Cluster Disruption Time-scale Research Articles

Star cluster evolution in dark matter dominated galaxies

We investigate the influence of the external tidal field of a dark matter halo on the dynamical evolution of star clusters using direct N-body simulations, where we assume that the halo is described by a Navarro, Frenk and White mass profile which has an inner density cusp. We assess how varying the mass and concentration of the halo affects the rate at which the star cluster loses mass and we find that increasing halo mass and concentration drives enhanced mass loss rates and in principle shorter cluster disruption timescales. In addition, we examine disruption timescales in a three-component model of a galaxy (bulge, disk and dark matter halo) and find good agreement with results based on an empirical model of the Galactic potential if we assume a halo mass of ∼1012M⊙. In general dark matter halos are expected to contribute significantly to the masses of galaxies and should not be ignored when modelling the evolution of star clusters. We extend our results to discuss how this can have a potentially profound effect on the disruption timescales of globular clusters, suggesting that we may underestimate the rate at which primordial globular clusters are disrupted.

The star clusters of the Magellanic System

AbstractMore than 50 years have elapsed since the first studies of star clusters in the Magellanic Clouds. The wealth of data accumulated since then has not only revealed a large cluster system, but also a diversified one, filling loci in the age, mass and chemical abundance parameter space which are complementary to Galactic clusters. Catalogs and photometric samples currently available cover most of the cluster mass range. The expectations of relatively long cluster disruption timescales in the Clouds have been confirmed, allowing reliable assessments of the cluster initial mass function and of the cluster formation rate in the Clouds. Due to their proximity to the Galaxy, Magellanic clusters are also well resolved into stars. Analysis of colour—magnitude diagrams (CMDs) of clusters with different ages, masses and metallicities are useful tools to test dynamical effects such as mass loss due to stellar evolution, two-body relaxation, stellar evaporation, cluster interactions and tidal effects. The existence of massive and young Magellanic clusters has provided insight into the physics of cluster formation. The magnitudes and colours of different stellar types are confronted with stellar evolutionary tracks, thus constraining processes such as convective overshooting, stellar mass-loss, rotation and pre main-sequence evolution. Finally, the Magellanic cluster system may contribute with nearby and well studied counterparts of recently proposed types of extragalactic clusters, such as Faint Fuzzies and Diffuse Star Clusters.

Star cluster evolution in the Magellanic Clouds revisited

AbstractThe evolution of star clusters in the Magellanic Clouds has been the subject of significant recent controversy, particularly regarding the importance and length of the earliest, largely mass-independent disruption phase (referred to as “infant mortality”). Here, we take a fresh approach to the problem, using a large, independent, and homogeneous data set of UBVR imaging observations, from which we obtain the cluster age and mass distributions in both the Large and Small Magellanic Clouds (LMC, SMC) in a self-consistent manner. We conclude that the (optically selected) SMC star cluster population has undergone at most ~30% (1σ) infant mortality between the age range from about 3–10 Myr, to that of approximately 40–160 Myr. We rule out a 90% cluster mortality rate per decade of age (for the full age range up to 109 yr) at a > 6σ level. Using a simple approach, we derive a “characteristic” cluster disruption time-scale for the cluster population in the LMC that implies that we are observing the initial cluster mass function (CMF). Preliminary results suggest that the LMC cluster population may be affected by <10% infant mortality.

The poorly constrained cluster disruption time-scale in the Large Magellanic Cloud

We use Monte Carlo simulations, combined with homogeneously determined age and mass distributions, based on multiwavelength photometry, to constrain the cluster formation history and the rate of bound cluster disruption in the Large Magellanic Cloud (LMC) star cluster system. We evolve synthetic star cluster systems formed with a power-law initial cluster mass function (ICMF) of spectral index a = -2 assuming different cluster disruption time-scales. For each of these cluster disruption time-scales, we derive the corresponding cluster formation rate (CFR) required to reproduce the observed cluster age distribution. We then compare, in a 'Poissonian' Χ 2 sense, model mass distributions and model two-dimensional distributions in log(mass) versus log(age) space of the detected surviving clusters to the observations. Because of the bright detection limit (M lim V ≃ -4.7 mag) above which the observed cluster sample is complete, one cannot constrain the characteristic cluster disruption time-scale for a 104 M ⊙ cluster, t dis 4 [where the disruption time-scale depends on cluster mass as t dis = t dis 4 (M cl /10 4 M ⊙ )ν, with γ ≃ 0.62], to better than a lower limit, t dis 4 > 1 Gyr. We conclude that the CFR has been increasing steadily from 0.3 clusters Myr -1 5 Gyr ago to a present rate of (20-30) clusters Myr -1 for clusters spanning a mass range of ∼100-10 7 M ⊙ . For older ages, the derived CFR depends sensitively on our assumption of the underlying CMF shape. If we assume a universal Gaussian ICMF, then the CFR has increased steadily over a Hubble time from ∼1 cluster Gyr -1 15 Gyr ago to its present value. On the other hand, if the ICMF has always been a power law with a slope close to a = -2, the CFR exhibits a minimum some 5 Gyr ago, which we tentatively identify with the well-known age gap in the LMC's cluster age distribution.

