Volume 4, Issue 26 March 2007
When Galaxies Collide
Astrophysical theorist Chung-Pei Ma making observations at the Keck Telescopes on the Big Island of Hawaii. Ma is one of the deputy directors of UC Berkeley's new Center for Cosmological Physics. Image credit: Alison Coil
Crash testing might seem more suited to a Detroit carmaker than a UC Berkeley academic. However, that hasn't stopped astronomy professor Chung-Pei Ma, who stages some of the most spectacular crashes imaginable. But cars and crash-test dummies aren't her area of expertise. Instead, this astrophysical theorist arranges collisions between entire galaxies.
In a cosmos that consists largely of vast stretches of nothingness, galaxies possess the strongest gravity around. This force not only keeps the planets, stars, dust, and gas in each galaxy together as they sail through space, it plows galaxies into one another like jalopies in a demolition derby.
The resulting collisions are awe-inspiring. Gases get accelerated to supersonic speeds, triggering the formation of new stars. Stars are hurled about at ever increasing speeds, until some are slung straight into outer space. Supermassive black holes, normally ensconced at the center of their own galaxies, end up circling one another like wary panthers. Their hulking movements set off powerful ripples in the membrane of spacetime known as gravitational waves.
Though such titanic pyrotechnics are intrinsically appealing, Ma has more serious reasons for studying galactic collisions. Her findings are revealing how some of the largest structures in the universe, elliptical galaxies and galaxy clusters, form and continue to evolve as the universe ages.
Four of the five galaxies in this image are involved in a violent collision. Located in a galaxy cluster known as Stephan's Quintet, the crash has produced one of the largest shock waves ever seen (green arc). Made up of superheated atoms of green gas, the wave covers an area greater than our own Milky Way. This false-color image was compiled using data from NASA's Spitzer Space Telescope and a ground-based telescope in Spain. Image credit: NASA/JPL-Caltech/Max-Planck Institute/P. Appleton (SSC/Caltech)
Elliptical galaxies are the most massive galaxies in the universe. They are shaped more like an ovoid vitamin pill than the pinwheel of spiral galaxies like our Milky Way. Many of the biggest elliptical galaxies observed sit at the center of galaxy clusters, agglomerations of thousands of galaxies gradually drawn together by the force of gravity.
"We believe that these larger galaxies came about not just by pulling in matter locally and condensing them through gravity," Ma says. "The first galaxies probably came about that way, but later on, bigger ones arose from the mergers of smaller ones. So there were a few startups when the universe was young, but you form the biggest ones gradually out of many generations of galaxy cannibalism."
When two galaxies collide, what transpires is very different from, say, one billiard ball smacking into another. Instead of ricocheting away in opposite directions, galaxies are much more likely to meld together. After all, Ma points out, "Galaxies are mostly empty, so the stars and dark matter mostly just pass each other by. The chances of two stars hitting each other is tiny." In fact, only one percent of the masses of these galaxies consists of matter we can see, such as stars and gases. The rest consists of dark matter—material we can't see but astronomers have inferred from many observations must exist.
Actual galaxy mergers are hard to find and even harder to view. So Ma is doing the next best thing—simulating galaxy collisions using computer models. This way, she can specify the types of mergers she wants to analyze—head-ons versus glancing blows; galaxies of different masses and shapes; even the occasional threesome—and analyze their fates with mathematical precision.
In her simulations, Ma and her UC Berkeley collaborators Michael Boylan-Kolchin and Eliot Quataert use virtual particles to represent chunks of galactic mass, taking into account their density profiles, shapes, and initial orbits. The computer then calculates the mutual gravity between every pair of particles in a simulation, and tracks their resulting velocities and positions. It takes thousands of iterations of this process on hundreds of interconnected computers to portray a merger event from start to finish.
At smaller scales, such as those between individual galaxies, gravity dominates the interactions between objects. But in the universe as a whole, other forces play pivotal roles in molding the present shape of the cosmos.
Ma is now trying to understand how these forces—in particular dark energy, believed to be responsible for accelerating the expansion of the universe—can affect individual galaxy mergers. For this, she uses a second type of model, known as a cosmological simulation, which recapitulates the large-scale evolution of the universe. "The cosmological simulation will tell us at the end of the day how many smaller young galaxies were gobbled up to form a galaxy of its present size and mass," Ma says.
Because tracking the many billions of particles needed to represent the entire universe taxes even the fastest supercomputers, cosmological models can only provide coarse-grained data about individual galaxies. To work around this problem, Ma and her collaborators are designing a scheme to combine large cosmological simulations with results from their more detailed individual galaxy collision simulations. The results should provide more realistic, higher-resolution depictions of what galaxy collisions really look like.
Learning how such large-scale structures have affected the overall formation of the universe, Ma says, is what keeps her arranging still more galactic smashups. "Galaxy mergers are exciting to me because they are the basic processes responsible for the formation of galaxies, the building blocks of the universe. At the same time, they connect so many different pieces of the cosmological puzzle."