Kinematics and excitation of the molecular hydrogen accretion disc in NGC 1275

We report the results of high spatial and spectral resolution integral-field spectroscopy of the central~3×3 arcsec2 of the active galaxy NGC 1275 (Perseus A), based on observations with the Near-infrared Integral Field Spectrograph (NIFS) and the ALTitude conjugate Adaptive optics for the InfraRed (ALTAIR) adaptive-optics system on the Gemini North telescope. The circum-nuclear disc in the inner R ~ 50 pc of NGC 1275 is seen in both the H2 and [Fe II] lines. The disc is interpreted as the outer part of a collisionally excited turbulent accretion disc. The kinematic major axis of the disc at a position angle of 68° is oriented perpendicular to the radio jet. A streamer-like feature to the south-west of the disc, detected in H2 but not in [Fe II], is discussed as one of possibly several molecular streamers, presumably falling into the nuclear region. Indications of an ionization structure within the disc are deduced from the He I and Brγ emission lines, which may partially originate from the inner portions of the accretion disc. The kinematics of these two lines agrees with the signature of the circum-nuclear disc, but both lines display a larger central velocity dispersion than the H2 line. The ro-vibrational H₂ transitions from the core of NGC 1275 are indicative of thermal excitation caused by shocks and agree with excitation temperatures of ~1360 and ~4290 K for the lower and higher energy H2 transitions, respectively. The data suggest X-ray heating as the dominant excitation mechanism of [Fe II] emission in the core, while fast shocks are a possible alternative. The [Fe II] lines indicate an electron density of ~4000 cm^-3. The H₂ disc is modelled using simulated NIFS data cubes of H2 emission from inclined discs in Keplerian rotation around a central mass. Assuming a disc inclination of 45° ± 10°, the best-fitting models imply a central mass of (8⁺⁷ _-2) × 108 M⊙ Taken as a black hole mass estimate, this value is larger than previous estimates for the black hole mass inNGC1275, but is in agreement with the M-σ relation within the rms scatter. However, the molecular gas mass in the core region is tentatively estimated to be non-negligible, which suggests that the central mass may rather represent an upper limit for the black hole mass. In comparison to other H2-luminous radio galaxies, we discuss the relative role of jet feedback and accretion in driving shocks and turbulence in the molecular gas component.