Annual Report 2011 Max Planck Institute for Astronomy

[a / Fe]
0.4
0.3
0.2
0.1
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III 25% II 22%
3%
I 50%
ter above the diagonal solid one-to-one line is indicative
of a significant dark matter fraction in the inner parts of
the explored sample of galaxies. Combining the stars and
gas to obtain (M/L) BAR can bring the spiral galaxies up to
20 % closer to the diagonal line, and adopting a Salpeter
IMF as more appropriate for elliptical galaxies increases
their (M/L) POP as indicated by the dashed line. Finally,
we are improving the dynamical model fits by including
a dark matter halo, but several galaxies will remain
with (M/L) DYN (M/L) BAR . Hence, significant amounts
of dark matter seem to be present in the inner parts of
galaxies, unless their distribution of stars is dominated
by either dwarf stars (bottom-heavy IMF) and/or dark
remnants (top-heavy IMF).
Dynamics in the Solar Neighborhood
In contrast to the study of nearby galaxies, the stars in
the Milky Way are resolved; thus we can estimate their
distances in addition to their sky positions, and we can
measure their proper motions in the plane of the sky
in addition to their line-of-sight velocities – often confusingly
called radial velocities. This means we have access
to all six phase space coordinates for at least a subset
of the stars. For example, we use G-type dwarf stars
from the Sloan Extension for Galactic Understanding
and Exploration (seGue DR7) survey, leading to a sample
of 13000 stellar tracers with galactocentric radius
7 R 9 kpc and height above the Galactic plane
0.5 |z| 2.5 kpc. Taking into account the selection of
spectroscopically-targeted stars from the color-selected
photometric sample, the left panel of Fig. III.3.3 shows
the number density of G dwarfs as function of their [α/
Fe] abundance – a proxy for age – and [Fe/H] metallicity.
III 13% II 17%
4% I 66%
Total (100%) Circular (65%) Eccentric (23%)
–1
III 53% II 31%
2% I 13%
–0.6 –0.2 –1 –0.6 –0.2 –1 –0.6 –0.2 1.2
[Fe/H] [Fe/H]
[Fe/H]
Fig. III.3.3: The number density of SDSS/seGue G-dwarf stars
in the Solar Neighborhood as function of their [α/Fe] abundance
and [Fe/H] metallicity. Left: The number density of
all G dwarfs shows a clear bi-modality that naturally inspires
a separation into α-young stars and α-old stars. Since all six
phase-space coordinates are measured, the orbits of all stars
can be computed in a Milky Way gravitational potential model.
III.3 Dynamics of Galaxies: inferring their mass distribution and formation history 59
600
500
400
300
200
100
Middle: Nearly all of the α-young stars are on near-circular
orbits as expected for thin-disk stars, but also a significant
fraction of the α-old stars, consistent with outward radial migration.
Right: The remaining α-old stars on eccentric orbits,
including nearly all old metal-poor stars, are difficult to explain
with radial migration alone, but might have formed through
early-on gas-rich mergers.
Given the stars’ current position and space motion,
we numerically integrate their orbits in a gravitational
potential Φ(R, z) of the Milky Way that includes a disk,
bulge and dark matter halo. As a measure of the orbital
eccentricity, we compute L z /L c the ratio of the conserved
z-component of the orbital angular momentum
to the maximum angular momentum when the orbit is
circular. Even if at their birth radii the stars move on
circular orbits with L z /L c 1, the current distribution
in L z /L c at the Solar Neighborhood is expected to extend
toward values below unity, both because of stars
scattering to more eccentric orbits, as well as due to
measurement errors, mainly in their distance and proper
motions. We choose L z /L c 0.85 for stars being on
(near-)circular orbits, and L z /L c 0.80 for stars being
on eccentric orbits, although the results are robust
against the precise limits adopted.
The middle panel of Fig. III.3.3 shows that nearly
all of the α-young stars are on circular orbits as expected
for thin-disk stars, but a significant fraction of
the α-old stars also follow circular orbits. The latter is
consistent with radial migration in which stars, due to
interaction at the co-rotation radius of non-axisymmetric
(transient) structures such as spiral arms and bars, are
efficiently moved in radius away from their birth radii
while remaining on near-circular orbits. Indeed, other
properties of the stars on circular orbits, such as the
vertical scale height, the average rotational velocity
and vertical velocity dispersion, show a smooth change
with [α/Fe] as expected from in-situ formation of the
thick disk through radial migration.
On the other hand, the α-old stars on eccentric orbits,
including nearly all old metal-poor stars, are difficult
to explain with radial migration alone. Their average
properties show no significant trends with nei-
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Star Count
Credit: Glenn van de Ven