arXiv subjects

Mark Gieles

Publications and source records attributed to Mark Gieles.

119 records · Page 7Linked to original sources

arXiv2008

Mass segregation in young star clusters: can it be detected from the integrated photometric properties?

We consider the effect of mass segregation on the observable integrated properties of star clusters. The measurable properties depend on a combination of the dynamical age of the cluster and the physical age of the stars in the cluster. To investigate all possible combinations of these two quantities we propose an analytical model for the mass function of segregated star clusters that agrees with the results of N-body simulations, in which any combination can be specified. For a realistic degree of mass segregation and a fixed density profile we find with increasing age an increase in the measured core radii and a central surface brightness that decreases in all filters more rapidly than what is expected from stellar evolution alone. Within a Gyr the measured core radius increases by a factor of two and the central surface density in all filters of a segregated cluster will be overestimated by a similar factor when not taking into account mass segregation in the conversion from light to mass. We find that the $V-I$ colour of mass segregated clusters decreases with radius by about 0.1-0.2 mag, which could be observable. From recent observations of partially resolved extra-galactic clusters a decreasing half-light radius with increasing wavelength was observed, which was attributed to mass segregation. These observations can not be reproduced by our models. We find that the differences between measured radii in different filters are always smaller than 5%.

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arXiv2008

An alternative method to study star cluster disruption

Many embedded star clusters do not evolve into long-lived bound clusters. The most popular explanation for this "infant mortality" of young clusters is the expulsion of natal gas by stellar winds and supernovae, which leaves up to 90% of them unbound. A cluster disruption model has recently been proposed in which this mass- independent disruption of clusters proceeds for another Gyr after gas expulsion. In this scenario, the survival chances of massive clusters are much smaller than in the traditional mass-dependent disruption models. The most common way to study cluster disruption is to use the cluster age distribution, which, however, can be heavily affected by incompleteness. To avoid this, we introduce a new method, based on size-of-sample effects, namely the relation between the most massive cluster, M_max, and the age range sampled. Assuming that clusters are sampled from a power-law initial mass function, with index -2 and that the cluster formation rate is constant, M_max scales with the age range sampled, such that the slope in a log(M_max) vs. log(age) plot is equal to unity. This slope decreases if mass-independent disruption is included. For 90% mass-independent cluster disruption per age dex, the predicted slope is zero. For the solar neighbourhood, SMC, LMC, M33, and M83, based on ages and masses taken from the literature, we find slopes consistent with the expected size-of-sample correlations for the first 100 Myr, hence ruling out the 90% mass-independent cluster disruption scenario. For M51, however, the increase of log(M_max) with log(age) is slightly shallower and for the Antennae galaxies it is flat. This simple method shows that the formation and/or disruption of clusters in the Antennae must have been very different from that of the other galaxies studied here, so it should not be taken as a representative case.

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arXiv2007

Star cluster life-times: dependence on mass, radius and environment

The dissolution time (t_dis) of clusters in a tidal field does not scale with the ``classical'' expression for the relaxation time. First, the scaling with N, and hence cluster mass, is shallower due to the finite escape time of stars. Secondly, the cluster half-mass radius is of little importance. This is due to a balance between the relative tidal field strength and internal relaxation, which have an opposite effect on t_dis, but of similar magnitude. When external perturbations, such as encounters with giant molecular clouds (GMC) are important, t_dis for an individual cluster depends strongly on radius. The mean dissolution time for a population of clusters, however, scales in the same way with mass as for the tidal field, due to the weak dependence of radius on mass. The environmental parameters that determine t_dis are the tidal field strength and the density of molecular gas. We compare the empirically derived t_dis of clusters in six galaxies to theoretical predictions and argue that encounters with GMCs are the dominant destruction mechanism. Finally, we discuss a number of pitfalls in the derivations of t_dis from observations, such as incompleteness, with the cluster system of the SMC as particular example.

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arXiv2006

Cluster Disruption: Combining Theory and Observations

We review the theory and observations of star cluster disruption. The three main phases and corresponding typical timescales of cluster disruption are: I) Infant Mortality (~10^7 yr), II) Stellar Evolution (~10^8 yr) and III) Tidal relaxation (~10^9 yr). During all three phases there are additional tidal external perturbations from the host galaxy. In this review we focus on the physics and observations of Phase I and on population studies of Phases II & III and external perturbations concentrating on cluster-GMC interactions. Particular attention is given to the successes and short-comings of the Lamers cluster disruption law, which has recently been shown to stand on a firm physical footing.

