Linda J. Smith
Space Telescope Science Institute
Observations with the Hubble Space Telescope have revolutionised the study
of extragalactic star cluster systems. I will review the contribution that
HST has made to our understanding of cluster formation and destruction, and
discuss the different models proposed for the evolution of clusters.
I will also present the latest news on Hubble, the new instruments and the servicing mission.
Mark Gieles
European Southern Observatory, Santiago, Chile
The luminosity functions (LFs) of star clusters in a wide range of
galactic environments can be well approximated by a power-law
function, with an index near -2.3 or -2. A fundamental property of
such a distribution function is that the luminosity of the brightest
object (L_max) scales (almost) linearly with the total number of
clusters, or the SFR of the host galaxy. This suggests that the
formation of Young Massive Clusters (YMCs) in interacting and merging
galaxies is simply a size-of-sample effect, i.e. due to the high SFR
in these enviroments it is statistically more likely to form YMCs
there. In this contribution I will provide evidence that the LFs of
the cluster populations of M51, M74 and M101 are not simple
power-laws, but show a steepening at high luminosity. From the
luminosity dependent index of the LF and its vertical offset it is
found that the underlying cluster IMF has a Schechter type truncation
around M*=(1-2)x105 M_sun and the cluster formation efficiency is
~10%, i.e. 10% of the stars form in bound star clusters. When
including mergers and dwarf galaxies in the L_max vs. SFR relation a
scaling of the truncation mass with the SFR of the form M* ~ SFR0.5
is needed to explain the data. Additional dependencies of these
results on various disruption scenarios will be discussed.
Brendan Mullan
The Pennsylvania State University
Using V- and I- band images from the Wide Field Planetary Camera 2 (WFPC2) on the Hubble Space Telescope (HST), we have found and studied compact stellar structures within the tidal tails of twelve different pairs of merging galaxies. These galaxies span the physical parameter space of interacting pairs (i.e. HI and stellar mass, pressure, and density within the tidal tail environments), allowing a refinement of the conditions in which star cluster formation is permitted. The twelve-object sample of interactions includes a diverse population of tail lengths, optical brightnesses, merging mass ratios, HI column densities, and stage on the Toomre (1977) sequence. Of this set, six images (NGC 4747, NGC 2782E, NGC 2535, NGC 1614, and NGC 6872) show significant excesses of star cluster formation in tidal tail debris. These have a wide range of merger stages, tail lengths, and optical brightnesses, but are HI-rich in all tails for which the HI column density is known. In all cases, we have used VI imaging to estimate ages of stellar populations in the tails, which are compared to merger dynamical time scales, the merger stage, colors of diffuse light in the tails, and data in the literature on star formation within the host galaxies.
KEYNOTE TALK: Overview of HST Results on Star Cluster Formation and Destruction
CONTRIBUTED TALK: Basic tools for studies on the formation and disruption of star clusters
CONTRIBUTED TALK: Tidal Tails in Merging Galaxies: Formation of Compact Stellar Structures