Max Planck Institute for Astronomy - Annual Report 2007

42 II. Highlights
on the giant branch and the point of departure from
the main sequence could be measured. After the data
were cleansed of field stars and background galaxies,
theoretical isochrones of stellar evolution could be
fitted to the color-brightness diagram, showing that
Canes Venatici I is 719,000 light-years away, its main
star population is poor in metals, and it is around 14.1
billion years old.
At the same time, a group of blue stars became discernible.
This group, which was also spatially separated,
was located at around 210 light-years east of the dwarf
galaxy’s center. Thus a possible interpretation of these
blue stars as so-called “blue stragglers” (these are double
stars which appear blue but which are not young stars) is
excluded. Because, however, there is no visible reason
for the assumption that a group of blue stragglers would
have gathered in a special part of a dwarf galaxy, the
simplest explanation remains that the blue objects are
young stars. According to model estimates, their age is
between 1.4 and 2 billion years. These young stars appear
somewhat more metal-rich than those of the older
generation.
A more exact analysis of the old star generation was
derived from total luminosity: 35,000 solar luminosities
for the old stars and 3,500 for the young. Converted, this
means that the young population contributes three to five
percent of the stellar mass of Canes Venatici I.
– 0
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All
stars
Fig. II.5.4 shows a contour representation of the old
(represented in red) and young (blue) star populations
observed with the LBT. The spatial distribution of the
young population revealed in the LBT study corresponded
well with the distribution found spectroscopically by
Ibata et al. Accordingly, one would have to assume that
both star groups also differ kinematically. Initial studies
thus reveal a complex evolution history in these far-lying
dwarf galaxies which will soon attract further observations.
The dwarf galaxy Leo T, also discovered within the
SDSS, poses serious challenges for the observers because,
at 1.38 million light-years, it is by far the most
distant known dwarf galaxy. Here too the images with the
LBT made a much deeper-reaching photometry possible.
The color-brightness diagram (Fig. II.5.5, left) proved to
be rather complex and could only be represented through
the overlaying of several populations. It follows that a
very young star generation, with an age between 400 million
and one billion years, co-exists with a significantly
older population. A more exact analysis points to the
fact that there was a long-lasting phase of star formation
Fig. II.5.4: Contour representation of the stellar distribution in
Canes Venatici I. Above: all stars, old stars, below: young blue
stars, both populations.
Red/old stars
Blue/young stars All stars
Blue/young stars
Red/old stars
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