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.
An image of the Cartwheel Galaxy taken with the NASA/ESA Hubble Space Telescope has been reprocessed using the latest techniques to mark the closure of the Space Telescope European Coordination Facility (ST-ECF), based near Munich in Germany, and to celebrate its achievements in supporting Hubble science in Europe over the past 26 years. Astronomer Bob Fosbury, who is stepping down as Head of the ST-ECF, was responsible for much of the early research into the Cartwheel Galaxy along with the late Tim Hawarden — including giving the object its very apposite name — and so this image was selected as a fitting tribute. The object was first spotted on wide-field images from the UK Schmidt telescope and then studied in detail using the Anglo-Australian Telescope. Lying about 500 million light-years away in the constellation of Sculptor, the cartwheel shape of this galaxy is the result of a violent galactic collision. A smaller galaxy has passed right through a large disc galaxy and produced shock waves that swept up gas and dust — much like the ripples produced when a stone is dropped into a lake — and sparked regions of intense star formation (appearing blue). The outermost ring of the galaxy, which is 1.5 times the size of our Milky Way, marks the shock wave’s leading edge. This object is one of the most dramatic examples of the small class of ring galaxies. Image: ESA/Hubble & NASA
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.
A rare and spectacular head-on collision between two galaxies appears in this NASA Hubble Space Telescope true-color image of the Cartwheel Galaxy, located 500 million light-years away in the constellation Sculptor. The new details of star birth resolved by Hubble provide an opportunity to study how extremely massive stars are born in large fragmented gas clouds. The striking ring-like feature is a direct result of a smaller intruder galaxy — possibly one of two objects to the right of the ring — that careened through the core of the host galaxy. Like a rock tossed into a lake, the collision sent a ripple of energy into space, plowing gas and dust in front of it. Expanding at 200,000 miles per hour, this cosmic tsunami leaves in its wake a firestorm of new star creation. Hubble resolves bright blue knots that are gigantic clusters of newborn stars and immense loops and bubbles blown into space by exploding stars (supernovae) going off like a string of firecrackers. The Cartwheel Galaxy presumably was a normal spiral galaxy like our Milky Way before the collision. This spiral structure is beginning to re-emerge, as seen in the faint arms or spokes between the outer ring and bulls-eye shaped nucleus. The ring contains at least several billion new stars that would not normally have been created in such a short time span and is so large (150,000 light-years across) our entire Milky Way Galaxy would fit inside. Image: NASA, ESA, and K. Borne (STScI)
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.
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.
Type: S pec (Ring)
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
Approximately 100 million years ago, a smaller galaxy plunged through the heart of Cartwheel galaxy, creating ripples of brief star formation. In this image, the first ripple appears as an ultraviolet-bright blue outer ring. The blue outer ring is so powerful in the GALEX observations that it indicates the Cartwheel is one of the most powerful UV-emitting galaxies in the nearby universe. The blue color reveals to astronomers that associations of stars 5 to 20 times as massive as our sun are forming in this region. The clumps of pink along the outer blue ring are regions where both X-rays and UV radiation are superimposed in the image. These X-ray point sources are very likely collections of binary star systems containing a blackhole (called Massive X-ray Binary Systems). The X-ray sources seem to cluster around optical/UV bright supermassive star clusters. The yellow-orange inner ring and nucleus at the center of the galaxy result from the combination of visible and infrared light, which is stronger towards the center. This region of the galaxy represents the second ripple, or ring wave, created in the collision, but has much less star for mation activity than the first (outer) ring wave. The wisps of red spread throughout the interior of the galaxy are organic molecules that have been illuminated by nearby low-level star formation. Meanwhile, the tints of green are less massive, older visible light stars. Although astronomers have not identified exactly which galaxy collided with the Cartwheel, two of three candidate galaxies can be seen in this image to the bottom left of the ring, one as a neon blob and the other as a green spiral. Previously, scientists believed the ring marked the outermost edge of the galaxy, but the latest GALEX observations detect a faint disk, not visible in this image, that extends to twice the diameter of the ring. Image: NASA/JPL-Caltech/P. N. Appleton (SSC/Caltech)
In this version, the insets above the composite show how each telescope individually views the Cartwheel galaxy. The colors represented in these insets do not correspond with the colors in the larger composite. Seen at the top left, the Chandra X-ray Observatory’s false color view of the Cartwheel galaxy was taken with the S-array on the telescope’s Advanced CCD Imaging Spectrometer. In the larger composite, Chandra’s X-ray data are also portrayed as purple. The Galaxy Evolution Explorer’s false color view (middle left) was taken with the telescope’s Far Ultraviolet detector. The mission’s contribution to the Cartwheel galaxy is shown as blue in the larger composite. The Hubble Space Telescope’s (middle right) false color view was taken with the B and I bands of the telescope’s Wide Field and Planetary Camera 2. In the larger composite, only Hubble’s B-band visible light data are shown. The mission’s contribution to the Cartwheel galaxy in the main image is depicted as green. The Spitzer Space Telescope’s false color view (top right) of the galaxy was taken at 8.0 microns with the telescope’s Infrared Array Camera. In the larger composite, Spitzer’s infrared observations are also shown as red. Image: NASA/JPL-Caltech/P. N. Appleton (SSC-Caltech)