Zusammenfassung
Metal production in galaxies traces star formation, and is therefore both
very patchy and highly concentrated toward the centers of galactic discs. This
would seem to suggest that galaxies should have highly inhomogeneous metal
distributions with strong radial gradients, but observations of present-day
galaxies typically show only shallow gradients with little to no azimuthal
variation, implying the existence of a redistribution mechanism. Unfortunately,
this mechanism is still poorly understood. We study the possible role of
gravitational instability-driven turbulence as a mixing mechanism by simulating
an unstable, isolated galactic disc at high resolution, including metal fields
treated as passive scalars. Since any cylindrical field can be decomposed into
a sum of Fourier-Bessel basis functions, we set up initial metal fields
characterized by these functions and study how different modes decay and mix.
We find that both shear and turbulence contribute to mixing, but that the
mixing rate strongly depends on the symmetries. Non-axisymmetric modes have
decay times smaller than the galactic orbital period because shear winds them
up to small spatial scales, where they are quickly erased by turbulence. In
contrast, the decay timescales for axisymmetric modes are greater than the
orbital period of the galaxy, although to all but the largest-scale
inhomogeneities the decay time is still short enough for significant mixing to
occur over cosmological time. The different timescales provides a natural
explanation for why galaxies retain metallicity gradients while there is almost
no variation at a fixed radius. Moreover the long timescales required for
mixing axisymmetric modes may explain the much greater diversity of metallicity
gradients observed in high redshift galaxies compared to local ones. The
high-redshift systems have not yet reached equilibrium, while most of the local
ones have.
Beschreibung
[1411.7585] Mixing and transport of metals by gravitational instability-driven turbulence in galactic discs
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