Max Planck Institute for Astronomy - Annual Report 2007

50 II. Highlights
Fig. II.7.2: A 22 times 22 square degree sized field of the SDSS
for stars (left) and galaxies (right). Ursa Major I and Willman I
(circles), two known satellite systems to our Milky Way, were
discovered by the program as well as two distant clusters of
galaxies (diamonds).
Current cosmological models describe the formation
of galaxies under the influence of cold, dark matter.
According to this scenario, in the early universe huge
clouds of dark matter existed, which acted as “gravitation
traps” for normal baryonic material. The galaxies
therefore formed in the center of large halos of dark
matter. However, these halos were not isolated structures
but they interacted with each other: Small halos merged
into larger ones. A part of the smaller halos continued to
exist, however, and orbited the larger halos on satellite
paths. While large galaxies formed in the central halos,
dwarf galaxies are supposed to have formed in the satellite
halos.
Computer simulations already pointed in 1999 to
the fact that a galaxy such as our Milky Way System
should be surrounded inside a radius of around two million
light years by approximately 50 dwarf galaxies; the
local group within a radius of five million light-years
should even contain up to 300 such satellite galaxies.
This prediction stood in clear contrast to observations:
Up until a few years ago, only nine dwarf galaxies were
known to exist.
On the theoretical side, many possibilities were discussed
in order to explain this discrepancy. For example,
it was assumed that in some satellite halos, star formation
was suppressed for unknown reasons which led to a
large number of invisible conglomerates of dark matter.
At the same time, however, it was also clear that the
dN/dM v
100
10
1
0.1
0
within 280kpc
–5 –10
Mv [mag]
–15 –20
Fig. II.7.3: Volume corrected luminosity function for a maximum
distance of 900,000 light years. Solid line with open circles:
brightness limit r 22.0 mag, dotted curve: brightness limit
r 22.5. The red and blue lines show the results from two
model calculations relating to the formation of dwarf galaxies.
discovery of dwarf galaxies was only possible to a limited
extent. After all, an adequate instrument to perform
such studies only became available it was only with the
SDSS, in which the MPIA is taking part.