Max Planck Institute for Astronomy - Annual Report 2007

The Search for Needles in the Hay Stack
– Four New Dwarf Galaxies
In fact, four new dwarf galaxies were already discovered
among the SDSS data in 2005 and 2006. They were, as
is generally so, named after the constellations in which
they were found. These were: Ursa Major I and II, Canes
Venatici, and Bootes. An international astronomer team
led by the MPIA and the University of Cambridge were
able during the reporting year to discover four more dwarf
galaxies. An additional faint object was added which was
very likely a globular cluster.
In the context of the SDSS, an area approximately the
size of 8000 square degrees around the galactic north pole
(corresponding to about one fifth of the entire celestial
sphere) was filmed in five photometric spectral ranges.
The dwarf galaxies discovered so far cannot be seen on
these images with the naked eye, rather they come to
light only after systematic, computer-supported searches
for over-densities of stars within specific brightness and
color ranges.
An important criterion in searching for dwarf galaxies,
and the later interpretation of data, is also the SDSS’s sensitivity.
Bright stars in the red-giant branch can be detected
at distances of up to three million light years; while very
faint stellar systems which hardly contain red giants can
only be detected at a distance of one million light years.
Thus the SDSS reached farther than any survey to date.
Under these boundary conditions, Sergej Koposov,
Hans-Walter Rix and Eric Bell at MPIA with their colleagues
developed a program to detect dwarf galaxies.
A program of this type works as follows: One searches
within an area of the sky for stars with selected colors
and brightness and compares their numbers with expected
values for background stars. A proven method in
the search for overdensities in stars or for other deviations
from average values within a particular region is
the application of a spatial kernel: average values for
all data points within a certain area of the sky (in our
case the colors and brightness of the stars) are sought.
The comparison with these average values will reveal
deviations such as the searched-for overdensities in the
angular scale of the selected kernel.
In order to test the efficiency of the program, the
values of “artificial” dwarf galaxies and globular clusters
were inserted. In this manner the search program’s
detection limit was determined for seven distance ranges
from 26 000 to 3.2 million light-years in dependence on
the searched for object’s brightness (Fig. II.7.1). The
size of the kernel played a significant role here: While a
smaller kernel allowed for the better discovery of faint
objects than a larger kernel did, this was at the expense
of galaxy size. A large kernel adds up more stars from
extended objects and therefore allows for the recognition
of objects with lower surface brightness than a smaller
kernel would have.
II.7. Dwarf Galaxies – the “Missing Satellaties” of the Milky Way 51
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Fig. II.7.4: Left column: SDSS images of indicated objects as
well as the positions of the su b a r u and INT fields. Right column:
Images filtered with a 4 kernel.
As a compromise, a kernel of 4 arc minutes was used
where the red limiting magnitude lay around 22.0 mag.
In the program’s trial runs, it turned out additionally that
HII regions and clusters of galaxies were erroneously
identified as dwarf galaxies (Fig. II.7.2). This problem
could be eliminated through a comparison to existing
catalogues of these objects.
Through this method, MPIA astronomers and their
colleagues were able to identify four as yet unknown
dwarf galaxies: Coma Berenices (Com), Canes Venatici
II (CVn II), Leo IV, and Hercules (Her). The fifth object,