ARCHIVED - NRC astronomers picture galactic cannibalism
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October 13, 2009— Ottawa, Ontario
Cannibal galaxies make for compelling stories, especially in the International Year of Astronomy. So when it was announced in Nature in September that the giant spiral galaxy, Andromeda (M31), was devouring its nearest small neighbour, Triangulum (M33), scientific and mainstream publications jumped on the story.
Spectacular photos showed Andromeda and Triangulum tugging a slender stream of stars between them like a strand of astronomical taffy.
"The level of interest in this is certainly very nice to see," admits Dr. Alan McConnachie a researcher at the NRC Herzberg Institute of Astrophysics (NRC-HIA) in Victoria. He leads the multinational Pan-Andromeda Archaeological Survey (PandAS), which photographed signs of a titanic past clash between the two galaxies - what he calls "fossil evidence" that Andromeda's gravity started to vacuum up the smaller Triangulum's stars and interstellar gas about 100 million years ago.
Triangulum has since torn away - for now. But both galaxies are headed for another collision in three to five billion years. It seems inevitable that Andromeda will "digest" Triangulum, grow, then move on to collide and merge with the similar sized Milky Way to create a larger galaxy yet.
PandAS is surveying Andromeda and Triangulum because they're closest to Earth - about 2.5 million light years away - and the largest galaxies visible in the Northern Hemisphere. With sight lines clearer than those for the Milky Way, in which earth is deeply embedded, they give great insight into the way stars and galaxies form.
For the past two summers, the PandAS team has photographed the Andromeda region using a powerful astronomical camera called MegaPrime/MegaCam. This work occurred at the Canada France Hawaii Telescope (CFHT) on Mauna Kea in Hawaii, which is run jointly by NRC, the Centre national de la recherche scientifique of France, and the University of Hawaii.
PandAS images give astronomers a unique reference to the galaxies, as well as new keys to the origins of galaxies and stars that should lead to insights into the birth of the universe. Andromeda's "astro-cannibalism" came to light after the first year of imaging in 2008 was analyzed. Dr. McConnachie says the survey stands on the shoulders of other astronomers, including many NRC-HIA astronomers who have worked with CFHT for years. PandAS observations support current cosmological theories that galaxies form and grow by merging with and absorbing their smaller galactic neighbours. Previous evidence existed, but this is richer.
"By going into much more detail, we can figure out how the Andromeda Galaxy formed," says Dr. McConnachie. "That gives us a really unique perspective on the fundamental physics of these models and how galaxies formed, which relates to dark matter and to which galaxies form stars in the first place."
Nobody has seen dark matter, but astronomers hypothesize its existence based on gravitational effects on visible matter. They theorize that dark matter may actually comprise most of the mass in the universe. It's thought to affect such things as the speeds at which galaxies spin, their orbits, and the heat of interstellar gas in galaxies and galaxy clusters.
The Pan-Andromeda Archaeological Survey (PandAS) is made possible by MegaPrime/MegaCam, which began operating in 2003. This camera was partly designed at NRC-HIA, built in France and mounted on the CFHT 3.6-metre telescope near the top of Mauna Kea, a dormant 4,200-metre volcano in Hawaii.
"At the moment, CFHT is in a rather unique position with this instrument,” says Dr McConnachie. “Canada has a very long history in what’s called ‘wide field imaging’ — doing very detailed studies of stars and galaxies over large areas. With MegaCam we’re seeing this come to fruition.”
MegaPrime/MegaCam uses a mosaic of 36 photographic detectors to photograph a square degree of sky with each exposure, a very big window in astronomical terms. The resolution of each exposure — 340 megapixels — is extremely high, too.
The PandAS project stitched together MegaPrime/MegaCam images to produce pictures that show Andromeda spans a diameter of nearly a million light years —more than 10 times the size of the bright disc that we usually imagine when we think of Andromeda. Each of the electronic image files it produces can comprise more than two-thirds of a terabyte of information.
When Andromeda and the Milky Way finally merge, anyone remaining on Earth would see blazing night skies, but few stars colliding outright. Galaxies are mostly vast stretches of dark matter and interstellar gas. But the colossal forces of the two galaxies merging will likely form new stars.
"We're now categorizing the types of stars in these structures in much more detail," Dr. McConnachie says. "Once we see there's a structure and a shape, and how the stars are moving, we can figure out where the stars have been - then we can start reverse-engineering to determine when all of these structures started forming."
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