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Register a new userEvolution of Nuclear Star Clusters
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David Merritt
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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.
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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.
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.
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 report on the discovery of several compact regions of mid-infrared emission in the starforming circum nuclear disk of the starburst/Seyfert2 galaxy NGC7582. The compact sources do not have counterparts in the optical and near-infrared, suggesting that they are deeply embedded in dust. We use the [NeII]12.8 micron line emission to estimate the emission measure of the ionized gas, which in turn is used to assess the number of ionizing photons. Two of the brighter sources are found to have ionizing fluxes of ~2.5x10^52, whereas the fainter ones have ~1x10^52 photons/s. Comparing with a one Myr old starburst, we derive stellar masses in the range (3-5)x10^5 Msun, and find that the number of O-stars in each compact source is typically (0.6-1.6)x10^3. We conclude that the compact mid-infrared sources are likely to be young, embedded star clusters, of which only a few are known so far. Our observation highlights the need for high resolution mid-infrared imaging to discover and study embedded star clusters in the proximity of active galactic nuclei.
We present a novel and flexible tensor approach to computing the effect of a time-dependent tidal field acting on a stellar system. The tidal forces are recovered from the tensor by polynomial interpolation in time. The method has been implemented in a direct-summation stellar dynamics integrator (NBODY6) and test-proved through a set of reference calculations: heating, dissolution time and structural evolution of model star clusters are all recovered accurately. The tensor method is applicable to arbitrary configurations, including the important situation where the background potential is a strong function of time. This opens up new perspectives in stellar population studies reaching to the formation epoch of the host galaxy or galaxy cluster, as well as for star-burst events taking place during the merger of large galaxies. A pilot application to a star cluster in the merging galaxies NGC 4038/39 (the Antennae) is presented.
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