Multiple components of the Jhelum stellar stream
  In simple models of the Milky Way, tidally disrupting satellites produce long
and thin---nearly one-dimensional---stellar streams. Using astrometric data
from the Gaia second data release and photometry from the Dark Energy Survey,
we demonstrate that the Jhelum stream, a stellar stream in the inner halo, is a
two-dimensional structure. The spatial distribution of highly probable Jhelum
members reveals a dense thin component and an associated diffuse, spatially
offset component. These two spatial components have indistinguishable proper
motions (at $\sigma\sim1\,\rm mas\,yr^{-1}$ level) and a similar ratio of blue
straggler to blue horizontal branch stars, which indicates a common origin for
the two components. The best-fit orbit to the narrow component (pericenter
$8\,\rm kpc$, apocenter $24\,\rm kpc$), however, does not explain the wide
component of the Jhelum stream. On the other hand, an older orbital wrap of
Jhelum's orbit traces the Indus stream, indicating a possible connection
between these two structures and additional complexity in Jhelum's formation.
Substructure in the Jhelum progenitor or precession of its tidal debris in the
Milky Way potential may explain the observed structure of Jhelum. Future
spectroscopic data will enable discrimination between these "nature" and
"nurture" formation scenarios. Jhelum adds to the growing list of cold stellar
streams that display complex morphologies and promise to reveal the dynamical
history of the Milky Way.
