CO band head spectroscopy of IC 342: Mass and age of the nuclear star cluster

Citation
T. Boker et al., CO band head spectroscopy of IC 342: Mass and age of the nuclear star cluster, ASTRONOM J, 118(2), 1999, pp. 831-842
Citations number
49
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Space Sciences
Journal title
ASTRONOMICAL JOURNAL
ISSN journal
0004-6256 → ACNP
Volume
118
Issue
2
Year of publication
1999
Pages
831 - 842
Database
ISI
SICI code
0004-6256(199908)118:2<831:CBHSOI>2.0.ZU;2-C
Abstract
We have used the NASA Infrared Telescope Facility (IRTF) to observe the nuc lear stellar cluster in the nearby, face-on, giant Scd spiral IC 342. From high-resolution (lambda/Delta lambda = 21,500) spectra of the (CO)-C-12 (2- 0) band head at 2.3 mu m, we derive a line-of-sight stellar velocity disper sion sigma = (33 +/- 3) km s(-1). To interpret this observation we construct dynamical models based on the Je ans equation for a spherical system. The light distribution of the cluster is modeled using an archival Hubble Space Telescope (HST) V-band image and a new ground-based K-band image. Under the assumption of an isotropic veloc ity distribution, the observed kinematics imply a K-band mass-to-light rati o M/L-K = 0.05, and a cluster mass M approximate to 6 x 10(6) M-circle dot. We compare the derived mass-to-light ratio with the "Starburst99" stellar population synthesis models of Leitherer and collaborators and infer a best -fitting cluster age in the range 10(6.8-7.8) yr. Although this result depe nds somewhat on a number of uncertainties in the modeling (e.g., the assume d extinction along the line of sight toward the nucleus, the initial mass f unction of the stellar population model, and the velocity dispersion anisot ropy of the cluster), none of the model parameters can be plausibly modifie d to yield a significantly larger age. Also, the inferred age is consistent with that found in our previous study based on the near-infrared absorptio n-line equivalent widths of the cluster (Boker, Forster-Schreiber, & Genzel ). Recent HST observations of large samples of spiral galaxies have shown that nuclear stellar clusters are very common in intermediate- to late-type spi rals. The cluster in IC 342 is more luminous than the clusters found in mos t other nearby spiral galaxies. If the nuclear stellar clusters in spiral g alaxies all have a mass similar to that of the cluster in IC 342, then stel lar population synthesis models indicate a median age for these clusters of several Gyr. This may be consistent with a scenario in which each spiral g alaxy has only one episode of nuclear star cluster formation. On the other hand, the incidence of young nuclear star clusters may be high enough to in dicate that the formation of these clusters is a recurring phenomenon. Age and population studies for a larger sample of galaxies are necessary to dis tinguish between these scenarios and to determine how these nuclear stellar clusters are related to the secular evolution of their environment. As a by-product of our analysis, we infer that IC 342 cannot have any centr al black hole more massive than 5 x 10(5) M-circle dot. This is similar to 6 times less massive than the black hole inferred to exist in our Galaxy, c onsistent with the accumulating evidence that galaxies with less massive bu lges harbor less massive black holes.