What a Tangled Web We Weave: Hermus as the Northern Extension of the
  Phoenix Stream
  We investigate whether the recently discovered Phoenix stream may be part of
a much longer structure that includes the previously discovered Hermus stream.
Using a simple model of the Galaxy with a disk, bulge, and a spherical dark
matter halo, we show that a nearly circular orbit, highly inclined with respect
to the disk, can be found that fits the positions, orientations, and distances
of both streams. While the two streams are somewhat misaligned in the sense
that they do not occupy the same plane, nodal precession due to the Milky Way
disk potential naturally brings the orbit into line with each stream in the
course of half an orbit. We consequently consider a common origin for the two
streams as plausible. Based on our best fitting orbit, we make predictions for
the positions, distances, radial velocities, and proper motions along each
stream. If our hypothesis is borne out by measurements, then at ~183 degrees
(~235 degrees with respect to the Galactic center) and ~76 kpc in length,
Phoenix-Hermus would become the longest cold stream yet found. This would make
it a particularly valuable new probe of the shape and mass of the Galactic halo
out to ~20 kpc.
