M81’s ‘halo’ offers insight into formation of galaxies

April 23rd, 2010 - 6:31 pm ICT by ANI  

Washington, April 23 (ANI): The ‘halo’ of M81, a galaxy in the constellation Ursa Major, has shed new light on galaxy formation.

Until now, ground-based telescopes have only observed individual stars in the haloes around the Milky Way and Andromeda Galaxies.

Differences in M81’s extended structure from the Milky Way’s halo may point to variations in the formation histories of spiral galaxies.

M81 is one of the largest galaxies in the M81 Group, a group of 34 galaxies located toward the constellation Ursa Major.

At 11.7 million light-years from Earth, it is one of the closest groups to the Local group, the group of galaxies that includes our own Milky Way.

Thanks to its proximity and similarity to the Milky Way, M81 provides anexcellent laboratory for testing galaxy formation models.

The most prominent of these models predicts that galaxies are built upfrom the merging and accretion of many smaller galaxies that orbitwithin their gravitational sphere of influence.

This chaotic, bottom-up growth leaves behind a halo of stars around massive spirals like the Milky Way. Do the findings about M81’s extended structure, possibly its halo, support this view?

True to its promise as an effective tool for the study of galaxy evolution, Subaru’s telescope has provided data to address this question.

The enormous light-gathering power of Subaru Telescope’s 8.2 meter primary mirror and the wide field-of-view of its Suprime-Cam enabled the telescope to provide evidence for a faint, extended structural component beyond M81’s bright optical disk.

It probed into space over one-hundred times darker than the night sky andimperceptible to the naked eye.

The telescope spotted individual stars and gathered enough of them to identify M81’s extended component and analyse its physical properties.

The results defy exact classification of the extended structure as a halo.

Although the spatial distribution of its stars resembles the Milky Way’s halo, M81’s “halo” differs from the Milky Way’s in other respects.

Measurements of the total light from all of its stars and analysis of their colours 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.

The results of this research contribute to the growing body of evidence that the outer structures of apparently similar galaxies are much more important and complex than astronomers have previously thought. (ANI)

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