An Eclectic View of our Milky Way Galaxy

Abstract

The nature of our Milky Way Galaxy is reexamined 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 [4,5], 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 $\sim 14$ km s$^{-1}$ rather than 20 km s$^{-1}$, the local circular velocity is $251 \pm 9$ 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 $R_0=8.34\pm0.27$ kpc, consistent with recent estimates which average $8.24\pm0.09$ 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 ($\sim 3 \times 10^{11}M_{\odot}$ and $\sim 4 \times 10^{11}M_{\odot}$) for the Milky Way and M31 subgroups of the Local Group, respectively. A cosinusoidal relation should yield identical results.