Max Planck Institute for Astronomy - Annual Report 2005

40 II. Highlights
log
M� /pc2 �e 6
5
4
3
2
4 6 8
log M/M �
Fig. II.6.2: Projected mass density versus total mass. The nuclear
clusters (filled squares) lie along a single sequence mainly defined
by galactic globular clusters. Clearly separated from them
are the spherical galaxies and the dwarf galaxies.
young ages. Therefore, the team tried to consequently
further develop the second and third methods. The high
quality of the data and the increasingly realistic synthetic
spectra were particularly useful for this purpose. The
third method was based on the finding that each population
with any evolutionary history can be described by a
series of instantaneous star formation events.
Nuclear star clusters
Globular clusters
Nuclei dE
Spheroids
Young massive clusters
Ultracompact dwarfs
10 12
Spectral model fits assuming single-ages yielded estimates
ranging from 10 7 to 3 � 10 9 years for the dominant
stellar population of the respective cluster. The results
were relatively independent of the metallicity assumed.
When extended star formation histories were used to fit
the spectra, somewhat higher mean ages of 4 � 10 7 to
10 10 years were obtained throughout. The metallicity is
generally slightly sub-solar but shows significant scatter
Table: II.6.1: Properties of nuclear clusters. Here d signifies the
distance, t the mean age, Z the metallicity, t min the time of the
last star formation episode, and M the mass.
Galaxy d [Mpc] log(t) Z log(t min ) log(M / M � )
NGC 0300 2.2 9.49 0.004 8.00 6.02 � 0.24
NGC 0428 6.1 9.27 0.02 7.48 6.51 � 0.14
NGC 1042 18.2 10.05 0.02 7.00 6.51 � 0.21
NGC 1493 11.4 9.76 0.008 7.00 6.38 � 0.14
NGC 2139 23.6 7.61 0.05 6.48 5.92 � 0.20
NGC 3423 14.6 9.75 0.008 7.00 6.53 � 0.14
NGC 7418 18.4 9.05 0.008 7.00 7.78 � 0.19
NGC 7424 10.9 9.11 0.008 7.48 6.09 � 0.14
NGC 7793 3.3 9.29 0.008 8.00 6.89 � 0.14