An international group of astronomers has used NASA’s Hubble Space Telescope to observe petal-like shells of the elliptical galaxy PGC 6240.
This image shows petal-like shells of the giant early-type galaxy PGC 6240. Image credit: ESA / Hubble team / NASA; acknowledgement: Judy Schmidt.
PGC 6240, also known as AM 0139-655, is giant early-type galaxy located in the southern constellation of Hydrus about 345 million light years away from Earth.
The galaxy resembles a white rose in the sky, with hazy shells of stars encircling a very bright center. Some of these shells are packed close to the center of the galaxy, while others are flung further out into space. Several wisps of material have been thrown so far that they appear to be almost detached from the galaxy altogether.
PGC 6240 is of great interest to scientists due to unusual clusters of stars that orbit around it – strong indication of a galactic merger in the recent past.
All the globular clusters around a certain galaxy form at approximately the same time, giving them all the same age. This is echoed within the clusters – all the stars within a single cluster form at around the same time, too. Because of this, most galaxies have cluster populations of pretty similar ages, both in terms of overall cluster, and individual stars.
However, PGC 6240 is unusual in that its clusters are varied – while some do contain old stars, as expected, others contain younger stars which formed more recently.
The most likely explanation for both the galaxy’s stacked shell structure and the unexpectedly young star clusters is that PGC 6240 merged with another galaxy at some point in the recent past.
Such a merger would send ripples through the galaxy and disrupt its structure, forming the concentric shells of material seen here.
It would also ignite a strong burst of star formation in the galaxy, which would then trigger similar activity in nearby space – leading to the creation of new, younger globular clusters around PGC 6240.
______
Bibliographic information: Malin DF and Carter D. 1983. A catalog of elliptical galaxies with shells. Astrophysical Journal, part 1, vol. 274, pp. 534-540; doi: 10.1086/161467
Aparna Maybhate et al. 2007. Evidence for Three Subpopulations of Globular Clusters in the Early-Type Poststarburst Shell Galaxy AM 0139−655. Astronomical Journal, 134, 1729; doi: 10.1086/521817
