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Dynamical Evolution and the Mass Function of Globular Cluster Systems

Dynamical Evolution and the Mass Function of Globular Cluster Systems. Steve Zepf and Enrico Vesperini Chris Waters Keith Ashman Arunav Kundu.

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Dynamical Evolution and the Mass Function of Globular Cluster Systems

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  1. Dynamical Evolution and the Mass Function of Globular Cluster Systems Steve Zepf and Enrico Vesperini Chris Waters Keith Ashman Arunav Kundu

  2. How to make globular cluster systems? •GC formation in starbursts, mergers, and the like is observed (as predicted) • Young GC systems (Antennae) have a power-law mass functions • Old GC systems have log-normal mass functions • Evaporation nicely explains such evolution, and gets the turnover mass and faint end slope right for GCs in the inner regions of galaxies (see Waters & Zepf poster). • Even know large majority of star clusters do not survive early mass loss (e.g. Zepf et al. 1999, Fall 2004). BUT

  3. How to make globular cluster systems? •GC formation in starbursts, mergers, and the like is observed (as predicted) • Young GC systems (Antennae) have a power-law mass functions • Old GC systems have log-normal mass functions • Evaporation nicely explains such evolution, and gets the turnover mass and faint end slope right for GCs in the inner regions of galaxies (see Waters & Zepf poster). • Even know large majority of star clusters do not survive early mass loss (e.g. Zepf et al. 1999, Fall 2004). BUT

  4. GC mass function ~constant with distance from the center of the host galaxy Problem: Evaporation naturally produces a radial gradient as mass loss occurs more rapidly for GCs in central regions because of smaller tidal radii (as discussed by many people here). Data: M87 conclusively shows the constancy of GCMF with distance from center of galaxy (Vesperini et al. 2003) Can this be explained by assuming all GCs have the same pericenter? NO – GC velocity data rule out such a strong radial anisotropy. What’s left?

  5. Ideas • Vesperini & Zepf (2003) – if GC systems begin with more massive GCs being more concentrated, then the massive GCs are more likely to survive early stellar mass loss than lower mass ones (following classic work on effects of concentration on GC survival, e.g. Chernoff & Weinberg).  can produce roughly lognormal mass function out of an initial power-law without radial dependence. Questions – Is the timescale too short compared to data indicating power-law mass function? Not yet, but more tests coming. Does it work when modeled with more realistic spatial distributions for young clusters? Even with great progress on GC formation, getting to observed old GC system mass function open question.

  6. M87 GC Mass Function(r) Vesperini, Zepf, Kundu, & Ashman 2003

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