How well do we know the age and mass distributions of the star cluster system in the Large magellanic Cloud?

R. Grijs, P. Anders

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The Large Magellanic Cloud (LMC) star cluster system offers the unique opportunity to independently check the accuracy of age (and the corresponding mass) determinations based on a number of complementary techniques. Using our sophisticated tool for star cluster analysis based on broad-band spectral energy distributions (SEDs), 'AnalySED', we re-analyse the Hunter et al. LMC cluster photometry. Our main aim is to set the tightest limits yet on the accuracy of absolute age determinations based on broad-band SEDs, and therefore on the usefulness of such an approach. Our broad-band SED fits yield reliable ages, with statistical absolute uncertainties within Δlog(Age/yr) ≃ 0.4 overall. The systematic differences we find with respect to previous age determinations are caused by conversions of the observational photometry to a different filter system, thus leading to systematically inaccurate results. The LMC's cluster formation rate (CFR) has been roughly constant outside of the well-known age gap between ∼3 and 13 Gyr, when the CFR was a factor of ∼5 lower. Using a simple approach to derive the characteristic cluster disruption time-scale, we find that log(tdis4/yr) = 9.9 ± 0.1 , where tdis=tdis4(Mcl/104 M⊙)0.62 . This long characteristic disruption time-scale implies that we are observing the initial cluster mass function (CMF). We conclude that while the older cluster (sub)samples show CMF slopes that are fully consistent with the α≃−2 slopes generally observed in young star cluster systems, the youngest mass and luminosity-limited LMC cluster subsets show shallower slopes (at least below masses of a few ×103 M⊙ ), which is contrary to dynamical expectations. This may imply that the initial CMF slope of the LMC cluster system as a whole is not well represented by a power law, although we cannot disentangle the unbound from the bound clusters at the youngest ages.
Original languageUndefined/Unknown
Pages (from-to)295-307
Number of pages13
JournalMonthly Notices of the Royal Astronomical Society
Issue number1
Publication statusPublished - 2006

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