Tidal imprints of a dark sub-halo on the outskirts of the Milky Way. II. Perturber azimuth

Abstract

We extend our analysis of the observed disturbances on the outskirts of the HI disk of the Milky Way. We employ the additional constraints of the phase of the modes of the observed HI image and asymmetry in the radial velocity to derive the azimuth of the perturber inferred to be responsible for the disturbances in the HI disk. Specifically, we carry out a modal analysis of the phase of the disturbances in the HI image and in high-resolution smoothed particle hydrodynamics (SPH) simulations of a Milky Way like galaxy tidally interacting with dark perturbers, the relative offset of which we utilize to derive the perturber azimuth. Under the assumption that the asymmetry in the radial velocity is due to the perturber, we derive the best fit to the radial velocity at l = 0, 180, and use this constraint to also derive the perturber azimuth. To make a direct connection with observations, we express our results in Sun-centered coordinates, predicting that the perturber responsible for the observed disturbances is between -50 ≲ l ≲ -10. We explicitly show that the phase of the disturbances in the outskirts of simulated galaxies, our primary metric for the azimuth determination, is relatively insensitive to the equation of state (for the range of gas fractions of local spirals). Our calculations here represent our continuing efforts to develop the "tidal analysis" method of Chakrabarti & Blitz (CB09). CB09 employed SPH simulations to examine tidal interactions between perturbing dark sub-halos and the Milky Way. They found that the amplitudes of the Fourier modes of the observed planar disturbances are best fit by a perturbing dark sub-halo with mass ∼one-hundredth that of the Milky Way, and a pericentric approach distance of 5-10 kpc. The overarching goal of this work is to attempt to outline an alternate procedure to optical studies for characterizing and potentially discovering dwarf galaxies - whereby one can approximately infer the azimuthal location of a perturber, its mass, and pericentric distance (CB09) from analysis of its tidal gravitational imprints on the HI disk of the primary galaxy. © 2011. The American Astronomical Society. All rights reserved.