The Edge of the Galaxy. (arXiv:2002.09497v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Deason_A/0/1/0/all/0/1">Alis J. Deason</a> (Durham), <a href="http://arxiv.org/find/astro-ph/1/au:+Fattahi_A/0/1/0/all/0/1">Azadeh Fattahi</a> (Durham), <a href="http://arxiv.org/find/astro-ph/1/au:+Frenk_C/0/1/0/all/0/1">Carlos S. Frenk</a> (Durham), <a href="http://arxiv.org/find/astro-ph/1/au:+Grand_R/0/1/0/all/0/1">Robert J. J. Grand</a> (MPA), <a href="http://arxiv.org/find/astro-ph/1/au:+Oman_K/0/1/0/all/0/1">Kyle A. Oman</a> (Durham), <a href="http://arxiv.org/find/astro-ph/1/au:+Garrison_Kimmel_S/0/1/0/all/0/1">Shea Garrison-Kimmel</a> (Factual), <a href="http://arxiv.org/find/astro-ph/1/au:+Simpson_C/0/1/0/all/0/1">Christine M. Simpson</a> (Chicago), <a href="http://arxiv.org/find/astro-ph/1/au:+Navarro_J/0/1/0/all/0/1">Julio F. Navarro</a> (UVic)
We use cosmological simulations of isolated Milky Way-mass galaxies, as well
as Local Group analogues, to define the “edge” — a caustic manifested in a
drop in density or radial velocity — of Galactic-sized haloes, both in dark
matter and in stars. In the dark matter, we typically identify two caustics:
the outermost caustic located at ~1.4r_200m corresponding to the “splashback”
radius, and a second caustic located at ~0.6r_200m which likely corresponds to
the edge of the virialized material which has completed at least two
pericentric passages. The splashback radius is ill defined in Local Group type
environments where the halos of the two galaxies overlap. However, the second
caustic is less affected by the presence of a companion, and seems a more
natural definition for the boundary of the Milky Way halo. Curiously, the
stellar distribution also has a clearly defined caustic, which, in most cases,
coincides with the second caustic of the dark matter. This can be identified in
both radial density and radial velocity profiles, and should be measurable in
future observational programmes. Finally, we show that the second caustic can
also be identified in the phase-space distribution of dwarf galaxies in the
Local Group. Using the current dwarf galaxy population, we predict the edge of
the Milky Way halo to be 292 +/- 61 kpc.
We use cosmological simulations of isolated Milky Way-mass galaxies, as well
as Local Group analogues, to define the “edge” — a caustic manifested in a
drop in density or radial velocity — of Galactic-sized haloes, both in dark
matter and in stars. In the dark matter, we typically identify two caustics:
the outermost caustic located at ~1.4r_200m corresponding to the “splashback”
radius, and a second caustic located at ~0.6r_200m which likely corresponds to
the edge of the virialized material which has completed at least two
pericentric passages. The splashback radius is ill defined in Local Group type
environments where the halos of the two galaxies overlap. However, the second
caustic is less affected by the presence of a companion, and seems a more
natural definition for the boundary of the Milky Way halo. Curiously, the
stellar distribution also has a clearly defined caustic, which, in most cases,
coincides with the second caustic of the dark matter. This can be identified in
both radial density and radial velocity profiles, and should be measurable in
future observational programmes. Finally, we show that the second caustic can
also be identified in the phase-space distribution of dwarf galaxies in the
Local Group. Using the current dwarf galaxy population, we predict the edge of
the Milky Way halo to be 292 +/- 61 kpc.
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