Astronomers using the Advanced Camera for Surveys on the NASA/ESA Hubble Space Telescope have imaged an unusual elliptical galaxy called NGC 3923.
This image shows NGC 3923, one of the most unusual objects in the Malin-Carter catalog of elliptical galaxies with shells. Image credit: ESA / Hubble / NASA / Judy Schmidt.
NGC 3923 is one of the most prominent examples of a shell galaxy with dozens of peculiar arc-like structures (shells), ranging from the outer parts of the galaxy down to its center.
The galaxy lies in the constellation Hydra. It is thought to be around 90 million light-years from Earth.
First discovered by British-Australian astronomer David Malin in 1977, shells around NGC 3923 are faint and it’s difficult to distinguish them from the bright background of the galaxy.
NGC 3923 has almost 30 shells, with only a few of the outer ones visible in the new image from Hubble and its shells are much more subtle than those of other shell galaxies.
The shells of NGC 3923 are also interestingly symmetrical, while other shell galaxies are more skewed.
Finding concentric shells of stars enclosing a galaxy is quite common and is observed in many elliptical galaxies.
In fact, every 10th elliptical galaxy exhibits this onion-like structure, which has never been observed in spiral galaxies.
The shell-like structures are thought to develop as a consequence of galactic cannibalism, when a larger galaxy ingests a smaller companion. As the two centers approach, they initially oscillate about a common centre, and this oscillation ripples outwards forming the shells of stars just as ripples on a pond spread when the surface is disturbed.
Astronomer Judy Schmidt submitted a version of the image to the Hubble’s Hidden Treasures image processing competition.
