The M 31 double nucleus probed with OASIS and HST

Abstract

Abstract. We present observations with the adaptive optics assisted integral field spectrograph OASIS of the M 31 double nucleus in the spectral domain around the Calcium triplet at a spatial resolution better than 0. 5 F W HM. These data are used to derive the two-dimensional stellar kinematics within the central 2 . Archival WFPC2/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 contributions of the bulge, the nucleus including P1 and P2, and the so-called UV peak. We then show, using the OASIS kinematical maps, that the axis joining P1 and P2, the two local surface brightness maxima, does not coincide with the kinematic major-axis, which is also the major-axis of the nuclear isophotes (excluding P1). We also confirm that the velocity dispersion peak is offset by ∼0. 2 from the UV peak, assumed to mark the location of the supermassive black hole. The newly reduced STIS/HST velocity and dispersion profiles are then compared to OASIS and other published kinematics. We find significant offsets with previously published data. Simple parametric models are then built to successfully reconcile all the available kinematics. We finally interpret the observations using new N-body simulations. The nearly Keplerian nuclear disk of M 31 is subject to a natural m = 1 mode, with a very slow pattern speed (3 km s −1 /pc for MBH = 7 10 7 M ), that can be maintained during more than a thousand dynamical times. The resulting morphology and kinematics of the mode can reproduce the M 31 nuclear-disk photometry and mean stellar velocity, including the observed asymmetries. It requires a central mass concentration and a cold disk system 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 candidates for the precursors of these types of structures, and may be common in central regions of galaxies.

m=1 mode instabilities in nuclear stellar disks around black holes

“…We find that m=1 mode instability is present in certain stellar disks harboring a SBH, which is not fixed, as long as we have Mass SBH /M ass disk 2.0. This mass range condition for the SBH is in accordance with recent numerical works (Bacon et al 2001, Jacobs & Sellwood 2001, Salow & Statler 2001. Our models also show that there is a strong correlation between the m=1 perturbation and the central mass displacement, suggesting a similar mechanism as the eccentric instability studied by other authors (Adams et al 1989, Taga & Iye 1998.…”

A lopsided chemically distinct nucleus in NGC 5055

“…Recently a growing amount of observational data has begun to confirm the reality of such structures, in particular in the centers of normal spiral galaxies. The most famous lopsided circumnuclear stellar disk belongs to M 31: its two brightness centers, one with high stellar velocity dispersion and the other dynamically cold, have been explained by Tremaine (1995) as an eccentric Keplerian disk around a supermassive black hole, and Bacon et al (2001) have argued that this disk must precess with an angular velocity of some 3 km s −1 pc −1 so suffering an m = 1 mode. In NGC 5055 there are too many asymmetries along the major axis: the brightness asymmetry, including [N II] and Hα emission lines, the rotation and stellar velocity dispersion asymmetries, and finally, the Mgb index distribution asymmetry found by us in this work.…”

Stellar dynamics around a massive black hole – I. Secular collisionless theory

“…The sketch of a model offered in Tremaine (1995) -for a lopsided Keplerian disc of stars around a MBH in the nucleus of M31 -has since been fleshed out in a number of numerical models (Statler 1999;Bacon et al 2001;Salow & Statler 2001;Sambhus & Sridhar 2002;Peiris & Tremaine 2003;Salow & Statler 2004;Kazandjian & Touma 2013;Brown & Magorrian 2013). This has brought to the forefront deep and interesting questions regarding the general nature of secular collisionless evolution.…”

“…Schwarzschild-type methods have been used to construct non-axisymmetric equilibria, both kinematic (Peiris & Tremaine 2003) and self-consistent (Poon & Merritt 2001;Sambhus & Sridhar 2002;Brown & Magorrian 2013)). Early numerical simulations (Bacon et al 2001;Jacobs & Sellwood 2001) showed collisionless relaxation of perturbed stellar discs. Touma, Tremaine & Kazandjian (2009) built a numerical algorithm which is particularly suited to the numerical exploration of secular clusters.…”