IAU Symposium 266
Star Clusters
Basic Galactic Building Blocks Throughout Time And Space

 

Scientific rationale

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SCIENTIFIC RATIONALE

 

It is widely accepted that stars do not form in isolation, but result from the fragmentation of molecular clouds, which in turn leads to star cluster formation. Over time, clusters dissolve or are destroyed by interactions with molecular clouds or tidal striping, and their members become part of the general field population. Star clusters are thus among the basic building blocks of galaxies.

In turn, star cluster populations, from young associations and open clusters to old globulars, are powerful tracers of the formation, assembly and evolutionary history of their parent galaxies. Although their importance (e.g., in mapping out the Milky Way) had been recognised for decades, major progress in this area has only become possible in recent years, both for Galactic and extragalactic cluster populations. This area has seen a major recent investment in time and effort, largely thanks to significant new resources in theory, simulations and observations, including breakthroughs in computational power, the advent of Hubble Space Telescope-driven science, deep and more precise data for large numbers of Galactic clusters, and an explosion of astrometric data (Hipparcos, UCAC2).

In the summer of 2009, several years will have passed since a number of very successful IAU symposia were held on aspects related to the build-up of galaxies and their basic building blocks. IAU Symposium 254 on "The Galactic Disk in Cosmological Context" (June 2008), IAU Symposium 241 on "Stellar populations as basic building blocks of galaxies" (December 2006) and IAU Symposium 235 on "Galaxy Evolution across the Hubble Time" (August 2006) addressed galaxy formation and evolution in a wider sense, while IAU Symposium 246 on "Dynamical Evolution of Dense Stellar Systems" (September 2007) and IAU Symposium 207 on "Extragalactic Star Clusters" (March 2001) focused on star clusters in their own right.

While most of these past Symposia have addressed individual aspects of galaxy formation and evolution, at the present time we feel the need for a new Symposium encompassing many of the individual aspects in a more general approach, and in particular focusing on star clusters (including the full range from very young associations and open clusters to young massive star clusters and old globulars), and star cluster populations in the wider context of their parent galaxies. Putting these results and new developments related to star clusters as individual entities into the broader context of galaxy evolution is the next logical step, which requires the combined efforts of theorists, observers and modelers working on a large variety of spatial scales, and spanning a very wide range of expertise. We are now reaching the stage that we are within reach of answering a number of fundamental questions that will have a significant impact on our understanding of numerous related issues as well, ranging from the formation, assembly and evolution of galaxies, to the details of the very process of star formation itself. These two issues are the backbone of research in modern astrophysics. Thus, we propose to focus on the role of star clusters, of any size and age, and their stellar populations in the overall context of galaxy evolution, across space (from local to high redshift) and time (from currently forming to fossil remnants).

Star clusters are the observational foundation for stellar astrophysics and evolution, provide essential tracers of galactic structure, and are unique stellar dynamical environments. Star formation, stellar structure, and stellar evolution continue to benefit and improve tremendously from the study of these systems. Additionally, fundamental quantities such as the initial mass function (IMF) can be successfully derived from modelling either the Hertzsprung-Russell diagrams or the integrated velocity structures (i.e., for massive clusters, the velocity dispersions, leading to dynamical mass estimates, and combined with integrated luminosities and independent age estimates) of, respectively, resolved and unresolved clusters and cluster populations. Star cluster studies thus span the fields of Galactic and extragalactic astrophysics, while heavily affecting our detailed understanding of the process of star formation in dense environments.

Globular clusters are regarded as fossil records of the earliest epochs of galaxy formation, including that of our own Milky Way. At the same time, the young massive star clusters currently being formed in the most violent starburst environments may be viable proto-globular clusters. Thus, star clusters form an ideal testbed for stellar evolution theories and are in fact among the best tools to study extreme stellar populations, such as X-ray binaries or "blue stragglers", both of which are likely products of dynamical interactions within clusters. However, dynamical modeling of clusters and entire cluster systems, at any age, still poses a considerable challenge for both theory and computational requirements.

Recent advances in instrumentation are driving a renaissance in the study of Galactic clusters, while extragalactic cluster studies are significantly aided by the development of new instrumentation supporting ever wider fields of view. New wide-field imaging cameras on several 4-8 meter telescopes offer the unique opportunity to study entire cluster populations to very faint magnitudes in a single pointing, both in the optical and near-infrared (e.g., VISTA). Complementing these photometric capabilities are a new generation of multi-object spectrographs and multiplexed integral-field units on 8-10 meter telescopes. In addition, from Chandra/XMM and GALEX at short wavelengths to the Spitzer Space Telescope in the mid and far-infrared, our observational window in which to study both the star clusters and their stellar populations are unsurpassed in terms of wavelength coverage and spatial resolution at the present time. With major efforts being expended on the planning for possible 50-70 meter "extremely large telescopes" (in both Europe and the USA), now is the opportune time to look forward to future progress in mapping a representative slice of the local Universe at the highest possible resolution and thus in the greatest detail ever achieved. Combining the emerging, unprecedented understanding of local stellar populations with observations of galaxies ("composite" stellar populations) at ever higher redshifts, we now have a fighting chance to constrain the evolution of the basic galactic building blocks throughout space and time.

With all of these exciting efforts ongoing at the present time, we believe that this is an opportune time to organise a Symposium as outlined in this proposal in the summer of 2009.