On the nature of the ring-like structure in the outer Galactic disk

Authors:
Amina Helmi,
Julio F. Navarro,
Andres Meza,
Matthias Steinmetz,
Vincent R. Eke

Comments: 4 pages, 2 figures, submitted to ApJ Letters. Higher resolution
Figure 2 available from this http URL

We examine the tidal disruption of satellite galaxies in a cosmological
simulation of the formation of a disk galaxy in the $Lambda$CDM scenario. We
find that the disruption of satellite galaxies in orbits roughly coplanar with
the disk leads naturally to the formation of ring-like stellar structures
similar to that recently discovered in the outer disk of the Milky Way. Two
interpretations appear plausible in this context. One is that the ring is a
transitory, localized radial density enhancement reflecting the apocenter of
particles stripped from a satellite during a recent pericentric passage (a
“tidal arc” reminiscent of the tidal arms seen in disk galaxy mergers). In
the second scenario, the ring is analogous to the “shells” found around some
elliptical galaxies, and would result from a minor merger that took place
several Gyr ago. The two interpretations differ in several ways. Tidal arcs (i)
are expected to span a limited longitude range; (ii) may carry a substantial
fraction of the original mass of the satellite; (iii) should exhibit a
significant velocity gradient with Galactic longitude; and (iv) are in all
likelihood asymmetric above and below the plane of the disk. Shells, on the
other hand, may comprise at most a small fraction of the original mass of the
satellite and, due to their more relaxed state, ought to be symmetric above and
below the plane, with no discernible velocity gradients across the structure.
If confirmed as a tidal feature, the ring discovered by SDSS in the outer
Galactic disk would strengthen the view–supported by numerical
simulations–that minor mergers and accretion events have played a significant
role building up not only the stellar halo, but also the disk components of the
Galaxy.

References and citations for this submission:

SLAC-SPIRES HEP (refers to ,
cited by, arXiv reformatted)