Cartwheel Galaxy

The Cartwheel Galaxy is a lenticular galaxy and ring galaxy located in the southern constellation Sculptor.

The galaxy lies at an approximate distance of 496 million light years from Earth. It is about 150,000 light years across in size, which makes it slightly larger than the Milky Way.

The galaxy’s unusual appearance resembles that of a wagon wheel and has earned it the nickname Cartwheel. The galaxy has been tidally distorted by a collision with another galaxy into the ring-and-hub or cartwheel structure for which it is known.

In the past, the Cartwheel Galaxy was a regular spiral galaxy until it collided with a smaller companion galaxy about 200 million years ago. When the smaller companion passed through the larger galaxy, the nearly head-on collision created an enormous shockwave through the Cartwheel Galaxy.

The shockwave travelled at high speeds, roughly 200,000 miles per hour, sweeping up dust and gas and triggering star forming activity around the galaxy’s central region. The galaxy’s centre itself was not affected, while the ring around the bright core is a starburst region.

The galaxy’s spiral structure is now starting to re-emerge, with faint arms or spokes appearing between the galaxy’s nucleus and the outer ring.
The intruder galaxy that passed through the larger Cartwheel is one of the smaller galaxies seen near the Cartwheel in images. It is likely the galaxy that appears disrupted and shows evidence of new star forming activity and young blue stars. It could, however, be the other companion, which has no gas, as it could have been stripped of gas during the encounter.

There is an alternative theory, the Ball-of-Light Particle Model, that may explain the galaxy’s unusual appearance. The model suggests that the large ball-of-light that originally made up the galaxy’s core eventually became unstable and split into two or three. At least one of these expelled balls-of-light became one of the smaller galaxies seen next to the Cartwheel. This would explain the bridge of material, a trail of neutral hydrogen gas, connecting the Cartwheel Galaxy and ones of the smaller galaxies nearby.

What was left of the core was highly unstable and a large electromagnetic field spun around it, inducing massive stars and expelling them in a ring pattern at high speed. The high velocity kept the ejected stars stable as it induced large gravitational forces within their cores. The surface of the Cartwheel’s core eventually started to become more stable, continuing to expel stars, which have become the ‘spokes’ seen in the galaxy.

The Cartwheel’s core later became unstable again and expelled another ring. This inner ring contains large balls-of-light that are decaying and leaving streams of smaller balls-of-light that create large structures with shapes similar to comets.
Starburst galaxies like the Cartwheel contain many extremely large and luminous newly formed stars. When massive stars end their lives in supernova explosions, they leave behind black holes and neutron stars. The neutron stars and black holes that have companion stars nearby pull matter off these stars and become strong X-ray sources.

The material is pulled from the stars and, because neutron stars and black holes have enormous gravitational fields, it forms accretion discs around them. The in-fall of material of the disc creates powerful X-rays. The Cartwheel Galaxy is notable for having a large number of X-ray sources identified as black holes in binary systems. About a dozen of such bright X-ray sources have been detected in the galaxy. In most galaxies, astronomers discover only one or two.

Recent observations detected a faint disk extending to twice the ring’s diameter, which would make the Cartwheel Galaxy 2.5 times larger than the Milky Way. Astronomers had previously believed that the ring marked the galaxy’s outermost edge.

The Cartwheel Galaxy allows astronomers to study the formation of extremely massive stars in large fragmented clouds of gas. The galaxy’s ring structure is home to several billion new stars, which could not ordinarily have been formed over such a relatively short period.

FACTS

The Cartwheel Galaxy was discovered by Fritz Zwicky in 1941. Zwicky considered the galaxy to be “one of the most complicated structures awaiting its explanation on the basis of stellar dynamics.”

The galaxy rotates at the speed of 217 km/s and is receding from us at 9,050 km/s. Its estimated mass is between 2.9 and 4.8 billion solar masses.

The Cartwheel Galaxy is one of the brightest sources of ultraviolet emission in the local universe.

As a result of the collision with the smaller galaxy, there are older stars in the Cartwheel Galaxy’s inner region, seen in ultraviolet wavelengths.

The images of the Cartwheel Galaxies reveal many faint, more distant galaxies, which form a large superstructure and lie near the Sculptor Wall, an enormous structure of galaxy clusters that extends outwards for more than a billion of light years.

Cartwheel Galaxy
Type: S pec (Ring)
Constellation: Sculptor
Coordinates: 00h 37m 41.1s (right ascension), -33°42’59” (declination)
Distance: 496 million light years (150 Mpc)
Apparent dimensions: 1′.1 x 0′.9
Apparent magnitude: 15.2
Mass: 2.9 – 4.8 billion solar masses
Designations: Cartwheel Galaxy, PGC 2248, ESO 350-40, AM0035-335, MCG-06-02-022a