Central kiloparsec region of Andromeda

Abstract

The Andromeda galaxy (M31) is the nearest giant spiral galaxy to our own, which offers an opportunity to study dynamical phenomena occurring in nuclear disks and bulges at high resolution to explain star formation quenching and galaxy evolution through collisions and tides. Multi-wavelength data have revealed strong dynamical perturbations in the central kiloparsec (kpc) region of M31, with an off-centered tilted disk and ring, coinciding with a dearth of atomic and molecular gas. Our goal is to understand the origin of these perturbations and, thus, we propose a dynamical model that reproduces the global features of the observations. We report on the integral field spectroscopy of the ionized gas with Hα and [N II] obtained with the Spectromètre Imageur à Transformée de Fourier pour l’Étude en Long et en Large de raies d’Émission (SITELLE), which is the optical imaging Fourier transform spectrometer (IFTS) at the Canada France Hawaii telescope (CFHT). Using the fully sampled velocity field of ionized gas, together with the more patchy molecular gas velocity field previously obtained with the CO lines at IRAM-30 m telescope and the dust photometry, we identified three dynamical components in the gas: the main disk, a tilted ring, and a nuclear warped disk. We computed a mass model for the central kpc, essentially from the stellar nuclear disk and bulge, with minimal contributions from the main stellar and gaseous disk, along with a dark matter halo. The kinematics of the ionized and molecular gas was then computed in this potential, and the velocity field confronted qualitatively to observations. The best fit helped us determine the physical parameters of the three identified gas components: size, morphology, and geometrical orientation. These results are qualitatively compatible with a recent head-on collision with a M-32 like galaxy, as previously proposed. The kinematical observations correspond to a dynamical re-orientation of the perturbed nuclear disk, through a series of warps and tearing of the disk into the ring, following the collision.