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Galaxy Dynamics: from the Early Universe to the Present
ASP Conference Series, Vol. XX, 2000
F. Combes, G. A. Mamon and V. Charmandaris, eds.
Mass and Age of the Nuclear Star Cluster in IC 342
Roeland P. van der Marel, Torsten B˜oker 1
Space Telescope Science Institute, 3700 San Martin Drive, Baltimore,
MD 21218, U.S.A.
William D. Vacca
Institute for Astronomy, Honolulu, HI 96822, U.S.A.
Abstract. Nuclear stellar clusters are common in intermediate to late­
type spirals. Here we summarize the results of a study of the mass and
age of the nuclear cluster in the nearby Scd spiral IC 342, based on 12 CO
(2­0) bandhead spectroscopy.
1. Introduction
Recent HST observations of large samples of spiral galaxies have shown that
nuclear stellar clusters are very common in intermediate to late­type spirals
(e.g., Carollo, Stiavelli, & Mack 1998; B˜oker et al. 1999). These clusters may
play an important role in the secular evolution of spiral galaxies. It is important
to study the masses, ages and other basic properties of these clusters, in order to
better understand their origin and the influence they have on their surroundings.
The nearby, giant Scd spiral IC 342 is an ideal target for such a study, because of
its proximity (distance 1.8 Mpc), face­on orientation, and bright nuclear cluster
(K=10.59 within a 1.2 ## diameter aperture).
2. Observations and Analysis
We used the NASA Infrared Telescope Facility (IRTF) to obtain high resolution
(#/## = 21500) 2.3µm spectra of the nuclear cluster in IC 342 at the 12 CO (2­0)
bandhead (B˜oker, van der Marel, & Vacca 1999). The observed spectrum implies
a line­of­sight stellar velocity dispersion # = (33 ± 3) km/s, cf. Figure 1a. To
interpret this result we constructed dynamical models based on the Jeans equa­
tion for an isotropic spherical system. The light distribution of the cluster was
modeled using an archival HST V­band image (Figure 1b), combined with a new
ground­based K­band image. The observed kinematics imply a K­band mass­
to­light ratio M/LK = 0.05 in solar units, and a cluster mass M # 6 â 10 6 M# .
Also, IC 342 cannot have any central black hole more massive than 5 â 10 5 M# .
The inferred M/LK can be compared with stellar population synthesis models
1 A#liated with the Astrophysics Division, Space Science Department, European Space Agency
1

2 van der Marel, B˜oker & Vacca
Figure 1. (a; left) Normalized 12 CO (2­0) bandhead spectrum of the
nuclear star cluster in IC 342 (top) compared to the spectra of four
late­type template stars (bottom). The heavy solid curve shows the
best Gaussian­convolved template fit to the galaxy spectrum (# = 33
km/s). (b; right) WFPC2/F606W (wide V­band) image of IC 342 from
the HST Data Archive. The nuclear cluster stands out on a faint di#use
stellar background, and is surrounded by a complicated pattern of dust
absorption and fainter, o#­nuclear star clusters.
(e.g., Leitherer et al. 1999) to estimate the age of the cluster. The low mass­to­
light ratio implies a young cluster age in the range 10 6.8-7.8 years (independent
of the assumed IMF), consistent also with the observed equivalent width of the
CO bandhead. However, an underlying older population cannot be ruled out,
since such a population would not contribute much to the observed luminosity.
3. Discussion
Several other nearby spiral galaxies also have young nuclear clusters. For exam­
ple, our own Galaxy has a cluster of only 10 6.5 yrs old, while both both M31 and
M33 have blue nuclei that are quite possibly young star clusters. So it may be
necessary to invoke scenarios with recurrent periods of star formation in spiral
nuclei to explain the high fraction of young clusters. To address this issue in
detail will require a determination of the mass and age distributions of nuclear
star clusters in a large sample of spiral galaxies.
References
B˜oker, T., van der Marel, R. P., & Vacca, W. D. 1999, AJ, 118, 831
B˜oker, T., et al. 1999, ApJS, in press
Carollo, C. M., Stiavelli, M., & Mack, J. 1998, AJ, 116, 68
Leitherer, C., et al. 1999, ApJS, 123, 3