The new Milky Way satellites: alignment with the VPOS and predictions for proper motions and velocity dispersions

Abstract

The evidence that stellar systems surrounding the Milky Way (MW) are distributed in a Vast Polar Structure (VPOS) may be observationally biased by satellites detected in surveys of the northern sky. The recent discoveries of more than a dozen new systems in the southern hemisphere thus constitute a critical test of the VPOS phenomenon. We report that the new objects are located close to the original VPOS, with half of the sample having offsets less than 20 kpc. The positions of the new satellite galaxy candidates are so well aligned that the orientation of the revised best-fitting VPOS structure is preserved to within 9 degrees and the VPOS flattening is almost unchanged (31 kpc height). Interestingly, the shortest distance of the VPOS plane from the MW center is now only 2.5 kpc, indicating that the new discoveries balance out the VPOS at the Galactic center. The vast majority of the MW satellites are thus consistent with sharing a similar orbital plane as the Magellanic Clouds, confirming a hypothesis proposed by Kunkel & Demers and Lynden-Bell almost 40 years ago. We predict the absolute proper motions of the new objects assuming they orbit within the VPOS. Independent of the VPOS results we also predict the velocity dispersions of the new systems under three distinct assumptions: that they (i) are dark-matter-free star clusters obeying Newtonian dynamics, (ii) are dwarf satellites lying on empirical scaling relations of galaxies in dark matter halos, and (iii) obey MOND.

Portrait of a Dark Horse: A Photometric and Spectroscopic Study of the Ultra-Faint Milky Way Satellite Pegasus Iii*

“…candidate member stars and identified seven, possibly eight, member stars in the system based on their radial velocities, where the member stars could be either red giants or AGB stars (red large dots in Figure 1) Table 5 in Pawlowski et al 2015), which supports the picture that Peg III is a satellite dwarf galaxy rather than a star cluster.…”

Stellar Streams Discovered in the Dark Energy Survey

“…We do not find any strong association of stream poles coinciding with the proposed vast polar structure (VPOS; Pawlowski et al 2012Pawlowski et al , 2015. Specifically, in the left panel of Figure 17 we plot the VPOS +new pole (Table 1 of Pawlowski et al 2015) transformed into galactocentric coordinates assuming a distance of 100 kpc. However, the limited sky coverage of the DES footprint will bias the observable distribution of stream poles.…”

New low surface brightness dwarf galaxies in the Centaurus group

“…However, a major challenge for the standard picture of structure formation with DM is now posed by the highly asymmetric features found in the distributions of dwarf galaxies in the Local Group (Kroupa et al 2005). There is the vast polar structure (VPOS; Pawlowski et al 2015Pawlowski et al , 2012, which is a thin (rms height ≈ 30 kpc) highly inclined, corotating substructure of faint satellite galaxies, young globular clusters, and streams, spreading in Galactocentric distance between 10 and 250 kpc. Following an earlier suggestion by Koch & Grebel (2006), a similar feature was found in the Andromeda galaxy surroundings (Metz et al 2007;Ibata et al 2013), called the Great Plane of Andromeda (GPoA).…”