Snakes on a Plane: do Milky Way stellar streams and globular clusters align in a Vast Polar Structure?. (arXiv:2001.11564v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Riley_A/0/1/0/all/0/1">Alexander H. Riley</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Strigari_L/0/1/0/all/0/1">Louis E. Strigari</a>
There is increasing evidence that a substantial fraction of Milky Way
satellite galaxies align in a rotationally-supported plane of satellites, a
rare configuration in cosmological simulations of galaxy formation. It has been
suggested that other Milky Way substructures (namely young halo globular
clusters and stellar/gaseous streams) similarly tend to align with this plane,
accordingly dubbed the Vast Polar Structure (VPOS). Using systemic proper
motions inferred from Gaia data, we find that globular cluster orbital poles
are not clustered in the VPOS direction, though the population with the highest
VPOS membership fraction is the young halo clusters (~30%). We additionally
provide a current census of stellar streams, including new streams discovered
using the Dark Energy Survey and Gaia datasets, and find that stellar stream
normals are also not clustered in the direction of the VPOS normal. We also
find that, based on orbit modeling, there is a likely association between NGC
3201 and the Gj”{o}ll stellar stream and that, based on its orbital pole, NGC
4147 is likely not a Sagittarius globular cluster. That the Milky Way’s
accreted globular clusters and streams do not align in the same planar
configuration as its satellites suggests that the plane of satellites is either
a particularly stable orbital configuration or a population of recently
accreted satellites. Neither of these explanations is particularly likely in
light of other recent studies, leaving the plane of satellites problem as one
of the more consequential open problems in galaxy formation and cosmology.
There is increasing evidence that a substantial fraction of Milky Way
satellite galaxies align in a rotationally-supported plane of satellites, a
rare configuration in cosmological simulations of galaxy formation. It has been
suggested that other Milky Way substructures (namely young halo globular
clusters and stellar/gaseous streams) similarly tend to align with this plane,
accordingly dubbed the Vast Polar Structure (VPOS). Using systemic proper
motions inferred from Gaia data, we find that globular cluster orbital poles
are not clustered in the VPOS direction, though the population with the highest
VPOS membership fraction is the young halo clusters (~30%). We additionally
provide a current census of stellar streams, including new streams discovered
using the Dark Energy Survey and Gaia datasets, and find that stellar stream
normals are also not clustered in the direction of the VPOS normal. We also
find that, based on orbit modeling, there is a likely association between NGC
3201 and the Gj”{o}ll stellar stream and that, based on its orbital pole, NGC
4147 is likely not a Sagittarius globular cluster. That the Milky Way’s
accreted globular clusters and streams do not align in the same planar
configuration as its satellites suggests that the plane of satellites is either
a particularly stable orbital configuration or a population of recently
accreted satellites. Neither of these explanations is particularly likely in
light of other recent studies, leaving the plane of satellites problem as one
of the more consequential open problems in galaxy formation and cosmology.
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