The M 31 double nucleus probed with OASIS and HST - A natural m=1 mode?

We present observations with the adaptive optics assisted integral field sp ectrograph OASIS of the M 31 double nucleus in the spectral domain around t he Calcium triplet at a spatial resolution better than 0:"5 FW H M. These d ata are used to derive the two-dimensional stellar kinematics within the ce ntral 2 ". ArchivalWFPC2/HST images in the F300W, F555W and F814W bands are revisited to perform a photometric decomposition of the nuclear region. We also present STIS/HST kinematics obtained from the archive. The luminosity distribution of the central region is well separated into the respective c ontributions of the bulge, the nucleus including P1 and P2, and the so-call ed UV peak. We then show, using the OASIS kinematical maps, that the axis j oining P1 and P2, the two local surface brightness maxima, does not coincid e with the kinematic major-axis, which is also the major-axis of the nuclea r isophotes (excluding P1). We also confirm that the velocity dispersion pe ak is offset by similar to0 "2 from the UV peak, assumed to mark the locati on of the supermassive black hole. The newly reduced STIS/HST velocity and dispersion profiles are then compared to OASIS and other published kinemati cs. We find significant offsets with previously published data. Simple para metric models are then built to successfully reconcile all the available ki nematics. We finally interpret the observations using new N-body simulation s. The nearly Keplerian nuclear disk of M 31 is subject to a natural m = 1 mode, with a very slow pattern speed (3 kms(-1)/pc for M-BH = 7 10(7) M-cir cle dot), that can be maintained during more than a thousand dynamical time s. The resulting morphology and kinematics of the mode can reproduce the M 31 nuclear-disk photometry and mean stellar velocity, including the observe d asymmetries. It requires a central mass concentration and a cold disk sys tem representing between 20 and 40% of its mass. Such a slow mode could be excited when interstellar clouds from the more external gaseous disk infall towards the centre. Nuclear disks formed from accreted gas are possible ca ndidates for the precursors of these types of structures, and may be common in central regions of galaxies.