The GRAVITY interferometer and the Milky Way’s nuclear star cluster

This thesis is divided into two parts: an instrumentation part and
an astrophysical part. The instrumentation part describes the
development and implementation of the fiber coupler and guiding
subsystems of the 2nd generation VLTI instrument GRAVITY. The
astrophysical part describes the derivation of the star formation
history of the Milky Way’s nuclear star cluster based on imaging
and spectroscopic data obtained at the Very Large Telescope. The
future VLTI instrument GRAVITY will deliver micro-arcsecond
astrometry, using the interferometric combination of four
telescopes. The instrument is a joint project of several European
institutes lead by the Max Planck Institut f¨ur extraterrestrische
Physik. The instrumental part of this thesis describes the fiber
coupler unit and the guiding system. They serve for beam
stabilization and light injection in GRAVITY. In order to deliver
micro-arcsecond astrometry, GRAVITY requires an unprecedented
stability of the VLTI optical train. We therefore developed a
dedicated guiding system, correcting the longitudinal and lateral
pupil wanderas well as the image jitter in VLTI tunnel. The
actuators for the correction are provided by four fiber coupler
units located in the GRAVITY cryostat. Each fiber coupler picks the
light of one telescope and stabilizes the beam. Furthermore each
unit provides field de-rotation, polarization adjustment as well as
atmospheric piston correction. A novel roof-prism design offers the
possibility of on-axis as well as off-axis fringe tracking. Finally
the stabilized beam is injected with minimized losses into
singlemode fibers via parabolic mirrors. We present lab results of
the first guiding- as well as the first fiber coupler prototype, in
particular the closed loop performance and the optical quality.
Based on the lab results we derive the on-sky performance of the
systems and the implications concerning the sensitivity of GRAVITY.
The astrophysical part of this thesis presents imaging and integral
field spectroscopy data for 450 cool giant stars within 1 pc from
Sgr A*. We use the prominent CO bandheads to derive effective
temperatures of individual giants. Additionally we present the
deepest spectroscopic observation of the Galactic Center so far,
probing the number of B9/A0 main sequence stars (2.2 − 2.8M) in two
deep fields. From spectro-photometry we construct a
Hertzsprung-Russell diagram of the red giant population and fit the
observed diagram with model populations to derive the star
formation history of the nuclear cluster. We find that (1) the
average nuclear star-formation rate dropped from an initial maximum
10Gyrs ago to a deep minimum 1-2Gyrs ago and increased again during
the last few hundred Myrs, and (2) that roughly 80% of the stellar
mass formed more than 5Gyrs ago; (3) mass estimates within R 1 pc
from Sgr A* favor a dominant star formation mode with a normal
Chabrier/Kroupa initial mass function for the majority of the past
star formation in the Galactic Center. The bulk stellar mass seems
to have formed under conditions significantly different from the
observed young stellar disks, perhaps because at the time of the
formation of the nuclear cluster the massive black hole and its
sphere of influence was much smaller than today.