Using the Subaru Telescope atop Maunakea, researchers have identified 11 dwarf galaxies and two star-containing halos in the outer region of a large spiral galaxy 25 million light-years away from Earth.
Stellar halos are spherical populations of stars and globular clusters thought to surround most disk galaxies and elliptical galaxies of cD class (found near the centers) of some rich galaxy clusters).
They are fascinating objects for astronomers but difficult to observe and study due to their low luminosity.
The team focused their attention on Galaxy NGC 4631, otherwise known as the Whale Galaxy because of its shape. They identified 11 dwarf galaxies in its outer region, some of which were already known and additionally, two tidal stellar streams orbiting the galaxy: one, called Stream SE, is located in front of it and the other, called Stream NW (the brighter of the pair) is nestled behind it.
Dwarf galaxies discovered by observation. Three color composites are characterized by HSC-g and HSC-i images. A pseudo-image with intermediate color is created from the averaged image of HSC-g and HSC-i images. The top right column (3) was previously thought to be a dwarf galaxy from an earlier study, but from the high resolution image taken by HSC, it is apparent that what was observed was an overlapping of foreground stars and background galaxies. (Credit: Tohoku University/NAOJ)
Astronomers believe it’s possible that they originated as a result of a gravitational interaction between the Whale Galaxy and a dwarf galaxy orbiting it.
Both streams are relatively fainter than other stellar streams that have been studied around galaxies close to the Milky Way.
It is thought that stellar halos are less common when a galaxy’s total stellar mass is smaller than the stellar mass of larger galaxies, such as the Triangulum Galaxy. As a result of their calculations, the researchers believe that the Whale Galaxy, although large, has a smaller mass than the Milky Way.
Future studies could help further clarify how stellar halos form around galaxies with relatively small masses, the researchers conclude.
This research is published in the Astrophysical Journal, 842, 127.