CONTENTS

Course: GLOBULAR CLUSTERS IN GALAXIES

Prof. William E. Harris
MacMaster University CANADA

EXTRAGALACTIC CLUSTERS

Globular clusters have been observed in all types of galaxies of practically all sizes, with the possible exception of the smallest dwarfs. In the case of the Milky Way, the clusters are spherically distributed about the Galactic Centre, where they are more densely distributed than in the outer regions of the Galaxy. Investigation of globular clusters has improved our knowledge of the structure of the Galaxy, the first stages of its formation and its chemical evolution, besides providing a huge impulse to the theory of stellar evolution. Obervations of this kind of object in other galaxies is of equally great interest; highly significant similarities and differences between the globular clusters of our own and other galaxies, such as Andromeda or the Magellanic Clouds, have been established, as explained by Professor William Harris of MacMaster University, Canada.

Globular cluster in a distant galaxy

"We can now look back over an entire century of astrophysics research. When we do that, I think the biggest long-range impact from the study of globular clusters has been in our understanding of the evolution of stars. In the first half of the century, the construction of stellar structure theory was strongly driven by the challenge presented by globular clusters: what were their stars? How massive, how old, how metal-poor, and how did they evolve? This pioneering work culminated in the early 1950’s, when there was an amazing convergence of theory with observation: it became possible simultaneously to detect the main-sequence turnoff in nearby globulars, and also to construct believable models that described the entire hydrogen-burning lifetime of a low-mass star. That work showed beyond a doubt that the universe had to be many Gigayears old, and that the earliest stars formed from pristine metal-poor material. These sweeping conclusions could not have come from any other type of object except the globular clusters! In more modern times, with the advent of high-speed computing, CCD detectors, and ultra-deep imaging from space, we have tremendously refined the whole grand drama of stellar evolution. It’s very fitting that, as the century (and millenium!) is drawing to an end, we are pushing to reach the end of the white-dwarf sequence — the very last evolutionary stage for globular cluster stars."

"The age calibrations for the globular clusters in the Milky Way still provide us with the strongest limit we have to the age of the Universe. 30 years ago, we did not know this quantity to a factor of two; now, we are arguing over differences of 25 percent. On the observational side, still better answers are going to depend on calibrating distances and metallicities to a few percent accuracy. On the theoretical side, we have to settle a long list of remaining uncertainties such as the degree of mixing and semiconvection, opacities and reaction rates, and the amount of helium diffusion."

"We think that all large galaxies have old-halo globular clusters, and so far, all the observations support this idea. In fact, a galaxy has to be very small — at the lower end of the dwarf sequence — to have none at all. Globular clusters do indeed seem to represent a common theme in the early history of galaxy formation. Modern imaging technology makes it easy to find the brightest globular clusters in quite remote galaxies; the current distance ‘record’ was set a few years ago for the central cD galaxy in Abell 2107 at a redshift of 12600 Km/s (or about 180 Megaparsecs if H=70), in which my colleagues and I detected the brightest globular clusters.

Individually, the globular clusters in other galaxies are surprisingly similar to the ones in our Milky Way in their range of luminosities, metallicities, and (apparently) in their formation history. However, there are striking differences as well, such as in total numbers – some cD galaxies may have 20,000 clusters compared with the 150 or so in our Galaxy. The way in which clusters are distributed in metallicity – the relative numbers at high and low [Fe/H] – also differ between otherwise similar galaxies in ways that are not yet understood."

"The most challenging problem is unquestionably to understand how globular clusters form. We know that they can be built in all kinds of conditions: within isolated dwarfs, in starbursts, in the halos of giant galaxies, in mergers between existing galaxies. All that we need, apparently, is a sufficiently large supply of gas collected together. But what governs the way that this gas turns itself into gravitationally bound systems of stars? What determines their near-universal formation efficiency of about 0.2%? What processes fix the mass distributions function, which again has a near-universal shape? These are large questions which have only the vaguest first order answers at the present time?"

William E. Harris was born November 28, 1947 in Edmonton, Canada. As an undergraduate, he studied mathematics at the University of Alberta and graduated there in 1969. Graduate school followed at the University of Toronto, with a Master’s degree in theoretical astrophysics in 1970 and PhD in astronomy in 1974, on "Globular Clusters in the Local Group Galaxies". After spending two years at Yale as a postdoctoral fellow, he moved to a faculty position at McMaster University in Hamilton, where he has been happy to stay conducting his teaching and research. As a graduate student, he developed a fascination with globular clusters and their place in galactic structure and galactic history which has lasted ever since. "No other subject in modern astrophysics has such a long history, or has re-invented itself so many times through technological advances and unexpected discoveries." Most of his current research is aimed at understanding the characteristics of globular cluster systems in giant elliptical galaxies, and at the ages and formation histories of globular clusters and their role in the earliest stages of galaxies.
Harris has found it a "terrific experience’’ to use telescopes all around the world during the period of rapid growth of astronomy beginning in the early 1970’s and continuing into today’s large-telescope era. He has chaired time allocation panels for the CFHT and HST, and was recently President of the Canadian Astronomical Society. In his free time, he enjoys choir singing (definitely as a bass), tennis in the summer, and in the winter the ice sport of curling — a strange and unique game invented long ago in Scotland but now played more in Canada than anywhere else.