June General Meeting
Syllabus Speaker Dr. Michael Brown, Monash University Physic and Astronomy Section
on the subject “The Growth of Galaxies over Cosmic Time”
In cosmic time it wasn’t that long ago that physics and astronomy were totally unrelated sections. Physics and Cosmology were treated as separate fields of study; Physics dealt with physical things on Earth, including particle physics, while Astronomy tried to pinpoint and explain the movements of lights in the night sky. Of course our understanding of Gravity and the Newtonian Laws brought the heavenly movements down to Earth, but in fact it was only the cosmic chemistry, the cosmological theories of the Steady State and Big Bang evolution in the 60s and 70s that started to link Physic and Astronomy subjects in the new term Astrophysics. The early universe theory offered a unique window for investigating high-energy phenomena that could not be recreated in the laboratory. And when, with the burst of the “cold war bubble” Particle Physics struck hard time funding cuts in the 60s, the discovery of quasars, pulsars and the cosmic microwave background provided a smooth transition for physics’ scientists and theorists into cosmic research. In a joint venture with Harvard University the Smithsonian Astrophysical Observatory in 1955 was one of the earliest to practice the “new astronomy”, as astrophysics was known then. Since then big bang cosmology and particle physics have shared in a healthy slice of the available research funding.
To those of us who grew up with Hubble’s simple classification of galaxies into the three visibly obvious branches: Elliptical, Spiral and Barred Spiral, it comes as kind of a shock to hear Michael Brown speak of hundreds of different galaxy types, graded by colour, luminosity, size, shape, age, quasar, black hole, redshift, active nuclei, jets, Dark Matter, mergers, mass anomaly and many more. Some 100 billion of them in the visible universe, ranging from 1000 to 100,000 parsec in diameter. Cosmology seems to have gone back to the drawing board, being re-written. We can no longer say that we know how the galaxies came to be as they are, how they evolve; we are not even sure how stars do form any more. It is an odd feeling, we are sure that today we know more than we did yesterday, and yet we know that we now know less about the functions of the universe than we did 50 years ago! Theories of Missing mass, Dark Matter, Dark Energy, Black Holes, inflation, an accelerating expansion, it all adds up to a big bag of unknowns. We do need bigger telescopes and more observational data.
Dr Michael Brown is an observational astrophysicist. His current research interests include the formation, assembly and evolution of galaxies over cosmic time. He uses large imaging and spectroscopic surveys, including the NOAO Deep Wide-field Survey (NDWFS), the Spitzer IRAC Shallow Survey, and the AGES spectroscopic survey, to measure the evolving space density and clustering of distant galaxies and their assembly history. In his most recent first author paper, The Evolving Luminosity Function of Red Galaxies, he traces the assembly history of red galaxies over the past 8 billion years. Blue star-forming galaxies are being transformed into red galaxies, with little ongoing star formation, between redshifts of z=1 and z=0. In contrast, the stellar masses of the most massive galaxies exhibit modest evolution over the same redshift range. While there is some ongoing assembly of red galaxies, the stellar masses of the most massive galaxies evolve slowly at low redshift. It is a large field of activity. In all Michael has been part of some 30 papers published in the last seven years.
On hand of a series of slides Michael demonstrated the change in space density over the past 7 billion years, the effect of mergers on the size of galaxies and the cause of their ageing. For some reason star formation seems to stop at some stage in a galaxy’s evolution. The bluish tint of new forming stars in them disappears and the galaxies take on the reddish hue of older stars. The puzzle is, what truncates the formation of new stars? Is there a limit to the maximum size any particular galaxy can grow to, apart from the availability of gas? What determines the initial size of galaxies? When were the massive red galaxies assembled? What are the progenitors of today’s most massive galaxies? These are some of the questions Michael left us with. Questions that need to be answered and confirmed by the study and interpretation of observational data. From the answers to those questions will grow our new understanding of the engines that drive the Cosmos.
The vote of appreciation was given to general acclaim by Peter Thomas with the traditional present of two bottles of ASV Red