ar X iv : a st ro - p h / 03 09 62 7 v 1 2 3 Se p 20 03 Why a ring of stars at r = 20 kpc ?

The recently discovered ring of stars near Galactocentric distance r = 20 kpc is interpreted as baryonic matter accreted onto the second caustic ring of dark matter in our galaxy. Caustic rings of dark matter were predicted in the Galactic plane at radii an ≃ 40 kpc/n where n = 1, 2, 3... They can reveal themselves by their gravitational influence on the distribution of baryonic matter. There is additional evidence for caustic rings of dark matter in the Milky Way from a series of sharp rises in the Galactic rotation curve. The positions of the rises are consistent at the 3% level with the above law for the caustic ring radii. Also, a triangular feature in the IRAS map of the galactic plane is consistent with the imprint of the caustic ring of dark matter nearest to us (n = 5) upon dust and gas in the Galactic plane. These observations imply that the dark matter in our neighborhood is dominated by a single flow whose density and velocity vector are estimated. 1 A Giant Stellar Ring in the Milky Way Plane Recently the discovery was announced of what appears to be a ring of stars, with radius of order 20 kpc, circling the Galaxy [1,2,3]. I will not discuss the evidence for the alleged ring of stars, but merely assume that it exists. The existence of such a ring is puzzling. In ref. [1], the ring is interpreted as the tidal stream from an accreted satellite galaxy. However, in this interpretation it is hard to account for the fact that the star population is confined to a nearly circular region [2]. Also, it would be an accident that the tidal stream lies in the Galactic plane. Ibata et al. [2] propose instead that the ring of stars is a perturbation of the Galactic disk population. They do not however explain what causes the perturbation. I would like to propose that the perturbation that causes the ring of stars at r = 20 kpc is the gravitational field of a caustic ring of dark matter. Several years ago caustic rings of dark matter were predicted in the Galactic plane at radii an ≃ 40 kpc/n where n = 1, 2, 3, .. [4]. The ring of stars at r = 20 kpc may be baryonic matter accreted onto the n = 2 caustic ring of dark matter. This interpretation would account for both its radius and its location in the Galactic plane. The existence of dark matter caustics in the halos of galaxies is a corollary of the observation [5] that cold dark matter particles lie on a three-dimensional sheet in six-dimensional phase-space. There are compelling reasons to believe that the dark matter of the universe is constituted in large part of non-baryonic collisionless particles with very small primordial velocity dispersion, such as axions and/or weakly interacting massive particles (WIMPs). Generically, such