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arXiv2006

Clusters in the solar neighbourhood: how are they destroyed?

We predict the survival time of initially bound star clusters in the solar neighbourhood taking into account: (1) stellar evolution, (2) tidal stripping, (3) shocking by spiral arms and (4) encounters with giant molecular clouds. We find that the predicted dissolution time is t_dis= 1.7 (M_i/10^4 M_sun)^0.67 Gyr for clusters in the mass range of 10^2 < M_i < 10^5 M_sun, where M_i is the initial mass of the cluster.. The resulting predicted shape of the logarithmic age distribution agrees very well with the empirical one, derived from a complete sample of clusters in the solar neighbourhood within 600 pc. The required scaling factor implies a star formation rate of 400 M_sun/Myr within 600 pc from the Sun or a surface formation rate of 3.5 10^-10 M_sun/(yr pc^2) for stars in bound clusters with an initial mass in the range of 10^2 to 3 10^4 M_sun.

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arXiv2006

The luminosity function of young star clusters: implications for the maximum mass and luminosity of clusters

We introduce a method to relate a possible truncation of the star cluster mass function at the high mass end to the shape of the cluster luminosity function (LF). We compare the observed LFs of five galaxies containing young star clusters with synthetic cluster population models with varying initial conditions. The LF of the SMC, the LMC and NGC 5236 are characterized by a power-law behavior NdL~L^-a dL, with a mean exponent of = 2.0 +/- 0.2. This can be explained by a cluster population formed with a constant cluster formation rate, in which the maximum cluster mass per logarithmic age bin is determined by the size-of-sample effect and therefore increases with log(age/yr). The LFs of NGC 6946 and M51 are better described by a double power-law distribution or a Schechter function. When a cluster population has a mass function that is truncated below the limit given by the size-of-sample effect, the total LF shows a bend at the magnitude of the maximum mass, with the age of the oldest cluster in the population, typically a few Gyr due to disruption. For NGC 6946 and M51 this implies a maximum mass of M_max = 5*10^5 M_sun. Faint-ward of the bend the LF has the same slope as the underlying initial cluster mass function and bright-ward of the bend it is steeper. This behavior can be well explained by our population model. We compare our results with the only other galaxy for which a bend in the LF has been observed, the ``Antennae'' galaxies (NGC 4038/4039). There the bend occurs brighter than in NGC 6946 and M51, corresponding to a maximum cluster mass of M_max = 2*10^6 M_sun (abridged).

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arXiv2005

Observational evidence for a truncation of the star cluster initial mass function at the high mass end

We present the luminosity function (LF) of star clusters in M51 based on HST/ACS observations taken as part of the Hubble Heritage project. The clusters are selected based on their size and with the resulting 5990 clusters we present one of the largest cluster samples of a single galaxy. We find that the LF can be approximated with a double power-law distribution with a break around M_V = -8.9. On the bright side the index of the power-law distribution is steeper (a = 2.75) than on the faint-side (a = 1.93), similar to what was found earlier for the ``Antennae'' galaxies. The location of the bend, however, occurs about 1.6 mag fainter in M51. We confront the observed LF with the model for the evolution of integrated properties of cluster populations of Gieles et al., which predicts that a truncated cluster initial mass function would result in a bend in, and a double power-law behaviour of, the integrated LF. The combination of the large field-of view and the high star cluster formation rate of M51 make it possible to detect such a bend in the LF. Hence, we conclude that there exists a fundamental upper limit to the mass of star clusters in M51. Assuming a power-law cluster initial mass function with exponentional cut-off of the form NdM ~ M^-b * exp(-M/M_C)dM, we find that M_C = 10^5 M_sun. A direct comparison with the LF of the ``Antennae'' suggests that there M_C = 4*10^5 M_sun.

