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. The
findings, published in The Astrophysical Journal, provide new insight
into how these 'tidal stellar streams' form around galaxies.
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.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.
The team also found that both streams are relatively fainter than
other stellar streams that have been studied around galaxies close to
the Milky Way. Stream NW is the brighter of the pair, and has a more
concentrated core. The researchers hypothesize that this brightness is
due to a dwarf galaxy, possibly embedded within it, and that this dwarf
had a gravitational interaction with the Whale Galaxy to form Stream SE.
Figure 2: 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)
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. Nonetheless, it is still in an active growth
phase, and so are its surrounding halos. 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 (Tanaka et
al. 2017, "Resolved Stellar Streams around NGC 4631 from a Subaru/Hyper
Suprime-Cam Survey", The Astrophysical Journal, 842, 127).
Link
Source: Subaru Telescope
