Hugh Couchman
Hugh Couchman
Professor Emeritus
ABB 317A

Hugh Couchman 640x192

Computer simulated galaxies

The universe took billions of years to evolve, but computers can simulate the process in a matter of hours or days. As part of the McMaster Unbiased Galaxy Survey (MUGS), astrophysics professor Hugh Couchman is using computational modelling to create a sample of 25 galaxies. "I try to model how these galaxies formed and why they have the structure they do," he explained. The model galaxies are being compared to what galaxies actually look like today.

The 25 galaxies in the survey are "unique members of the population of galaxies, which we've chosen according to unbiased criteria," said Couchman. In order to qualify for the survey, the galaxies only needed to have a mass close to that of the Milky Way galaxy and avoid the chaotic environments of dense clusters of galaxies.

Three hundred thousand years after the Big Bang, the universe had cooled and was very "smooth" (the density of matter was evenly distributed). If the universe had remained completely smooth, nothing would have changed, but very small fluctuations in the density allowed matter to group together. "Those tiny little ripples in the matter density slowly grow," said Couchman. "When you have a slightly over-dense region, it has a slightly stronger gravitational attraction, so it tends to pull matter towards it and gets even more over-dense."

Astronomers can measure the cosmic microwave background emission that was produced 400,000 years after the Big Bang to determine the spectrum of fluctuations, which is used as an input in computational modelling to see if these fluctuations produce the same types of galaxies that exist today.

"The biggest challenge at the moment is getting the physics right," said Couchman. "A galaxy is a complicated ecosystem. It's a balance between gravity and other significant processes." In addition to taking gas dynamics and dark matter into account, astrophysical processes such as star formation greatly influence the development of galaxies.

Massive stars use up their energy and burn out more quickly than smaller stars. When a massive star evolves, it releases huge amounts of radiation; when it dies, it explodes as a supernova, sending an enormous blast wave into the interstellar medium. These processes regulate how gas cycles through the star formation process. "There's a complex interaction between stars and the gas in galaxies," said Couchman. "That's turned out to be incredibly challenging to model." The range of scales involved makes it impossible to incorporate stars into the model directly because they are so much smaller than galaxies.

The collaboration between astronomers and physicists in the Department of Physics and Astronomy is one of the features that drew Couchman to McMaster. "One of the reasons I wanted to come here is because astronomy is part of physics," he said. "In many places, the astronomers are either in their own separate department, or quite separated from the physicists. Here, we share offices on the same floor and talk to each other. I really like that about this department."

Growth of structure in the universe via gravitational N-body simulations

Hugh Couchman
Department of Physics & Astronomy
McMaster University

Dear Prospective Graduate Student,

This letter is to introduce myself and to describe my group and the kind of research that we do.

I am interested in post recombination cosmology which is the study of the universe and its contents after the universe cooled and first became neutral about 100,000 years after the Big Bang. In particular I investigate how structure - galaxies, clusters of galaxies and large-scale structure - grows from small density ripples present at recombination. Much of this work involves computer simulation and is a field which has been named as one of the "Grand Challenges" of the physical sciences!

A Master's student, Sam Bromley, got his degree in September and Todd Fuller, a student of mine at the University of Western Ontario, successfully defended his Ph.D. in May. I expect to have openings for two and possibly three new students this coming year. There are currently two postdoctoral fellows working with me: Rob Thacker - who is investigating the formation of spiral galaxies, a much sought-after goal in cosmology - and James Wadsley - who works with me and Ralph Pudritz on areas of common interest between star formation theorists and cosmologists. Ralph Pudritz and I are hoping to hire another postdoc this coming year.

The group is very informal and interacts closely. The postdocs, in particular, are a great asset and frequently collaborate with and help students. I usually suggest several projects and encourage students to pick one which excites them. My group forms part of the wider theoretical astrophysics group with Alison Sills' and Ralph Pudritz' groups and I have close ties with the other theorists in Physics and with those in other departments, often because we all use computers to do science (it's amazing the range of science you come into contact with this way). Student have their own workstations and access to a wide range of other computers.

I am part of several external collaborations, including "Virgo" which is based in Durham, UK and with connections in Munich, Germany and with "C4" which is a collaboration between Victoria and McMaster, as well as ties with the Astronomy groups in Sussex, UK, the University of Washington in Seattle and the Canadian Institute for Theoretical Astrophysics at the University of Toronto. My students can expect to be involved with this joint work and will have opportunities for travel to meetings and conferences.

Please do not hesitate to contact me at couchman@mcmaster.ca if you think this is a research area in which you might be interested or simply if you have any questions.

Best wishes,
Hugh Couchman

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