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arXiv2005

Hierarchical Star Formation in M51: Star/Cluster Complexes

We report on a study of young star cluster complexes in the spiral galaxy M51. Recent studies have confirmed that star clusters do not form in isolation, but instead tend to form in larger groupings or complexes. We use {\it HST} broad and narrow band images (from both {\it WFPC2} and {\it ACS}), along with {\it BIMA}-CO observations to study the properties and investigate the origin of the e complexes. We find that the complexes are all young ($< 10$ Myr), have sizes between $\sim$85 and $\sim$240 pc, and have masses between 3-30 $\times 10^{4} M_{\odot}$. Unlike that found for isolated young star clusters, we find a strong correlation between the complex mass and radius, namely $M\propto R^{2.33 \pm 0.19}$. This is similar to that found for giant molecular clouds (GMCs). By comparing the mass-radius relation of GMCs in M51 to that of the complexes we can estimate the star formation efficiency within the complexes, although this value is heavily dependent on the assumed CO-to-H$_2$ conversion factor. The complexes studied here have the same surface density distribution as individual young star clusters and GMCs. If star formation within the complexes is proportional to the gas density at that point, then the shared mass-radius relation of GMCs and complexes is a natural consequence of their shared density profiles. We briefly discuss possibilities for the lack of a mass-radius relation for young star clusters. We note that many of the complexes show evidence of merging of star clusters in their centres, suggesting that larger star clusters can be produced through the build up of smaller clusters.

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arXiv2005

The Star Cluster Population of M51: III. Cluster disruption and formation history

In this work we concentrate on the evolution of the cluster population of the interacting galaxy M51 (NGC 5194), namely the timescale of cluster disruption and possible variations in the cluster formation rate. We present a method to compare observed age vs. mass number density diagrams with predicted populations including various physical input parameters like the cluster initial mass function, cluster disruption, cluster formation rate and star bursts. If we assume that the cluster formation rate increases at the moments of the encounters with NGC 5195, we find an increase in the cluster formation rate of a factor of 3, combined with a disruption timescale which is slightly higher then when assuming a constant formation rate (t_4 = 200 Myr vs. 100 Myr). The measured cluster disruption time is a factor of 5 shorter than expected on theoretical grounds. This implies that the disk of M51 is not a preferred location for survival of young globular clusters, since even clusters with masses of the order of 10^6 M_sun will be destroyed within a few Gyr.

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arXiv2005

An analytical description of the disruption of star clusters in tidal fields with an application to Galactic open clusters

We present a simple analytical description of the disruption of star clusters in a tidal field, which agrees excellently with detailed N-body simulations. The analytic expression can be used to predict the mass and age histograms of surviving clusters for any cluster initial mass function and any cluster formation history. The method is applied to open clusters in the solar neighbourhood, based on the new cluster sample of Kharchenko et al. From a comparison between the observed and predicted age distributions in the age range between 10 Myr to 3 Gyr we find the following results: (1) The disruption time of a 10^4 M_sun cluster in the solar neighbourhood is about 1.3+/-0.5 Gyr. This is a factor 5 shorter than derived from N-body simulations of clusters in the tidal field of the galaxy. (2) The present starformation rate in bound clusters within 600 pc from the Sun is 5.9+/-0.8 * 10^2 M_sun / Myr, which corresponds to a surface star formation rate in bound clusters of 5.2+/-0.7 10^(-10) M_sun/yr/pc^2. (3) The age distribution of open clusters shows a bump between 0.26 and 0.6 Gyr when the cluster formation rate was 2.5 times higher than before and after. (4) The present star formation rate in bound clusters is half as small as that derived from the study of embedded clusters. The difference suggests that half of the clusters in the solar neighbourhood become unbound within 10 Myr. (5) The most massive clusters within 600 pc had an initial mass of 3*10^4 M_sun. This is in agreement with the statistically expected value based on a cluster initial mass function with a slope of -2, even if the physical upper mass limit is as high as 10^6 M_sun.

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arXiv2004

Disruption time scales of star clusters in different galaxies

The observed average lifetime of the population of star clusters in the Solar Neighbourhood, the Small Magellanic Cloud and in selected regions of M51 and M33 is compared with simple theoretical predictions and with the results of N-body simulations. The empirically derived lifetimes (or disruption times) of star clusters depend on their initial mass as t_dis ~ Mcl^0.60 in all four galaxies. N-body simulations have shown that the predicted disruption time of clusters in a tidal field scales as t_dis^pred ~ t_rh^0.75 t_cr^0.25, where t_rh is the initial half-mass relaxation time and t_cr is the crossing time for a cluster in equilibrium. We show that this can be approximated accurately by t_dis^pred ~ M_cl^0.62 for clusters in the mass range of about 10^3 to 10^6 M_sun, in excellent agreement with the observations. Observations of clusters in different extragalactic environments show that t_dis also depends on the ambient density in the galaxies where the clusters reside. Linear analysis predicts that the disruption time will depend on the ambient density of the cluster environment as t_dis ~ rho_amb^-0.5. This relation is consistent with N-body simulations.

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