A re-interpretation of the Triangulum-Andromeda stellar clouds: a
  population of halo stars kicked out of the Galactic disk
  The Triangulum-Andromeda stellar clouds (TriAnd1 and TriAnd2) are a pair of
concentric ring- or shell-like over-densities at large $R$ ($\approx$ 30 kpc)
and $Z$ ($\approx$ -10 kpc) in the Galactic halo that are thought to have been
formed from the accretion and disruption of a satellite galaxy. This paper
critically re-examines this formation scenario by comparing the number ratio of
RR Lyrae to M giant stars associated with the TriAnd clouds with other
structures in the Galaxy. The current data suggest a stellar population for
these over-densities ($f_{\rm RR:MG} < 0.38$ at 95% confidence) quite unlike
any of the known satellites of the Milky Way ($f_{\rm RR:MG} \approx 0.5$ for
the very largest and $f_{\rm RR:MG} >>1$ for the smaller satellites) and more
like the population of stars born in the much deeper potential well inhabited
by the Galactic disk ($f_{\rm RR:MG} < 0.01$). N-body simulations of a
Milky-Way-like galaxy perturbed by the impact of a dwarf galaxy demonstrate
that, in the right circumstances, concentric rings propagating outwards from
that Galactic disk can plausibly produce similar over-densities. These results
provide dramatic support for the recent proposal by Xu et al. (2015) that,
rather than stars accreted from other galaxies, the TriAnd clouds could
represent stars kicked-out from our own disk. If so, these would be the first
populations of disk stars to be found in the Galactic halo and a clear
signature of the importance of this second formation mechanism for stellar
halos more generally. Moreover, their existence at the very extremities of the
disk places strong constraints on the nature of the interaction that formed
them.
