5.2 Dwarf Galaxies
Nearby dwarf galaxies allow to resolve their stellar population into
individual stars. This is especially true for images obtained with the
optical and near infrared cameras aboard the Hubble Spece telescope.
Doing deep multi-band photometry of the individual stars of those
dwarf galaxies allows to reconstruct the star formation history of the
host galaxies when comparing the observed color magnitude diagrams
with simulated ones based on stellar evolutionary codes. Low
metallicity systems like Leo A and a couple of Blue Compact Dwarf
galaxies have been studied, among them galaxies which have been
suspected to be young, e.g. building their first stellar generation
nowadays. In all cases, a stellar population have been revealed which
is as old as the globular clusters of the Milky Way galaxy, so more
than 10 Gyr. The star formation rate of all studied objects is
variable by a factor of a few. This project is a long standing
collaboration with R.-E. Schulte-Ladbeck (Pittsburg), L. Greggio
(Padua), M.M. Crone (Saratoga Springs), and I. Drozdovsky (Caltech).
One of the major concerns when digging into the fossil records of
local galaxies as described above is the precise distance of the dwarf
galaxies. Besides the tip of the red giant branch variable stars are a
well established tool to derive reliable distances. As the relevant
variables are among the brightest stars of the dwarfs, a small
telescope can be used for finding and monitoring these variable stars
provided good seeing and excellent software tools are available. We
are use the Wendelstein 80 cm telescope for monitoring (supplement by
some Calar Alto 1.23m telescope data). Difference imaging techniques
are applied (as in the pixel lensing project). Delta Cepheii stars
and Mira stars have been detected in Leo A confirming its Local Group
membership. Five other dwarfs are also monitored (project in
progress).
last update:
10/2004, editor of this page: Roberto Philip Saglia
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