An updated detailed characterization of planes of satellites in the MW and M31

Abstract

ABSTRACT We present a detailed characterization of planes of satellite galaxies in the Milky Way (MW) and M31. For a positional analysis, we introduce an extension to the ‘4-galaxy-normal density plot’ method from Pawlowski, Kroupa & Jerjen. It finds the normal directions to the predominant planar configurations of satellites of a system, yielding for each a collection of planes of increasing member satellites. This allows to quantify the quality of planes in terms of population (Nsat) and spatial flattening (c/a). We apply this method to the latest data for confirmed MW and M31 satellite samples, with 46 and 34 satellites, respectively. New MW satellites form part of planes previously identified from a smaller sample of Nsat= 27: we identify a new plane with Nsat = 39 as thin as the VPOS-3 (c/a ∼ 0.2), and with roughly the same normal direction; so far the most populated plane that thin reported in the Local Group. We introduce a new method to determine, using kinematic data, the axis of maximum co-orbitation of MW satellites. Interestingly, this axis approximately coincides with the normal to the former plane: $\ge 45\pm 5{{\ \rm per\ cent}}$ of satellites co-orbit. In M31 we discover a plane with Nsat = 18 and c/a ∼ 0.15, i.e. quality comparable to the GPoA, and perpendicular to it. This structure is viewed face-on from the Sun making it susceptible to M31 satellite distance uncertainties. An estimation of the perpendicular velocity dispersion suggests that it is dynamically unstable. Finally we find that mass is not a property determining a satellite’s membership to good quality planes.