Cosmic Halo May Help Explain Variety Among Galaxies
Visible light image of spiral galaxy M81 taken by Subaru Telescope's Prime Focus Camera.
Credit: National Astronomical Observatory of Japan

A halo-like cosmic structure around a well-known spiral galaxy may help uncover clues into the formation of galaxies, though it has also raised new questions over how the galaxy differs from our own Milky Way.

The extended structure of the Messier 81 (M81) galaxy, which has many hallmarks of a galactic halo, was spotted by astronomers using the Subaru Telescope at the National Astronomical Observatory of Japan.

Until now, ground-based telescopes have only observed individual stars in the halos around the Milky Way and Andromeda galaxies. But the enormous light-gathering power of Subaru Telescope's 27-foot (8.2-meter) primary mirror and the wide field-of-view of its Prime Focus Camera (Suprime-Cam) enabled the telescope to provide evidence of a faint, halo-like structure extending beyond M81's bright disk of stars.

The Subaru telescope's detectors can pick up light in the night sky that is more than 100 times fainter than what can be seen by the unaided human eye. That sensitivity allowed the telescope to spot individual stars, observe enough of them to identify what could be M81's halo, and analyze its physical properties. ?

A galaxy's halo of stars is usually spherical in shape and appears to extend beyond the galaxy's main disk. By studying the differences in this extended structure in M81, when compared to the Milky Way's halo, astronomers can analyze the variations in the birth histories of different spiral galaxies.

So far, the findings of the Subaru telescope defy exact classification of M81's extended structure as a halo in the traditional sense. And while the spatial distribution of its stars resembles the Milky Way's halo, M81's "halo" also differs from the Milky Way's in other respects.

Measurements of the total light from all of M81's stars, and analysis of their colors, point to estimates that M81's "halo" could be several times brighter and contain more processed materials ? nearly twice as much mass in the form of metals (all elements heavier than helium), than the Milky Way's halo.

Astronomers hope to use observations of both galaxies to determine how the differences between them arise and decide if the fundamental definition of a galactic halo needs to be updated.

M81 is one of the largest galaxies in the M81 Group, a group of 34 galaxies located toward the Ursa Major constellation. Located 11.7 million light-years away from Earth, it is one of the closest groups to the Local Group, which is the collection of galaxies that includes our own Milky Way. ?

The most prominent of the existing models predicts that galaxies are built up when they merge with many other smaller galaxies that orbit within their gravitational sphere of influence. This rather chaotic growth leaves behind a halo of stars around massive spirals like in the Milky Way.

The differences between the Milky Way's halo and the structure around M81 add more support to the growing body of evidence that the outer structures of apparently similar galaxies are actually much more important and complex than they had previously thought, Subaru telescope astronomers said in a statement.