Max Planck Institute for Astronomy - Annual Report 2007

56 II. Highlights
II.8 Unique Galay Portraits with Th i n g s
Under MPIA’s leadership using the Very Large Array
(VLA), an international team of astronomers studied a
total of 34 nearby galaxies in the light of atomic hydrogen’s
21 cm line. The VLA is a radio interferometer
located near Socorro (New Mexico). The project was
designated “The HI Nearby Galaxy Survey” (Th i n g s) and
has been the largest program of this type at the VLA. It
delivered data that are unique from many points of view
and which will be the foundation for many systematic
studies. In an initial series of publications, MPIA astronomers
and colleagues from other institutions studied
the rotation curves of spiral and dwarf galaxies in order
to understand more about the puzzling dark matter halos.
In a subsequent study, they researched the connection
between the density of interstellar matter and star formation.
Observations of atomic hydrogen’s (HI) 21 cm line have
been considered for decades to be one of the best methods
to study the structure and kinematics of the interstellar
medium – both in our Milky Way system as well as
in other galaxies. Compared to other radiation, this has a
series of advantages. HI emission experiences no extinction
through interstellar dust and is in almost all cases
optically thin, so that one can immediately derive the
density of the hydrogen gas from the measured intensity.
In addition, the kinematics of the hydrogen gas can be
ascertained by looking at the Doppler effect.
Selecting Galaxies for Th i n g s
Because of the comparatively large wavelength, one
needs large telescope apertures to obtain a sufficient spatial
resolution. The VLA interferometer (Fig. II.8.1) offers
unique possibilities in this regard. The VLA worked
for tH i N g s in three different configurations with antenna
separations of between 35 m and 11.4 km. A high spectral
resolution between 1.3 and 5.2 km/s and a high spatial
resolution of 6 arc seconds was decisive for the project.
Thus, astronomers had to go to the technical limits of the
instrument.
The principal goal of tH i N g s is to study key characteristics
of galaxies across the entire Hu b b l e sequence (with
the exception of starburst galaxies). Galaxy morphology,
star formation and evolution, as well as mass distribution
are the main areas of interest (compare 2005 Annual
Report, p. 82).
The selection of galaxies followed several criteria.
They cover a wide range in their star formation rates,
absolute luminosities, and metallicities. Almost all gal-
axies are targets of the sp i t z e r Infrared Nearby Galaxies
Survey (si N g s) project’s catalogue in which the MPIA is
also participating. So far, the Na s a sp i t z e r space telescope
is the most efficient infrared observatory. si N g s
studies the near and middle infrared dust characteristics
of galaxies. At the same time, for many of the tH i N g s
galaxies, observations by the American ga l e x space telescope
and CO observations from the ir a m telescope are
available. Thus, star formation rates can be ascertained
and compared with the characteristics of neutral hydrogen
(see below). Most of these observation data have
about the same angular resolution of 6 arcseconds and
can therefore easily be compared to each other.
There were also other criteria in selecting the galaxies.
For example, all of them are in a distance range of 6 to 50
million light years, resulting in a spatial resolution of 300
to 1600 light years – which is significantly better than
earlier observations. Galaxies from the Local Group were
not included because their angular size in the sky is too
large. In the end, 34 galaxies were selected requiring 500
hours observing time for the detailed study of their light
on the 21cm line (Fig. II.8.2). On the following pages we
present two already published results.
Rotation Curves and Models for Dark Matter Halos
Since the 1970s, there have been more and more indications
that galaxies are embedded in extended clouds
(halos) of dark matter. This was derived from the galaxies’
rotation curves. If the galaxies consisted exclusively
of visible matter, then the stars and gas clouds would
orbit the center in Keplerian orbits on which the orbital
velocity would decrease as the distance from the center
grew. Yet, after a certain radius, one observes an almost
constant velocity extending into the outskirts of the galaxies,
our Milky Way also has such a rotation curve. This
phenomenon is explained by the gravitational effect of
the dark matter halo.
This dark matter must have already played a decisive
role in the formation of galaxies. In the early universe
it created potential troughs in which normal, baryonic
material accumulated and further condensed into galaxies.
Modern computer simulations take into account the
gravitational effect of the dark matter, though its further
physical characteristics are widely unknown. However,
a problem has recently emerged in these cosmological
calculations: The theory predicted that the density, and
thus also the gravitational potential of dark matter halos,
increases very steeply toward the center (with r –1 to
r –1.5 ). This is known as the “cuspy core” model. Galaxy
rotation curves however can best be explained with a