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

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.

Star cluster formation and evolution in nearby starburst galaxies -- II. Initial conditions

We use the ages, masses and metallicities of the rich young star cluster systems in the nearby starburst galaxies NGC 3310 and NGC 6745 to derive their cluster formation histories and subsequent evolution. We further expand our analysis of the systematic uncertainties involved in the use of broad-band observations to derive these parameters by examining the effects of a priori assumptions on the individual cluster metallicities. The age (and metallicity) distributions of both the clusters in the circumnuclear ring in NGC 3310 and of those outside the ring are statistically indistinguishable, but there is a clear and significant excess of higher-mass clusters IN the ring compared to the non-ring cluster sample; it is likely that the physical conditions in the starburst ring may be conducive for the formation of higher-mass star clusters, on average, than in the relatively more quiescent environment of the main galactic disc. For the NGC 6745 cluster system we derive a median age of ~10 Myr. NGC 6745 contains a significant population of high-mass "super star clusters", with masses in the range 6.5 <= log(M_cl/M_sun) <= 8.0. This detection supports the scenario that such objects form preferentially in the extreme environments of interacting galaxies. The age of the cluster populations in both NGC 3310 and NGC 6745 is significantly lower than their respective characteristic cluster disruption time-scales. This allows us to obtain an independent estimate of the INITIAL cluster mass function slope, alpha = 2.04(+- 0.23)(+0.13)(-0.43) for NGC 3310, and 1.96(+- 0.15)(+- 0.19) for NGC 6745, respectively, for masses M_cl >= 10^5 M_sun and M_cl >= 4 x 10^5 M_sun. These mass function slopes are consistent with those of other young star cluster systems in interacting and starburst galaxies.

The Missing Link in Star Cluster Evolution

The currently most popular models for the dynamical evolution of star clusters predict that the power-law cluster luminosity functions (CLFs) of young star cluster systems will be transformed rapidly into the universal Gaussian CLFs of old Milky Way-type "globular" cluster systems. Here, we provide the first evidence for a turnover in the intermediate-age ~1 Gyr old CLF in the center of the nearby starburst galaxy M82, which very closely matches the universal CLFs of old Milky Way-type globular cluster systems. Thiors provides an important test of both cluster disruption theories and hierarchical galaxy formation models. It also lends strong support to the scenario that these young cluster systems may eventually evolve into old Milky Way-type globular cluster systems. M82's proximity, its shortest known cluster disruption timescale of any galaxy, and its well-defined peak of cluster formation make it an ideal candidate to probe the evolution of its star cluster system to fainter luminosities, and thus lower masses, than has been possible for any galaxy before.

Star cluster formation and disruption time-scales -- II. Evolution of the star cluster system in the fossil starburst of M82

We obtain new age and mass estimates for the star clusters in the fossil starburst region B of M82, based on improved fitting methods. Our new age estimates confirm the peak in the age histogram attributed to the last tidal encounter with M81; we find a peak formation epoch at slightly older ages than previously published, log(t p e a k yr - 1 ) = 9.04, with a Gaussian a of Δ log(t w i d t h ) = 0.273. The actual duration of the burst of cluster formation may have been shorter because uncertainties in the age determinations may have broadened the peak. Our improved mass estimates confirm that the (initial) masses of the M82 B clusters with V ≤ 22.5 mag are mostly in the range 10 4 -10 6 M O ., with a median mass of M c l = 1.08 x 10 5 M O .. The formation history of the observed clusters shows a steady decrease towards older ages. This indicates that cluster disruption has removed a large fraction of the older clusters. Adopting the expression for the cluster disruption time-scale of t d i s (M) = t d i s 4 (M/10 4 M O .) γ with γ ≃ 0.62 (Paper I), we find that the ratios between the real cluster formation rates in the pre-burst phase [log(t yr - 1 ) ≥ 9.4], the burst phase [8.4 < log(t yr - 1 ) < 9.4] and the post-burst phase [log(t yr - 1 ) ≤ 8.4] are approximately 1: 2: 1/40. The formation rate during the burst may have been higher if the actual duration of the burst was shorter than adopted. The mass distribution of the clusters formed during the burst shows a turnover at log(M c 1 /M O .) ≃ 5.3 that is not caused by selection effects. This distribution can be explained by cluster formation with an initial power-law mass function of slope a = 2 up to a maximum cluster mass of M m a x = 3 x 10 6 M O . and cluster disruption with a normalization time-scale t d i s 4 /t b u r s t = (3.0 ′ 0.3) x 10 - 2 . For a burst age of 1 x 10 9 yr, we find that the disruption time-scale of a cluster of 10 4 M O . is t d i s 4 ∼ 3 x 10 7 yr, with an uncertainty of approximately a factor of 2. This is the shortest disruption time-scale known in any (disc region of a) galaxy.