Cartwheel Galaxy

The Cartwheel Galaxy (catalogue ESO 350-40 and PGC 2248) is a lenticular ring galaxy about 500 million light-years away in the constellation Sculptor. It has a D₂₅ isophotal diameter of 57.69 kiloparsecs (188,200 light-years), and a mass of about 2.9–4.8 billion solar masses; its outer ring has a circular velocity of 217 km/s.

It was discovered by Fritz Zwicky in 1941. Zwicky considered his discovery "one of the most complicated structures awaiting its explanation on the basis of stellar dynamics."

The Third Reference Catalogue of Bright Galaxies (RC3) measured a D₂₅ isophotal diameter for the Cartwheel Galaxy at about 60.9 arcseconds, giving it a diameter of 57.69 kiloparsecs (188,200 light-years) based on a redshift-derived distance of 132.2 megaparsecs (431 million light-years). This diameter is slightly larger than that of the Andromeda Galaxy.

The large Cartwheel Galaxy is the dominant member of the Cartwheel Galaxy group, consisting of four physically associated spiral galaxies. The three companions are referred to in several studies as G1, the smaller irregular blue Magellanic spiral; G2, the yellow compact spiral with a tidal tail; and G3, a more distant spiral often seen in wide field images.

One supernova has been observed in the Cartwheel Galaxy. SN 2021afdx (Type II, mag. 18.796) was discovered by ATLAS on 23 November 2021.

The structure of the Cartwheel Galaxy is noted to be highly complicated and heavily disturbed. The Cartwheel consists of two rings: the outer ring, the site of massive ongoing star formation due to gas and dust compression; and the inner ring that surrounds the galactic center. A ring of dark absorbing dust is also present in the nucleic ring. Several optical arms or "spokes" are seen connecting the outer ring to the inner. Observations show the presence of both non-thermal radio continuum and optical spokes, but the two do not seem to overlap.

The galaxy was once a normal spiral galaxy before it apparently underwent a head-on "bullseye" style collision with a smaller companion approximately 200–300 million years prior to how we see the system today. When the nearby galaxy passed through the Cartwheel Galaxy, the force of the collision caused a powerful gravitational shock wave to expand through the galaxy. Moving at high speed, the shock wave swept up and compressed gas and dust, creating a starburst region around the galaxy's center portion that went unscathed as it expanded outwards. This explains the bluish ring around the center, which is the brighter portion. It can be noted that the galaxy is beginning to retake the form of a normal spiral galaxy, with arms spreading out from a central core. These arms are often referred to as the cartwheel's "spokes".

Alternatively, a model based on the gravitational Jeans instability of both axisymmetric (radial) and nonaxisymmetric (spiral) small-amplitude gravity perturbations allows an association between growing clumps of matter and the gravitationally unstable axisymmetric and nonaxisymmetric waves which take on the appearance of a ring and spokes. Based on observational data, however, this theory of ring galaxy evolution does not appear to apply to this specific galaxy.