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A Slowly Precessing Disk in the Nucleus of M31 as the Feeding Mechanism for a Central Starburst


We present a kinematic study of the nuclear stellar disk in M31 at infrared wavelengths using high spatial resolution integral field spectroscopy. The spatial resolution achieved, FWHM = 0.″12 (0.45 pc at the distance of M31), has only previously been equaled in spectroscopic studies by space-based long-slit observations. Using adaptive-optics-corrected integral field spectroscopy from the OSIRIS instrument at the W. M. Keck Observatory, we map the line-of-sight kinematics over the entire old stellar eccentric disk orbiting the supermassive black hole (SMBH) at a distance of r < 4 pc. The peak velocity dispersion is 381 ±55 km s-1, offset by 0.″13 ±0.″03 from the SMBH, consistent with previous high-resolution long-slit observations. There is a lack of near-infrared (NIR) emission at the position of the SMBH and young nuclear cluster, suggesting a spatial separation between the young and old stellar populations within the nucleus. We compare the observed kinematics with dynamical models from Peiris & Tremaine. The best-fit disk orientation to the NIR flux is [θ l, θ i, θ a] = [-33° ±4°, 44° ±2°, -15° ±15°], which is tilted with respect to both the larger-scale galactic disk and the best-fit orientation derived from optical observations. The precession rate of the old disk is ΩP = 0.0 ±3.9 km s-1 pc-1, lower than the majority of previous observations. This slow precession rate suggests that stellar winds from the disk will collide and shock, driving rapid gas inflows and fueling an episodic central starburst as suggested in Chang et al.

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