Anne’s Picture of the Day: The Cartwheel Galaxy

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October 26, 2012

The Cartwheel Galaxy, a ring galaxy in Sculptor

Image Credit: ESA/Hubble & NASA

The Cartwheel Galaxy (ESO 350-40) is a ring galaxy of some 150,000 light-years across (which is slightly larger than our Milky Way galaxy), located 496 million light-years away in the southern constellation Sculptor. It is speeding away from us at about 9050 kilometers per second and rotates at 217 kilometers per second. The galaxy has a mass of about 3.85 billion solar masses.

Previously, scientists believed the bright blue ring marked the outermost edge of the galaxy, but the latest observations detect a faint disk, not visible in this image, that extends to twice the diameter of the ring. This means the Cartwheel is a monstrous 2.5 times the size of the Milky Way.

The blue ring of starburst reveals billions of newly born stars, caused by a rare and spectacular head-on collision with a smaller companion approximately 200 million years ago (i.e., 200 million years prior to the image).

When the nearby intruder galaxy — possibly one of the two galaxies on the left side of the image — passed through the Cartwheel Galaxy, the force of the collision caused a powerful shock wave through the galaxy, much like the ripples produced when a stone is dropped into a lake.

Moving at the high speed of more than 300,000 km per hour, the shock wave swept up gas and dust, producing the expanding starburst ring around the galaxy’s center. The most recent star burst (star formation due to compression waves) has lit up the outermost ring of the Cartwheel.

Star formation via starbursts result in the formation of massive and extremely luminous stars. When these stars explode as supernovae, they leave behind neutron stars and black holes. Some of these neutron stars and black holes have nearby companion stars, and become powerful X-ray sources as they pull matter off their companions.

While most galaxies have only one or two bright X-ray sources, the Cartwheel contains an exceptionally large number of them. The brightest X-ray sources in the Cartwheel are likely black holes with companion stars, and appear as the white dots that lie along the rim of the image, because many massive stars formed in the rim.

The Cartwheel Galaxy was once a normal spiral galaxy similar to our home galaxy, the Milky Way, before it underwent the collision, with spiral arms winding outward from the galaxy’s center. The galaxy is beginning to take the form of a normal spiral galaxy again, as seen in the faint arms or spokes between the outer ring and the bulls-eye shaped nucleus.

Usually a galaxy is brighter toward the center, but the collision actually smoothed out the interior of the galaxy, concentrating older stars and dust into the inner regions, like the calm after the storm of star formation.

The Cartwheel Galaxy is one of the most dramatic examples of the small class of ring galaxies, and it is an unusual opportunity to study new stars, because many stars would ordinarily take much longer to form.

This image was taken with the NASA/ESA Hubble Space Telescope.

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