Halo Spin from Primordial Inner Motions. (arXiv:1904.03201v1 [astro-ph.CO])
<a href="http://arxiv.org/find/astro-ph/1/au:+Neyrinck_M/0/1/0/all/0/1">Mark C. Neyrinck</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Aragon_Calvo_M/0/1/0/all/0/1">Miguel A. Aragon-Calvo</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Falck_B/0/1/0/all/0/1">Bridget Falck</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Wang_J/0/1/0/all/0/1">Jie Wang</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Szalay_A/0/1/0/all/0/1">Alexander S. Szalay</a>
We reexamine how angular momentum arises in dark-matter haloes. The standard
tidal-torque theory (TTT), in which an ellipsoidal protohalo is torqued up by
the tidal field, is an approximation to a mechanism which is more accurate, and
that we find to be more pedagogically appealing. In the initial conditions,
within a collapsing protohalo, there is a random gravity-sourced velocity
field; the spin from it largely cancels out, but with some excess. Velocities
grow linearly, giving a sort of conservation of angular momentum (in a
particular comoving linear-theory way) until collapse. Then, angular momentum
is conserved in physical coordinates. This picture is more accurate in detail
than the TTT, which is not literally valid, although it is useful for many
predictions. Protohaloes do not uniformly torque up; instead, their inner
velocity fields retain substantial dispersion. We also discuss how this picture
is applicable to rotating filaments, and the relation between halo mass and
spin. We also explain that an aspherical protohalo in an irrotational flow
generally has nonzero angular momentum, entirely from its aspherical outskirts.
We reexamine how angular momentum arises in dark-matter haloes. The standard
tidal-torque theory (TTT), in which an ellipsoidal protohalo is torqued up by
the tidal field, is an approximation to a mechanism which is more accurate, and
that we find to be more pedagogically appealing. In the initial conditions,
within a collapsing protohalo, there is a random gravity-sourced velocity
field; the spin from it largely cancels out, but with some excess. Velocities
grow linearly, giving a sort of conservation of angular momentum (in a
particular comoving linear-theory way) until collapse. Then, angular momentum
is conserved in physical coordinates. This picture is more accurate in detail
than the TTT, which is not literally valid, although it is useful for many
predictions. Protohaloes do not uniformly torque up; instead, their inner
velocity fields retain substantial dispersion. We also discuss how this picture
is applicable to rotating filaments, and the relation between halo mass and
spin. We also explain that an aspherical protohalo in an irrotational flow
generally has nonzero angular momentum, entirely from its aspherical outskirts.
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