The Evolution of Galaxies

X CANARY ISLANDS WINTER SCHOOL OF ASTROPHYSICS "Globular Clusters"

Course: STELLAR DYNAMICS IN GLOBULAR CLUSTERS

Prof. Rebecca A. W. Elson
Institute of Astronomy, Cambridge UNITED KINGDOM

THE EVOLUTION OF GALAXIES

A typical globular cluster contains from a hundred thousand to a million stars bound together by the gravitational force they exert on one another. The density of stars in a cluster increases as we approach the centre, where it can reach values thousands of times greater than the density of stars in the solar neighbourhood. The clusters associated with our Galaxy are spherically distributed about the Galactic Centre, in the direction of Sagittarius, at about 8 kiloparsecs from the Sun. About two hundred are known and each one moves in an elongated orbit with a period of revolution of about two hundred million years that takes it through the Galactic plane twice each revolution. These crossings raise gravitational tides within the clusters, which lose some of their outer stars to the Galaxy. The orbital velocity of a cluster can be derived from the total mass of its host galaxy and its distance from the centre of the galaxy. The dynamics of globular clusters provides one of the most reliable means of studying the evolution and structure of galaxies.

Globular Cluster NGC1818, in the Large Magellanic Cloud

What do you think are the most important contributions of the study of globular clusters to our knowledge of the Universe? What type of key information may be found when studying globular clusters exclusively?

"The globular clusters in the halo of our Galaxy give us the strictest lower limit on the age of the Universe. Globular clusters in our own and other galaxies provide a unique opportunity to study the early stages of evolution in galaxies of various types. They are also ideal laboratories for testing theories of stellar evolution, of the dynamical evolution of large-N self-gravitating systems, and for quantifying the stellar initial mass function, and investigating its universality. This last issue is crucial for interpreting the integrated light of distant galaxies and drawing conclusions about the star formation history of the Universe"

How old are the globular clusters? By how much do you expect this age to vary?

"The terms ‘globular’ and ‘open’ clusters were coined to distinguish ancient, rich clusters in the halo of our Galaxy, from the sparser, younger clusters in the disk. It has become apparent that these categories are not altogether appropriate for the cluster systems of other galaxies. The Large Magellanic Cloud, for example, contains clusters rich enough to merit the description ‘globular’, but not necessarily ancient. Massive star clusters have also been observed forming in mergers of distant galaxies. I would therefore say a globular cluster can be any age! Recent results using Hipparcos parallaxes find an allowable range of ages for the oldest globular clusters of 9.5 – 13.9 Gyr, with a most probable value around 11.8 Gyr."

Taking into account the results of dynamic models in regard to the survival of clusters in different environments, would it be possible to obtain information, or an indication, of these systems' ages, from the present population of their clusters?

"It would be difficult to estimate the ages of globular cluster systems from assessments of their survival rates. There are many unknowns, including the cluster orbits, the kind of dynamical stresses they would have encountered as the galaxy was forming, the initial range of masses and densities of the clusters, and their efficiency of formation from their progenitor clouds. It would be easier to turn the question around and, using the relatively accurate ages measured for globular clusters in our own Galaxy, for example, place constraints on the kinds of disruptive processes they could have been subjected to, and survived, in their lifetimes."

In relation to the subject of this Winter School, what is the most interesting problem in your present research?

"Globular cluster systems in elliptical galaxies appear to have colour distributions that vary significantly from galaxy to galaxy. Some have only blue, or metal poor clusters, while others have both metal poor an metal rich populations. Such bimodality may be the result of mergers or of a multi-phase collapse of the parent galaxy, and contains important clues about the early stages of galaxy formation. Accurate photometry for more systems is needed to form a more complete picture, and this is one of the goals of my current research. A second question my research addresses is the universality of the stellar initial mass function (IMF). Large Magellanic Cloud clusters provide an ideal laboratory for exploring this, and I am leading a large HST project to determine deep IMFS in 8 clusters with a wide range of ages and central densities."

PROFILE

REBECCA A. W. ELSON was born in Montreal (Canada) on 2 January 1960.
She now lives in the United Kingdom but retains double Canadian and American nationality.
After graduating in Astronomy at Smith College (USA), she obtained a master's degree at the University of British Columbia (Canada) and concluded her work on dynamical friction models.
In 1986 she completed her doctorate at the Institute of Astronomy (Cambridge), the subject of her thesis being the structure and evolution of rich stellar clusters in the Large Magellanic Cloud, a topic on which she continued to work during her stay as a postdoc at the Institute of Advanced Studies, Princeton, New Jersey (USA).
From 1989 onwards, her work has centred more on the formation and early evolution stellar clusters. During 1989-90 she gave a course at Harvard University on "Science and Ethics".
Since 1991, she has been an Associate Postdoctoral Researcher at the Institute of Astronomy, Cambridge (United Kingdom), working mainly with Hubble Space Telescope data in various projects related to galactic and extragalactic stellar populations, globular clusters and globular cluster systems.