Discovering the Origins of Galaxies’ Halos

Using a Japanese telescope on top of the sacred Hawaiian mountain of Mauna Kea, 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. The findings, published in The Astrophysical Journal, provide new insight into how these ‘tidal stellar streams’ form around galaxies

Eleven dwarf galaxies and two star-containing halos were identified in the outer region of the nearby Whale Galaxy.

Copyright : Tohoku University / National Astronomical Observatory
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Researchers from Tohoku University and colleagues used an ultra-wide field of view camera on the Subaru Telescope to develop a better understanding of stellar halos. These ring-shaped collections of stars orbit large galaxies and can often originate from smaller dwarf galaxies nearby.

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. Dwarf galaxies are not easily detected because of their small sizes, masses and low brightness. The team also found two tidal stellar streams orbiting the galaxy: one, called Stream SE, is located in front of it and the other, called Stream NW, is nestled behind it.

Based on calculations aiming to estimate the metallic content of the stellar streams, the team believes it’s possible that they originated as a result of a gravitational interaction between the Whale Galaxy and a dwarf galaxy orbiting 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.

Citation: Title: Resolved Stellar Streams around NGC 4631 from a Subaru/Hyper Suprime-Cam SurveyAuthors: Mikito Tanaka, Masashi Chiba, and Yutaka Komiyama
Journal: The Astrophysical Journal
DOI: 10.3847/1538-4357/aa6d11