Research Note

Spectroscopic constraints on the form of the stellar cluster mass function

¹ Excellence Cluster Universe, Boltzmannstr. 2, 85748 Garching, Germany
e-mail: bastian@usm.lmu.de
² Department of Astronomy and Astrophysics, The Pennsylvania State University, University Park, PA 16802, USA
³ Gemini Observatory, Casilla 603, La Serena, Chile
⁴ Giant Magellan Telescope Organization, 251S. Lake Ave., Pasadena 91001, USA
⁵ Department of Astronomy, Box 351580, University of Washington, Seattle, WA 98195, USA
⁶ Department of Astrophysics, IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
⁷ Institut für Astronomie und Astrophysik der Universität München, Scheinerstraße 1, 81679 München, Germany
⁸ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge, CB3 0HA, UK

Received: 20 February 2012
Accepted: 23 March 2012

Abstract

This contribution addresses the question of whether the initial cluster mass function (ICMF) has a fundamental limit (or truncation) at high masses. The shape of the ICMF at high masses can be studied using the most massive young (<10 Myr) clusters, however this has proven difficult due to low-number statistics. In this contribution we use an alternative method based on the luminosities of the brightest clusters, combined with their ages. The advantages are that more clusters can be used and that the ICMF leaves a distinct pattern on the global relation between the cluster luminosity and median age within a population. If a truncation is present, a generic prediction (nearly independent of the cluster disruption law adopted) is that the median age of bright clusters should be younger than that of fainter clusters. In the case of an non-truncated ICMF, the median age should be independent of cluster luminosity. Here, we present optical spectroscopy of twelve young stellar clusters in the face-on spiral galaxy NGC 2997. The spectra are used to estimate the age of each cluster, and the brightness of the clusters is taken from the literature. The observations are compared with the model expectations of Larsen (2009, A&A, 494, 539) for various ICMF forms and both mass dependent and mass independent cluster disruption. While there exists some degeneracy between the truncation mass and the amount of mass independent disruption, the observations favour a truncated ICMF. For low or modest amounts of mass independent disruption, a truncation mass of 5−6 × 10⁵ M_⊙ is estimated, consistent with previous determinations. Additionally, we investigate possible truncations in the ICMF in the spiral galaxy M 83, the interacting Antennae galaxies, and the collection of spiral and dwarf galaxies present in Larsen (2009, A&A, 494, 539) based on photometric catalogues taken from the literature, and find that all catalogues are consistent with having a truncation in the cluster mass functions. However for the case of the Antennae, we find a truncation mass of a few  × 10⁶M_⊙ , suggesting a dependence on the environment, as has been previously suggested.