Max Planck Institute for Astronomy - Annual Report 2005

log
M� / (pc3 (km/h) 3 fe )
2
0
–2
–4
–6
–8
–10
between the individual clusters (Table II.6.1). Modeling
extended formation histories, with multiple ages, always
yielded better fits to the spectra than a single population.
From these fits, the team was able to conclude that
nuclear clusters, in contrast to globular clusters, generically
have multi-age stellar populations.
Moreover, modeling with several stellar populations
allowed us to determine the time of the last star formation
episode tmin for each cluster. The average is 34
million years; none of the last episodes was more than
100 million years ago.
Obviously these nuclear clusters underwent repeated
star formation episodes in the past and still do today.
This again raises the question of how these compact,
massive star clusters formed in the centers of so many
spiral galaxies. Which processes induce recurrent star
formation episodes? And it is still a mystery as to why
no nuclear star clusters are found in the centers of some
spiral galaxies that seem to be no different than others
that do have them. Therefore, the team carried out more
observations that will contribute to a better understanding
of the fundamental properties of galactic centers
in the future.
II.6. Giant Star Clusters in the Centers of Spiral Galaxies 41
Nuclear star clusters
Globular clusters
Nuclei dE
Spheroids
Young massive clusters
Ultracompact dwarfs
2 4 6 8 10 12 14
log M/M �
Fig. II.6.3: Phase space densities versus total mass for different
types of stellar systems. Again a clear gap between the nuclear
star clusters and globular clusters on the one hand and the
spherical (dwarf) galaxies on the other hand is discernible. The
dotted line marks the location of clusters fulfilling the virial
theorem and having a radius of 10 ly. The solid line is the locus
of systems meeting the virial theorem and a Faber-Jackson type
relationship between mass and velocity dispersion.
(Jakob Walcher, Hans-Walter Rix, Nadine Neumayer,
in cooperation with:
Space Telescope Science Institute, Baltimore;
eSa/eSteC, Noordwijk;
Observatories of the Carnegie
Institution of Washington, Pasadena;
Oxford Astrophysics, Oxford;
Ohio University, Athens, USA;
MPI für Astrophysik, Garching,
and Institut d�Astrophysique de Paris)