August General Meeting Report
Guest Speaker: Professor Duncan A Forbes, Swinburne Centre for Astrophysics
on “ Globular Clusters – what are they good for”
It sounded like a philosophical question at first. The meaning of Life sort of thing. Of what earthly use could Globular Clusters (GC) be to us, light-years away from Earth, you can’t even see them with the
naked eye. Not at all like the Sun, which gives us life-sustaining light and energy. But then, like the
proverbial “new-born baby” question, Professor Duncan Forbes let the GC potential evolve before us
as he listed their newly discovered characteristics. What the Cepheid Variables were to our knowledge of the extent of the Universe, the GC now seem to be to the measure of their parent galaxy: the reference, the knowable absolute reference, to the total mass of a galactic system. Indeed a truly
astronomical milestone. So, what is a Globular Cluster?
• A Globular Cluster is a bound homogenous collection of up to 106 stars
• to first order are single stellar populations
• all galaxies with Mv<-15 have at least one Globular Cluster, some have over 10,000
• they have a universal luminosity function
• they provide a unique probe of galaxy evolution
A Globular Cluster is a spherical collection of stars that orbits a galactic core as a satellite. GC are very tightly bound by gravity, which gives them their spherical shapes and relatively high stellar densities toward their centres. GC are found in the halo of a galaxy, they contain considerably more
stars and are much older than the less dense Galactic (or Open) Clusters found in the galactic disk.
GC are fairly common; there are about 158 currently known GC in the Milky Way. Andromeda may have as many as 500. Some giant elliptical galaxies, such as M87, can have as many as 10,000 GC. They orbit the galaxy out to 130 thousand light-years or more.
On Hand of numerous slides and computer simulations Duncan Forbes explained the on-going research into the properties and populations of GC. It is well-known how clear their individual stars shine. They sparkle in the field of the telescope like a handful of diamonds dropped onto a dark velvet
scarf. No dust lanes, no nebulas and, strange as it may seem, apparently no Dark Matter. GC provide a unique method for tracing the formation and evolution of their host galaxies. As single stellar
populations they are far easier to interpret than the multi-population complexity of galaxy field stars.
The scaling properties of globular clusters and their Globular Cluster Luminosity Function can become a new Distance Candle that gives the Hubble function to within 10%, and provide important limits on the hierarchical assembly history of galaxies. Amongst the oldest stellar systems available for study, GC can be used to constrain the formation of galaxy bulges, in particular the role of mergers vs secular evolution.
Globular Cluster research requires work at technology’s cutting edge: The Hubble Space Telescope for imaging and the 10m Keck plus Gemini telescopes for spectroscopy. Even there it is not uncommon to have 3 hours exposure times to get results. Hypotheses on the GC origin have to deal with their mainly single stellar population, their defined age brackets, the apparent absence of Dark Matter, and of course their globular shape. Computer models simulating galactic interaction seem to
favour the theory of them being the remnants of small galaxies penetrating the central bulge of a larger galaxy. Stripped of all gas and dust, new star formation would have ceased, leaving the existing, uniform star population throughout. Relics of disrupted satellite galaxies around the Milky Way and Andromeda have been found, but direct evidence of a satellite galaxy in the early stages of being disrupted remained elusive until 2003. A dwarf satellite galaxy in the process of being torn apart by gravitational tidal forces has been found, 2 billion light years away, as it merges with a larger galaxy's dark matter halo. The results illustrate the morphological transformation of dwarf galaxies by tidal interaction and the continued build-up of galaxy halos. Ten billion years ago (a popular age of GC) galaxy collisions would have been a frequent occurrence as galaxies competed for living space. The
prominent age-gap between 10 and 2 billion year old GC on some galaxies is just one of the puzzles that still challenge this new field of studies.
Following question time Duncan Forbes was thanked and presented with the traditional appreciation, to general acclaim.
Professor Duncan A. Forbes is currently the Deputy Director, Centre for Astrophysics& Supercomputing, Swinburne University, Australia. He has some 138 publications to his credit or in association. A quick Resume lists the following: Duncan graduated BSc(hons) University of
Canterbury NZ, made his PhD in Cambridge UK, Postdoctoral Fellow Lick Observatory USA, then
Senior Lecturer in Birmingham UK, and in 2000 he joined the Swinburne University as Associate Professor. His research interests include Globular Cluster and galaxy formation.
To conclude the evening’s program Barry Clark gave a short report on his solar eclipse journey to Siberia, complete with traditional Russian headgear.