An eclectic view of our Milky Way galaxy

Abstract

The nature of our Milky Way galaxy is re-examined from an eclectic point of view. Evidence for a central bar, for example, is not reflected in the distribution of RR Lyrae variables in the central bulge (Majaess. Acta Astron. 60, 55 (2010); Dékány et al. arXiv:1309.5933. 2013), and it is not clear if either a 2-armed or 4-armed spiral pattern is appropriate for the spiral arms. Radial velocity mapping of the Galaxy using radio H I, H II, or CO observations is compromised by the assumptions adopted for simple Galactic rotation. The Sun’s local standard of rest (LSR) velocity is ∼14 km s−1 rather than 20 km s−1, the local circular velocity is 251 ± 9 km s−1 rather than 220 km s−1, and young groups of stars exhibit a 10–20 km s−1 “kick” relative to what is expected from Galactic rotation. By implication, the same may be true for star-forming gas clouds affected by the Galaxy’s spiral density wave, raising concerns about their use for mapping spiral arms. Proper motion data in conjunction with the newly-estimated velocity components for the Sun’s motion imply a distance to the Galactic centre of R0 = 8.34 ± 0.27 kpc, consistent with recent estimates, which average 8.24 ± 0.09 kpc. A cosinusoidal Galactic potential is not ruled out by observations of open star clusters. The planetary nebula cluster Bica 6, for example, has a near-escape orbit for a Newtonian potential, but a near-normal orbit in a cosinusoidal potential field. The nearby cluster Collinder 464 also displays unusually large tidal effects consistent with those expected for a cosinusoidal potential. A standard Newtonian version of the Virial theorem for star clusters yields very reasonable masses (∼3 × 1011 M⊙ and ∼4 × 1011 M⊙) for the Milky Way and M31 subgroups of the Local Group, respectively. A cosinusoidal relation should yield identical results.