From the sun to the Galactic Center

The centers of galaxies are their own ultimate gravitational sinks.
Massive black holes and star clusters as well as gas are especially
likely to fall into the centers of galaxies by dynamical friction
or dissipation. Many galactic centers harbor supermassive black
holes (SMBH) and dense nuclear (star) clusters which possibly
arrived there by these processes. Nuclear clusters can be formed in
situ from gas, or from smaller star clusters which fall to the
center. Since the Milky Way harbors both an SMBH and a nuclear
cluster, both can be studied best in the Galactic Center (GC),
which is the closest galactic nucleus to us. In Chapter 1, I
introduce the different components of the Milky Way, and put these
into the context of the GC. I then give an overview of relevant
properties (e.g. star content and distribution) of the GC.
Afterwards, I report the results of four different studies about
the GC. In Chapter 2, I analyze the limitations of astrometry, one
of the most useful methods for the study of the GC. Thanks to the
high density of stars and its relatively small distance from us it
is possible to measure the motions of thousands of stars in the GC
with images, separated by few years only. I find two main
limitations to this method: (1) for bright stars the not perfectly
correctable distortion of the camera limits the accuracy, and (2)
for the majority of the fainter stars, the main limitation is
crowding from the other stars in the GC. The position uncertainty
of faint stars is mainly caused by the seeing halos of bright
stars. In the very center faint unresolvable stars are also
important for the position uncertainty. In Chapter 3, I evaluate
the evidence for an intermediate mass black hole in the small
candidate cluster IRS13E within the GC. Intermediate mass black
holes (IMBHs) have a mass between the two types of confirmed black
hole: the stellar remnants and the supermassive black holes in the
centers of galaxies. One possibility for their formation is the
collision of stars in a dense young star cluster. Such a cluster
could sink to the GC by dynamical friction. There it would consist
of few bright stars like IRS13E. Firstly, I analyze the SEDs of the
objects in IRS13E. The SEDs of most objects can be explained by
pure dust emission. Thus, most objects in IRS13E are pure dust
clumps and only three young stars. This reduces the significance of
the 'cluster' IRS13E compared to the stellar background. Secondly,
I obtain acceleration limits for these three stars. The
non-detection of accelerations makes an IMBH an unlikely scenario
in IRS13E. However, since its three stars form a comoving
association, which is unlikely to form by chance, the nature of
IRS13E is not yet settled. In the third study (Chapter 4) I measure
and analyze the extinction curve toward the GC. The extinction is a
contaminant for GC observations and therefore it is necessary to
know the extinction toward the GC to determine the luminosity
properties of its stars. I obtain the extinction curve by measuring
the flux of the HII region in the GC in several infrared HII lines
and in the unextincted radio continuum. I compare these ratios with
the ratios expected from recombination physics and obtain
extinctions at 22 different lines between 1 and 19 micron. For the
K-band I derive A_Ks=2.62+/-0.11. The extinction curve follows a
power law with a steep slope of -2.11+/-0.06 shortward of 2.8
micron. At longer wavelengths the extinction is grayer and there
are absorption features from ices. The extinction curve is a tool
to constrain the properties of cosmic dust between the sun and the
GC. The extinction curve cannot be explained by dust grains
consisting of carbonaceous and silicate grains only. In addition
composite particles, which also contain ices are necessary to fit
the extinction curve. In the final part of this thesis (Chapter 5)
I look at the properties of most of the stars in the GC. These are
the old stars that form the nuclear cluster of the Milky Way. I
obtain the mass distribution and the light distribution of these
stars. I find that the flattening of the stellar distribution
increases outside 70''. This indicates that inside a nearly
spherical nuclear cluster dominates and that the surrounding light
belongs mostly to the nuclear disk. I dissect the light in two
components and obtain for the nuclear cluster L_Ks=2.7*10^7 L_sun.
I obtain proper motions for more than 10000 stars and radial
velocities for more than 2400 stars. Using Jeans modeling I combine
velocities and the radial profile to obtain within 100'' (4 pc) a
mass of 6.02*10^6 M_sun and a total nuclear cluster mass of
12.88*10^6 M_sun. The Jeans modeling and various other evidence
weakly favor a core in the extended mass compared to a cusp. The
old star light shows a similar core. The mass to light ratio of the
old stars of the nuclear cluster is consistent with the usual
initial mass function in the Galaxy. This suggests that most stars
in GC formed in the usual way, in a mode different from the origin
of the youngest stars there.