Credit: ESA/Hubble & NASAAcknowledgement: D. Calzetti (University of Massachusetts) and the LEGUS Team
This bundle of bright stars and dark dust is a dwarf spiral galaxy
known as NGC 4605, located around 16 million light-years away in the
constellation of Ursa Major (The Great Bear). This galaxy’s spiral
structure is not obvious from this image, but NGC 4605 is classified as
an SBc type galaxy — meaning that it has sprawling, loosely wound arms and a bright bar of stars cutting through its centre.
NGC 4605 is a member of the Messier 81 group of galaxies, a gathering of bright galaxies including its namesake Messier 81 (heic0710), and the well-known Messier 82 (heic0604a).
Galaxy groups like this usually contain around 50 galaxies, all loosely
bound together by gravity. This group is famous for its unusual
members, many of which formed from collisions between galaxies. With
its somewhat unusual form, NGC 4605 fits in well with the family of
perturbed galaxies in the M81 group, although the origin of its
abnormal features is not yet clear.
The Messier 81 group is one of the nearest groups to our own, the
Local Group, which houses the Milky Way and some of its well-known
neighbours, including the Andromeda Galaxy and the Magellanic Clouds.
Galaxy groups provide environments where galaxies can evolve through
interactions like collisions and mergers. These galaxy groups are then
lumped together into even larger gatherings of galaxies known as
clusters and superclusters. The Local and Messier 81 groups both belong
to the Virgo Supercluster, a large and massive collection of some 100 galaxy groups and clusters.
With so many galaxies swarming around, NGC 4605 may seem
unremarkable. However, astronomers are using this galaxy to test our
knowledge of stellar evolution. The newly-formed stars in NGC 4605 are
being used to investigate how interactions between galaxies affect the
formation, evolution, and behaviour of the stars within, how bright
stellar nurseries come together to form stellar clusters and stellar
associations, and how these stars evolve over time.
And that's not all — NGC 4605 is also proving to be a good testing ground for dark matter.
Our theories on this hypothetical type of matter have had good success
at describing how the Universe looks and behaves on a large scale — for
example at the galaxy supercluster level — but when looking at
individual galaxies, they have run into problems. Observations of NGC
4605 show that the way in which dark matter is spread throughout its
halo is not quite as these models predict. While intriguing,
observations in this area are still inconclusive, leaving astronomers to
ponder over the contents of the Universe.
