All spun up: Image shows a simulated polar ring galaxy (left) next to a real one (NGC 4650A) for comparison. The central spinning galactic discs are surrounded by much larger spinning rings of material, perpendicular to the galactic plane.
Credit: University of Washington/Astrophysical Journal
SYDNEY: Polar ring galaxies are strange galaxies in which an outer ring of gas and stars rotates over the poles, making them appear from a distance like spinning tops. But just how they grow has long puzzled astronomers.
Now, U.S. and Canadian astronomers say the oddly-shaped galaxies grow like normal galaxies, but suck in gas and dust spinning in a plane at an angle to the galactic disk. See a video of the formation of a polar ring galaxy, created by the astronomers, here.
Galactic flavours
Galaxies come in two flavours, irregularly-shaped, and spiral, like our own Milky Way Galaxy. Polar ring galaxies look like spiral galaxies but, further out, also have a larger orbiting ring of gas, dust and stars, perpendicular to the spiral arms of the galaxy.
There are two possible theories of formation of these galaxies on the table; either they form when two spiral galaxies collided (creating a spiral disk and polar 'ring'), or they form by stripping material from a passing galaxy (feeding into the polar ring).
Now a team of astrophysicists, led by Chris Brook from the University of Washington in Seattle, have used supercomputers to simulate high-resolution observations of spiral galaxies in different stages of formation, and say that neither model fits their simulations.
Slow rain
Their simulations show that instead, these oddly shaped galaxies may form just like normal galaxies do, by the gradual accretion of matter from surrounding space. But in the case of polar galaxies, the rain of material that feeds the disk is at a high angle to the direction of the galactic plane.
"The irresistible conclusion of our simulation is that the formation processes of polar disk galaxies are the same as those for the outer disk regions of normal disk galaxies, except that the polar disk forms in a different plane with respect to the central galaxy," the authors write in the current issue of the Astrophysical Journal.
If confirmed, their results could be crucial to understanding the nature of dark matter, the elusive material that makes up about one-fifths of the universe's matter and plays a major role in how galaxies form. Dark matter is thought to dictate how galaxies grow, so understanding these rare galaxies helps place constraints on how dark matter shapes them.
