and Cosmology

2.6 The Galactic Center
83
Fig. 2.38. Determination of the mass M(r) within a radius r
from Sgr A ∗ , as measured by the radial velocities and proper
motions of stars in the central cluster. Mass estimates obtained
from individual stars (S14, S2, S12) are given by the points
with error bars for small r. The other data points were derived
from the kinematic analysis of the observed proper motions
of the stars, where different methods have been applied. As
can be seen, these methods produce results that are mutually
compatible, so that the mass profile plotted here can be
regarded to be robust. The solid curve is the best-fit model,
representing a point mass of 2.9 × 10 6 M ⊙ plus a star cluster
with a central density of 3.6 × 10 6 M ⊙ /pc 3 (the mass profile
of this star cluster is indicated by the dash-dotted curve). The
dashed curve shows the mass profile of a hypothetical cluster
with a very steep profile, n ∝ r −5 , and a central density of
2.2 × 10 17 M ⊙ pc −3
the same modulation time-scale. Observing the Galactic
center simultaneously in the NIR and the X-rays
revealed a clear correlation of the corresponding light
curves; for example, simultaneous flares were found in
these two wavelength regimes. These flares have similar
light profiles, indicating a similar origin of their
radiation. The consequences of these observations for
the nature of the central black hole will be discussed
in Sect. 5.4.6, after we have introduced the concept of
black holes in a bit more detail. Flares were also observed
at mm-wavelengths; their time-scale appears to
be longer than that at higher frequencies, as expected
if the emission comes from a more extended source
component.
2.6.5 The Proper Motion of Sgr A ∗
From a series of VLBI observations of the position of
Sgr A ∗ , covering eight years, the proper motion of this
compact radio source was measured with very high precision.
To do this, the position of Sgr A ∗ was determined
relative to two compact extragalactic radio sources. Due
to their large distances these are not expected to show