On the mass of dense star clusters in starburst galaxies from spectrophotometry

Abstract

The mass of unresolved young star clusters derived from spectrophotometric data may well be off by a factor of 2 or more once the migration of massive stars driven by mass segregation is accounted for. We quantify this effect for a large set of cluster parameters, including variations in the stellar initial mass function (IMF), the intrinsic cluster mass, and mean mass density. Gas‐dynamical models coupled with the Cambridge stellar evolution tracks allow us to derive a scheme to recover the real cluster mass given measured half‐light radius, one‐dimensional velocity dispersion and age. We monitor the evolution with time of the ratio of real to apparent mass through the parameter η. When we compute η for rich star clusters, we find non‐monotonic evolution in time when the IMF stretches beyond a critical cut‐off mass of 25.5 M⊙. We also monitor the rise of colour gradients between the inner and outer volume of clusters: we find trends in time of the stellar IMF power indices overlapping well with those derived for the Large Magellanic Cloud cluster NGC 1818 at an age of 30 Myr. We argue that the core region of massive Antennae clusters should have suffered from much segregation despite their low ages. We apply these results to a cluster mass function, and find that the peak of the mass distribution would appear to observers shifted to lower masses by as much as 0.2 dex. The star formation rate derived for the cluster population is then underestimated by from 20 to 50 per cent.

Extranuclear Hα-emitting complexes in low-z(U)LIRGs: precursors of tidal dwarf galaxies?

“…However, some studies have shown that η is not a constant, and can vary with time (η ∼ 3−10), depending on, for example, the degree of mass segregation and the binary fraction of the cluster (Fleck et al 2006;Kouwenhoven & de Grijs 2008). Since we do not have this information, we use η = 9.75.…”

Young star clusters in interacting galaxies – NGC 1487 and NGC 4038/4039

“…As described in Fleck et al (2006) their models indicate that the first 10 Myr of cluster evolution -at least for dense clusters like those in our sample 1.4 ± 0.2 300 0.91 1.5 ± 0.4 300 0.85 1.4 ± 0.2 300 S1_1 3.6 ± 0.3 15-300 0.70 3.6 ± 0.3 150 S1_2 3.6 ± 0.5 300 0.69 3.6 ± 0.2 15-300 0.74 3.6 ± 0.4 300 2000_1 b 3.6 ± 1.0 300 S2_1 2.0 ± 0.6 300 0.68 3.7 ± 0.5 152 0.71 3.7 ± 0.5 150 S2_2 2.0 ± 0.5 300 0.75 2.9 ± 0.4 100-300 0.66 2.5 ± 0.5 300 S2_3 2.7 ± 0.5 300 0.68 3.9 ± 0.3 30-300 0.77 3.0 ± 1.0 300 S1_5 b was assigned the same size and concentration as the two clusters in its vicinity (S1_1 and S1_2), because the three clusters appear comparable in size in the K-band images, and the cluster is undetected in the HST images.…”

The internal dynamics of stellar clusters

“…Under virial equilibrium conditions, one finds η 8 for a good range of King models. The catch is that while η may remain constant for mass-weighted quantities, in reality flux-weighted quantities will evolve on the same or a similar timescale as the massive stars for several important cases (see Boily et al 2005;Fleck et al 2006 for a fuller exploration of single-star models). As a result, η may well vary by a factor of up to three in the case of single-star populations.…”

“…Note that the idea of converting light to mass is used by observers, for instance see McCrady & Graham (2007) orÖstlin et al (2007, yet theorists have been slow to convert their model data into observable quantities. Some work has attempted to draw consequences for full globular cluster mass functions by deconvolving the mass of model clusters of a given initial mass and age (Fleck et al 2006;Kruijssen & Portegies Zwart 2009), although much remains to be done in this area given the oversimplification of the stellar populations modelled. Yet, it is clear that η is a key parameter to evaluate as it may hold the answer to cluster survival and the true mass function (de Grijs & Parmentier 2007).…”