KEYNOTE TALK: Overview of HST Results on Star Cluster Formation and Destruction

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.

CONTRIBUTED TALK: Basic tools for studies on the formation and disruption of star clusters

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.

CONTRIBUTED TALK: Tidal Tails in Merging Galaxies: Formation of Compact Stellar Structures

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.

CONTRIBUTED TALK: Cluster Formation in Tidal Tails and Intergalactic Space

Iraklis Konstantopoulos

University College London

Star cluster formation is not limited to the 'classical' environments of galactic spiral arms and bright bulges. Observations find them in all types of quiescent milieus, supporting the existence of a single 'mode' of star formation. We present a study of young star clusters in the tidal tails of NGC 6872 and the intergalactic space of Stefan's Quintet. We will discuss our thorough spectroscopic measurements of age, metallicity, extinction and mass that were obtained using the Trancho method. Interestingly, we find very little that is 'interesting' in these environments - that is to say, they do not offer any special parameters as one might expect given their place of creation; the real interest lies in the way they relate to their galaxy-disk counterparts. From these results we draw implications on the cluster formation and evolution process.

CONTRIBUTED TALK: Galaxy Mergers And Star Clusters -- An HST Perspective

Rupali Chandar

University of Toledo

One trademark of gas rich galaxy mergers is a firestorm of star formation. Many (and possibly most) of the stars in these systems form in compact star clusters, some of which reach extremely high masses. The spectacular images returned by the Hubble Space Telescope (HST) allow us to detect and study individual star clusters in nearby galaxy mergers in detail. I will summarize recent results on the star cluster systems in 4 nearby galaxy mergers: the Antennae, NGC520, NGC2623, and NGC3256, based on multi-band imaging taken with HST.