The young star cluster system in the Antennae: evidence for a turnover in the luminosity function

Abstract

The luminosity functions (LFs) of star cluster (SC) systems (i.e. the number of clusters per luminosity interval) are vital diagnostics to probe the conditions of SC formation. Early studies have revealed a clear dichotomy between old globular clusters and young clusters, with the former characterized by Gaussian-shaped LFs, and the latter following a power law. Recently, this view was challenged by studies of galaxy merger remnants and post-starburst galaxies. In this paper, we re-evaluate the young ( few hundreds of Myrs, with the majority few tens of Myrs) SC system in the ongoing spiral-spiral major merger system NGC 4038/39, the 'Antennae' galaxies. The Antennae galaxies represent a very active and complex star-forming environment, which hampers cluster selection and photometry as well as the determination of observational completeness fractions. A main issue of concern is the large number of bright young stars contained in most earlier studies, which we carefully exclude from our cluster sample by accurately determining the source sizes. The resulting LFs are fitted both with Gaussian and with power-law distributions, taking into account both the observational completeness fractions and the photometric errors, and compared using a likelihood-ratio test. The likelihood-ratio results are rigidly evaluated using Monte Carlo simulations. We perform a number of additional tests, for example, with subsets of the total sample, all confirming our main result: that a Gaussian distribution fits the observed LFs of clusters in this preferentially very young cluster system significantly better than a power-law distribution, at a (statistical) error probability of less than 0.5 per cent.

Global survey of star clusters in the Milky Way

“…Larsen 2002;de Grijs et al 2003;Gieles et al 2006;Anders et al 2007;Mora et al 2009;Whitmore et al 2014). However, studies of the CLF in the Milky Way are more informative, since local clusters have a number of advantages over those in extragalactic systems.…”

Gemini Spectroscopic Survey of Young Star Clusters in Merging/Interacting Galaxies. Iii. The Antennae

“…Early WFPC HST imaging revealed a large population of young clusters (Whitmore & Schweizer 1995) whose properties have been the subject of many subsequent studies (e.g. Whitmore et al 1999, Mengel et al 2005, Anders et al 2007). The currently forming clusters have masses up to a few times 10 6 M ⊙ , and are distributed throughout the galaxy (Zhang et al 2001).…”

Early dynamical evolution of star cluster systems

“…That most observed mass functions of young clusters are power laws with a spectral index of −2 [see Parmentier & Gilmore (2007), and references therein; but see also Anders et al (2007) for a possible turnover in the cluster mass function of the Antennae merger, NGC 4038/39] therefore tells us something about cluster formation conditions which are common to different types of cluster environments. The high sensitivity to the external tidal field of high-mass cores of constant surface density may be a hint that, contrary to theoretical expectations, the index β of the mass-radius relation r c ∝ m β c of cluster-forming cores is shallower than β = 1/2.…”