The Sculptor’s ‘Silver Coin’

Located around 10.5 million light-years away, in the constellation of the Sculptor, is a vibrant spiral galaxy known as NGC 253. Also referred to as the Sculptor Galaxy, or the ‘Silver Coin’, NGC 253 lies almost edge-on to us and can be viewed in the Southern Hemisphere with a good set of binoculars. In this image, taken by NASA’s Wide-field Infrared Survey Explorer (WISE), the Sculptor Galaxy is shown in varying infrared hues from all four of the telescope’s infrared detectors.

The red image, captured at wavelengths of 22-microns, exhibits the galaxy’s activity as new stars heat up their dusty surroundings. Star formation in the core is so intense that it created diffraction spikes – a telescopic feature observable around the brightest stars. The green image, captured at wavelengths of 12 microns, shows freshly created stars in the core and spiral arms of the galaxy. The ultraviolet light emitted by these stars is absorbed by the dusty particles which remained following the stars’ formation, causing these particles to glow with infrared light. The blue image, captured at wavelengths of 3.4 and 4.6 microns, reveals stars of all ages present throughout the entire galaxy. These images indicate that NGC 253 is undergoing a phase of extreme star formation, particularly within its core and spiral arms. As a result, it has been classified as a ‘starburst galaxy’.

A galaxy is considered to be a starburst galaxy when there is an unusually high concentration of stars forming at an extremely high rate. This surge of activity is generally a result of galaxy mergers or other events which led to an increase in galactic material. NGC 253 is believed to be so incredibly active due to a collision with a dwarf galaxy around 200 million years ago. While all stars form within dense clouds of dust and gas, scientists believe that starburst galaxies are distinguishable from other star-forming regions due to the composition of its gas clouds.

Adam Leroy (of the Ohio State University) and his colleagues used the giant ALMA radio telescope to examine 10 distinct molecular clouds where star formation in NGC 253 was highest. They identified 40 signatures created by “tracer molecules”, allowing them to map out these starburst regions. They found that the presence of hydrogen cyanide (HCN) indicated dense areas of star formation, while molecules such as H13CN and H13CO+ pointed towards areas in which the density was significantly higher. By comparing the densities, motions and distributions of molecules within the 10 star-forming clouds, Leroy and his team observed that these regions are much larger, denser and more chaotic than similar clouds in typical spiral galaxies.

Through the examination of starburst galaxies, such as NGC 253, and an analysis of the molecular composition of their star-forming regions, we can learn more about the evolution of the different galaxies in our universe. It could also lead to discoveries as to why different regions are more adept star-producers, and whether they are capable of producing different types of stars to the ones in our own Milky Way.

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