and Cosmology

2. The Milky Way as a Galaxy
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been obtained from radio-, IR-, and X-ray radiation.
Since the GC is nearby, and thus serves as a prototype
of the central regions of galaxies, its observation is of
great interest for our understanding of the processes
taking place in the centers of galaxies.
2.6.1 Where is the Galactic Center?
The question of where the center of our Milky Way is
located is by no means trivial, because the term “center”
is in fact not well-defined. Is it the center of mass of the
Galaxy, or the point around which the stars and the gas
are orbiting? And how could we pinpoint this “center”
accurately? Fortunately, the center can nevertheless be
localized because, as we will see below, a distinct source
exists that is readily identified as the Galactic center.
Radio observations in the direction of the GC show
a relatively complex structure, as is displayed in Fig.
2.34.
A central disk of HI gas exists at radii from several
100 pc up to about 1 kpc. Its rotational velocity yields
a mass estimate M(R) for R 100 pc. Furthermore,
radio filaments are observed which extend perpendicularly
to the Galactic plane, and also a large number of
supernova remnants are seen. Within about 2 kpc from
the center, roughly 3 × 10 7 M ⊙ of atomic hydrogen is
found. Optical images show regions close to the GC
towards which the extinction is significantly lower. The
best known of these is Baade’s window – most of the
microlensing surveys towards the bulge are conducted
in this region. In addition, a fairly large number of globular
clusters and gas nebulae are observed towards the
central region. X-ray images (Fig. 2.35) show numerous
X-ray binaries, as well as diffuse emission by hot gas.
The innermost 8 pc contain the radio source Sgr A
(Sagittarius A), which itself consists of different
components:
• A circumnuclear molecular ring, shaped like a torus,
which extends between radii of 2 pc R 8pcand
is inclined by about 20 ◦ relative to the Galactic
disk. The rotational velocity of this ring is about
∼ 110 km/s, nearly independent of R. This ring has
a sharp inner boundary; this cannot be the result of an
equilibrium flow, because internal turbulent motions
would quickly (on a time-scale of ∼ 10 5 yr) erase
this boundary. Probably, it is evidence of an energetic
event that occurred in the Galactic center within
the past ∼ 10 5 years. This interpretation is also supported
by other observations, e.g., by a clumpiness
in density and temperature.
• Sgr A East, a non-thermal (synchrotron) source of
shell-like structure. Presumably this is a supernova
remnant (SNR), with an age between 100 and 5000
years.
• Sgr A West is located about 1. ′ 5 away from Sgr
A East. It is a thermal source, an unusual HII region
with a spiral-like structure.
• Sgr A ∗ is a strong compact radio source close to the
center of Sgr A West. Recent observations with mm-
VLBI show that its extent is smaller than 3 AU. The
radio luminosity is L rad ∼ 2 × 10 34 erg/s. Except for
the emission in the mm and cm domain, Sgr A ∗ is
a weak source. Since other galaxies often have a compact
radio source in their center, Sgr A ∗ is an excellent
candidate for being the center of our Milky Way.
Through observations of stars which contain a radio
maser 14 source, the astrometry of the GC in the radio
domain was matched to that in the IR, i.e., the position
of Sgr A ∗ is also known in the IR. 15 The uncertainty in
the relative positions between radio and IR observations
is only ∼ 30 mas – at a presumed distance of the GC of
8 kpc, one arcsecond corresponds to 0.0388 pc, or about
8000 AU.
2.6.2 The Central Star Cluster
Density Distribution. Observations in the K-band
(λ ∼ 2 μm) show a compact star cluster that is centered
on Sgr A ∗ . Its density behaves like ∝ r −1.8 in the
distance range 0.1pc r 1 pc. The number density
14 Masers are regions of stimulated non-thermal emission which show
a very high surface brightness. The maser phenomenon is similar to
that of lasers, except that the former radiate in the microwave regime
of the spectrum. Masers are sometimes found in the atmospheres of
active stars.
15 One problem in the combined analysis of data taken in different
wavelength bands is that astrometry in each individual wavelength
band can be performed with a very high precision – e.g., individually
in the radio and the IR band – however, the relative astrometry
between these bands is less well known. To stack maps of different
wavelength precisely “on top of each other”, knowledge of exact relative
astrometry is essential. This can be gained if a population of
compact sources exists that is observable in both wavelength domains
and for which accurate positions can be measured.