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Almost everyone can appreciate the beauty of the night sky. It was once wisely stated, “When it is dark enough, you can see the stars.” For some, however, the beauty comes with what we can’t see – the dark matter – that comprises most of the mass out in space.

The unique focus on dark matter investigations at Queen’s University was one of many attractive features that lured astrophysicist Stéphane Courteau to Kingston in 2004. Already in the 1990s, Queen’s had initiated an intensive campaign at the deep-underground to track some of the dark matter produced by the Sun – the elusive, nearly massless, and invisible neutrino particles. (In 1999, SNO scientists solved the three-decade old “solar neutrino problem,” to significant international acclaim.) Courteau’s own search for dark matter is of a different yet complementary nature. Rather than conduct his investigations underground, he and his team look up into space with large ground-based telescopes, and use images and spectra of galaxies taken at different wavelengths to map out the distribution of visible and invisible light in, and around, galaxies.

Every known galaxy, like two of his favorites (the Andromeda and Sombrero galaxies), is surrounded by a halo of dark matter that accounts for more than 90% of its total mass. Because the dark matter is invisible, its presence is inferred through the activity and behaviour of surrounding visible objects. For example, by measuring the movement of stars, which are being accelerated by dark matter, we can infer the amount of dark matter. It can also be measured by looking at visible distortions caused by gravitational lensing (a light-bending process first proposed by Einstein and Zwicky in 1936). Understanding where the dark matter is located and how much there is allows Courteau, his team, and collaborators (including Larry Widrow and Kristine Spekkens of Queen’s) to create models for the typical mass distribution in galaxies and clusters of galaxies. These can, in turn, be compared to theories of galaxy formation and evolution to understand how galaxies like our own have emerged, and also test models for predicting the nature of the invisible mass which SNOLAB* scientists are also actively chasing.

Courteau collects data on some of the largest telescopes in the world, located in remote, dry, mountaintop locations away from light pollution, such as those in the Chilean Atacama desert or atop the Mauna Kea extinct volcano on the Big Island of Hawaii. His observational campaigns always involve students. For instance, two of his current PhD students, Jonathan Sick and Nathalie Ouellette, have pursued some of the most extensive studies of the nearby Andromeda galaxy and the Virgo cluster of galaxies (see slideshow photos) to date. Besides mapping the dark matter, they can identify the different stellar populations (that is, for example, the age and/or chemical composition of each stellar group) within galaxies that ultimately constrain how they evolve. Together with the SNOLAB group, Queen’s astrophysicists like Courteau, and their students, form one of the most active centres for research on dark matter in the world.

*With the solar neutrino problem essentially solved, dark matter detections in the revamped SNOLAB observatory now focus on measuring other properties of neutrinos, as well as the detection of the so-called “cold dark matter” particles that are more massive than the neutrino but far harder to detect. SNOLAB is the premier laboratory of its kind in the world.