared integral field data obtained using adaptive optics with the Gemini Near-infrared Integral Field Spectrograph (NIFS). When our models attempt to fit both the NIFS kinematics and additional low spatial resolution kinematics, our results depend sensitively on how χ² is computed—probably a consequence of complex bar kinematics that manifest immediately outside the nuclear region. The most robust results are obtained when only the high spatial resolution kinematic constraints in the nuclear region are included in the fit. Our best estimates for the black hole mass and H-band mass-to-light ratio are M _BH ~ 3.76 ± 1.15 × 10⁷ M _⊙ (1σ error) and Upsilon _H ~ 0.34 ± 0.03 M _⊙/L _⊙ (3σ error), respectively (the quoted errors reflect the model uncertainties). Our black hole mass measurement is consistent with estimates from both reverberation mapping (3.57^+0.45_-0.37× 10⁷ {{M}_⊙ }) and gas kinematics (3.0^+0.75_-2.2× 10⁷ {{M}_⊙ }; 1σ errors), and our best-fit mass-to-light ratio is consistent with the photometric estimate of Upsilon _H = 0.4 ± 0.2 M _⊙/L _⊙. The NIFS kinematics give a central bulge velocity dispersion σ _c = 116 ± 3 km s^-1, bringing this object slightly closer to the M _BH-σ relation for quiescent galaxies. Although NGC 4151 is one of only a few Seyfert 1 galaxies in which it is possible to obtain a direct dynamical black hole mass measurement—and thus, an independent calibration of the reverberation mapping mass scale—the complex bar kinematics makes it less than ideally suited for this purpose.