Cartwheel Galaxy

The Cartwheel Galaxy, cataloged as ESO 350-40 and PGC 2248, is a striking lenticular ring galaxy. It is located roughly 500 million light-years away within the constellation Sculptor. This galaxy is the dominant member of the Cartwheel Galaxy group. This group consists of four physically associated spiral galaxies. The Cartwheel is quite large, with a diameter of approximately 150,000 light-years. This makes it slightly larger than our neighbor, the Andromeda Galaxy.

The galaxy possesses a highly complicated and disturbed structure. It is defined by two main rings. The outer ring is a site of massive, ongoing star formation. This happens because a shock wave has compressed gas and dust. This compression creates a starburst region, which makes the ring appear bright and bluish. Inside this is an inner ring that surrounds the galactic center. This inner ring also contains a ring of dark, absorbing dust. Connecting these two rings are several optical arms known as "spokes." While both radio and optical spokes exist, they do not seem to overlap.

This unique shape is the result of a violent cosmic event. Originally, the Cartwheel was a normal spiral galaxy. Approximately 200 to 300 million years ago, it underwent a head-on collision. A smaller companion galaxy passed through it in a "bullseye" style impact. This collision generated a powerful gravitational shock wave. As this wave moved outward at high speed, it swept up gas and dust. This process created the bright starburst ring we see today. The central portion of the galaxy remained largely unscathed as the wave expanded.

Astronomers have long studied this complex system. Fritz Zwicky discovered the galaxy in 1941. He described it as one of the most complicated structures requiring an explanation based on stellar dynamics. Today, scientists use HI (neutral hydrogen) tail mapping to understand these collisions. Hydrogen is the lightest and most abundant gas in galaxies. It is easily torn away by gravitational forces. By mapping these tails, researchers can identify the "culprit" galaxy that caused the collision.

In the Cartwheel group, there are three companions labeled G1, G2, and G3. G1 is a smaller, irregular blue Magellanic spiral. G2 is a yellow, compact spiral that features a tidal tail. G3 is a more distant spiral often seen in wide-field images. Most evidence suggests that G3 is the "bullet" galaxy. It is located about 88 kiloparsecs, or 287,000 light-years, away. An HI tail connects G3 to the Cartwheel, supporting this theory. This is more likely than G1 or G2 being the intruder, as they remain much closer to the Cartwheel.

The intense star formation in the ring leads to extreme phenomena. The massive stars created in the starburst eventually explode as supernovas. For example, the ATLAS system discovered a Type II supernova, SN 2021afdx, on November 23, 2021. These explosions leave behind neutron stars and black holes. Some of these objects become ultra or hyperluminous X-ray sources. This happens when a black hole pulls matter off a nearby companion star. The Cartwheel contains an exceptionally large number of these X-ray sources along its rim.

The future of the Cartwheel Galaxy involves a slow return to normalcy. Over the next few hundred million years, the current ring structure is expected to disintegrate. The remaining gas, dust, and stars will begin to fall back toward the center. If the companion galaxies G1, G2, and G3 remain distant, the galaxy will likely regain a spiral shape. This recovery depends on the reformation of spiral density waves. The galaxy's evolution shows how massive gravitational interactions can temporarily transform the fundamental shape of a galactic system.