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Register a new userCore radius evolution of star clusters
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Mark I. Wilkinson
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We use N-body simulations of star clusters to investigate the possible dynamical origins of the observed spread in core radius among intermediate-age and old star clusters in the Large Magellanic Cloud (LMC). Two effects are considered, a time-varying external tidal field and variations in primordial hard binary fraction. Simulations of clusters orbiting a point-mass galaxy show similar core radius evolution for clusters on both circular and elliptical orbits and we therefore conclude that the tidal field of the LMC has not yet significantly influenced the evolution of the intermediate-age clusters. The presence of large numbers of hard primordial binaries in a cluster leads to core radius expansion; however, the magnitude of the effect is insufficient to explain the observations. Further, the range of binary fractions required to produce significant core radius growth is inconsistent with the observational evidence that all the LMC clusters have similar stellar luminosity functions.
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Cool-core clusters are characterized by strong surface brightness peaks in the X-ray emission from the Intra Cluster Medium (ICM). This phenomenon is associated with complex physics in the ICM and has been a subject of intense debate and investigation in recent years. In order to quantify the evolution in the cool-core cluster population, we robustly measure the cool-core strength in a local, representative cluster sample, and in the largest sample of high-redshift clusters available to date. We use high-resolution Chandra data of three representative cluster samples spanning different redshift ranges: (i) the local sample from the 400 SD survey with median z = 0.08, (ii) the high redshift sample from the 400 SD Survey with median z=0.59, and (iii) 15 clusters drawn from the RDCS and the WARPS, with median z = 0.83. Our analysis is based on the measurement of the surface brightness concentration, c_SB, which allows us to characterize the cool-core strength in low signal-to-noise data. We also obtain gas density profiles to derive cluster central cooling times and entropy. In addition to the X-ray analysis, we search for radio counterparts associated with the cluster cores. We find a statistically significant difference in the c_SB distributions of the two high-z samples, pointing towards a lack of concentrated clusters in the 400 SD high-z sample. Taking this into account, we confirm a negative evolution in the fraction of cool-core clusters with redshift, in particular for very strong cool-cores. This result is validated by the central entropy and central cooling time, which show strong anti-correlations with c_SB. However, the amount of evolution is significantly smaller than previously claimed, leaving room for a large population of well formed cool-cores at z~1.
Two-body relaxation times of nuclear star clusters are short enough that gravitational encounters should substantially affect their structure in 10 Gyr or less. In nuclear star clusters without massive black holes, dynamical evolution is a competition between core collapse, which causes densities to increase, and heat input from the surrounding galaxy, which causes densities to decrease. The maximum extent of a nucleus that can resist expansion is derived numerically for a wide range of initial conditions; observed nuclei are shown to be compact enough to resist expansion, although there may have been an earlier generation of low-density nuclei that were dissolved. An evolutionary model for NGC 205 is presented which suggests that the nucleus of this galaxy has already undergone core collapse. Adding a massive black hole to a nucleus inhibits core collapse, and nuclear star clusters with black holes always expand, due primarily to heat input from the galaxy and secondarily to heating from stellar disruptions. The expansion rate is smaller for larger black holes due to the smaller temperature difference between galaxy and nucleus when the black hole is large. The rate of stellar tidal disruptions and its variation with time are computed for a variety of initial models. The disruption rate generally decreases with time due to the evolving nuclear density, particularly in the faintest galaxies, assuming that scaling relations derived for luminous galaxies can be extended to low luminosities.
We study the evolution of embedded clusters. The equations of motion of the stars in the cluster are solved by direct N-body integration while taking the effects of stellar evolution and the hydrodynamics of the natal gas content into account. The gravity of the stars and the surrounding gas are coupled self consistently to allow the realistic dynamical evolution of the cluster. While the equations of motion are solved, a stellar evolution code keeps track of the changes in stellar mass, luminosity and radius. The gas liberated by the stellar winds and supernovae deposits mass and energy into the gas reservoir in which the cluster is embedded. We examine cluster models with 1000 stars, but we varied the star formation efficiency (between 0.05-0.5), cluster radius (0.1-1.0 parsec), the degree of virial support of the initial population of stars (0-100%) and the strength of the feedback. We find that an initial star fraction $M_star/M_{rm tot} > 0.05$ is necessary for cluster survival. Survival is more likely if gas is not blown out violently by a supernova and if the cluster has time to approach virial equilibrium during out-gassing. While the cluster is embedded, dynamical friction drives early and efficient mass segregation in the cluster. Stars of $m gtrsim 2,M_odot$ are preferentially retained, at the cost of the loss of less massive stars. We conclude that the degree of mass segregation in open clusters such as the Pleiades is not the result of secular evolution but a remnant of its embedded stage.
Until now it has been impossible to observationally measure how star cluster scale height evolves beyond 1Gyr as only small samples have been available. Here we establish a novel method to determine the scale height of a cluster sample using modelled distributions and Kolmogorov-Smirnov tests. This allows us to determine the scale height with a 25% accuracy for samples of 38 clusters or more. We apply our method to investigate the temporal evolution of cluster scale height, using homogeneously selected sub-samples of Kharchenko et al. (MWSC), Dias et al. (DAML02), WEBDA, and Froebrich et al. (FSR). We identify a linear relationship between scale height and log(age/yr) of clusters, considerably different from field stars. The scale height increases from about 40pc at 1Myr to 75pc at 1Gyr, most likely due to internal evolution and external scattering events. After 1Gyr, there is a marked change of the behaviour, with the scale height linearly increasing with log(age/yr) to about 550pc at 3.5Gyr. The most likely interpretation is that the surviving clusters are only observable because they have been scattered away from the mid-plane in their past. A detailed understanding of this observational evidence can only be achieved with numerical simulations of the evolution of cluster samples in the Galactic Disk. Furthermore, we find a weak trend of an age-independent increase in scale height with galactocentric distance. There are no significant temporal or spatial variations of the cluster distribution zero point. We determine the Suns vertical displacement from the Galactic Plane as $Z_odot=18.5pm1.2$pc.
We use HST/ACS observations of the spiral galaxy M51 in F435W, F555W and F814W to select a large sample of star clusters with accurate effective radius measurements in an area covering the complete disc of M51. We present the dataset and study the radius distribution and relations between radius, colour, arm/interarm region, galactocentric distance, mass and age. We select a sample of 7698 (F435W), 6846 (F555W) and 5024 (F814W) slightly resolved clusters and derive their effective radii by fitting the spatial profiles with analytical models convolved with the point spread function. The radii of 1284 clusters are studied in detail. We find cluster radii between 0.5 and ~10 pc, and one exceptionally large cluster candidate with a radius of 21.6 pc. The median radius is 2.1 pc. We find 70 clusters in our sample which have colours consistent with being old GC candidates and we find 6 new faint fuzzy clusters in, or projected onto, the disc of M51. The radius distribution can not be fitted with a power law, but a log-normal distribution provides a reasonable fit to the data. This indicates that shortly after the formation of the clusters from a fractal gas, their radii have changed in a non-uniform way. We find an increase in radius with colour as well as a higher fraction of redder clusters in the interarm regions, suggesting that clusters in spiral arms are more compact. We find a correlation between radius and galactocentric distance which is considerably weaker than the observed correlation for old Milky Way GCs. We find weak relations between cluster luminosity and radius, but we do not observe a correlation between cluster mass and radius.
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