Azimuthal metallicity variations, spiral structure, and the failure of radial actions based on assuming axisymmetry
arxiv(2024)
摘要
We study azimuthal variations in the mean stellar metallicity, <[Fe/H]>, in a
self-consistent, isolated simulation in which all stars form out of gas. We
find <[Fe/H]> variations comparable to those observed in the Milky Way and
which are coincident with the spiral density waves. The azimuthal variations
are present in young and old stars and therefore are not a result of recently
formed stars. Similar variations are present in the mean age and
alpha-abundance. We measure the pattern speeds of the <[Fe/H]>-variations and
find that they match those of the spirals, indicating that they are at the
origin of the metallicity patterns. Because younger stellar populations are not
just more [Fe/H]-rich and alpha-poor but also dynamically cooler, we expect
them to more strongly support spirals, which is indeed the case in the
simulation. However, if we measure the radial action, J_R, using the Stackel
axisymmetric approximation, we find that the spiral ridges are traced by
regions of high J_R, contrary to expectations. Assuming that the passage of
stars through the spirals leads to unphysical variations in the measured J_R,
we obtain an improved estimate of J_R by averaging over a 1 Gyr time interval.
This time-averaged J_R is a much better tracer of the spiral structure, with
minima at the spiral ridges. We conclude that the errors incurred by the
axisymmetric approximation introduce correlated deviations large enough to
render the instantaneous radial action inadequate for tracing spirals.
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