January General Meeting
A special invitation to country members was again part of the ASV January General Meeting this year, a show-and-tell-and-look week-end on 14-15 January. It included a friendly get-together on Friday night at the ASV Lodge and an Open Day barbeque cum sun-spotting afternoon at the Melbourne Observatory site on Saturday. My wife Ursula and I, living close to 100km from Melbourne took advantage of the Saturday opportunity to meet country friends again and, after a delicious meal interspersed with chasing sunspots through a hazy cloud cover (see photo collection next page), joined the crowd in the Herbarium for the January general meeting.
The speaker for the evening was Glen Mackie, lecturer at the Centre for Astrophysics and Supercomputing, Swinburne University of Technology in Melbourne, and the subject for his talk was Mining the 2dF Galaxy Redshift Survey.
With a series of PowerPoint slides Glen gave us some background to his chosen subject and took us through its development. The 2 degree Field Galaxy Redshift Survey (2dF), five years in the making, finished in 2003 with over 245,000 spectra of objects (mainly galaxies) obtained across two regions of the southern sky. It was a major astronomical undertaking at the time, with a number of ingenious innovations emerging from the project and in the process giving a new lease of life to the Anglo-Australian Telescope (AAT) at Siding Springs (refer report on Dr Fred Watson’s talk in CRUX 17:1 by Ray Colton). The 2dF system at the AAT was at the time arguably the world's most complex astronomical instrument. Its design allowed the acquisition of up to 400 simultaneous spectra of objects down to magnitude 19.5 anywhere within a two degree field in the sky. The hardware consists of a wide field corrector, an atmospheric dispersion compensator, a robot gantry for positioning the optical fibres and two spectrographs, each accepting 200 of the fibres to produce medium resolution spectra. After a series of technical improvements to the positioner, going from one field configuration to the next could be accomplished in one hour; that is, a total of 400 individual moves at about 6-7 seconds per fibre, with a positioning accuracy down to ~15 microns, equivalent to ~0.25 arcsec.
The initial motivation for the project was, off course, to shed light onto the long-standing controversy about the large scale structure of the cosmos. Ever since Kepler and Newton gave the heavens above a three-dimensional form, the random distribution of matter, mass, stars and galaxies in the sky (as well as their relative motions) has troubled astronomers. Einstein, with a cosmological sleight of hand, introduced a constant called lambda and tried to bring back stability into what was then still considered the heavenly domain (die göttliche Ordnung). The discovery of the Cosmic Microwave Background radiation (CMB) finally tipped the balance from a “steady state” theory to one with a violent beginning, a big bang. But that in itself did not explain the obvious localised lumpiness nor the large scale structure of the universe. What was needed was a detailed, three dimensional map showing the position and distribution of the visible indicators of mass. The peculiar clustering of galaxies seemed to confirm a nagging suspicion that there was more to astrophysics then what meets the eye. Out of the paired information of the 2dF survey, including the following expanded Northern Hemisphere Sloan project (nicknamed ‘pi in the sky’ in a parody on its scientific parameters) and the CMB pictures of the COBE project, the concept of dark matter and dark energy was born.
Until 1998 astrophysicists were debating whether gravity was slowing the expansion enough to eventually cause the universe to collapse in a Big Crunch. “Mining” the 2dF survey for distance, speed and intrinsic brightness correlations and comparing the result with the data from microwave charts of the early universe brought scientists to the inescapable conclusion that only an accelerating universe would have allowed it to grow to today's size. An accurate measurement of galaxy clustering on scales up to 300Mpc allowed precise assumptions of the total mass density of the universe and the baryon fraction. This places a strong new upper limit on the total neutrino mass and, in making possible precise measurements of the Hubble value, it supports the idea of some form on non-zero cosmological constant. Careful sifting of the vast survey archives furthermore revealed a series of so-called Bright Centre Galaxies. BCGs are so large that 100 Milky Ways (the galaxy we reside in) end to end could easily fit inside one of them. BCGs dominate the centre of richly populated clusters of galaxies that can have anywhere from 100 to 1000 galaxy members. Why are these galaxies important? Studies of close-by BCGs have shown that they possess a remarkably narrow range of luminosities. By assuming all BCGs have the same absolute brightness and then measuring their apparent brightness, their distance can be calculated. The BCGs large luminosities allows them to be viewed with the Hubble Space Telescope across cosmologically significant distances of the universe. While previous studies had assumed that BCGs were spatially centrally and dynamically centred on their clusters, new results reveal that some 1/3 of BCGs have velocities in excess of 300km/s with respect to their clusters. This new result is in agreement with hierarchical merger models, and confirms how galaxies and clusters form and continue to grow even at present epochs. – Way beyond the original intend, the 2dF surveys have become a priceless source of information for astrophysical researchers and a stepping stone for them to the fabric of the cosmos.
In conclusion Glen gave a plug for the educational opportunities available at Swinburne Astronomy Online, (of which he is the Assistant Coordinator) where online degree programs are on offer in Graduate Certificate, Diploma and Master of Science, as well as a short, six week non-assessed course in astronomy. Alfred Klink “