A New Take on M81’s Halo

Sometimes, amid all the attention lavished on space observatories like the Hubble Space Telescope, we forget that the big scopes here on terra firma are expanding astronomers' horizons too.

Messier 81's halo
Visible-light image of spiral galaxy M81 taken in 2005 by the Subaru Telescope and its Suprime-Cam imager. Click here for a larger version.
Subaru Telescope
Take, for example, this recently released image of Messier 81 (its northern half, anyway). Taken with Japan's Subaru telescope atop Mauna Kea, the image reveals a diffuse glow (appearing bluish here) skirting beyond the galaxy's beautiful spiral disk. It's part of a curious gas-rich ring known as Arp's Loop, which likely resulted from an interaction with neighboring M82 and NGC 3077 some 200 to 300 million years ago.

The Hubble Space Telescope recorded a nice view of the entire galaxy a few years ago. But with an aperture of 27 feet (8.2 m), Subaru and its prime-focus camera resolved many individual stars hovering near M81's disk. A team led by astronomer Michael K. Barker (University of Edinburgh) wanted to know whether they constitute part of a halo surrounding M81, much as a cocoon of stars and globular clusters surrounds our Milky Way.

The suspicion is that halos are evidence of a galaxy's appetite for devouring some of its neighbors — in the Milky Way's case, unfortunate victims that used to hover nearby in the Local Group. When two galaxies merge, the ensuing tug of war for gravitational control throws out many stars (or hydrogen gas that turns into stars) out into looping, inclined orbits. These far-flung suns serve as a smoking gun of the merged systems' violent encounter.

Barker's team analyzed 40,000 stars beyond the galaxy's visible disk and concludes that M81's halo is distinctly different from the Milky Way's. It's both brighter and more abundant in "metals" (astronomer-speak for any element heavier than helium). These differences must be telling us something about how M81 evolved — but the story line isn't complete yet. Perhaps this giant pinwheel feasted on more small galaxies than the Milky Way did, or maybe the starry envelope is evidence of tidal tugging from M82 and NGC 3077. The researchers aren't even sure if they're seeing a true halo or just an extension of the visible disk.

In any case, M81 is one of the easiest galaxies to spot in a backyard telescope. Located some 12 million light-years away, it has the total light of a 7th-magnitude star. These evenings you'll find it high overhead near the Big Dipper's bowl. Click here for a nice finder chart published several years ago in Night Sky magazine.

3 thoughts on “A New Take on M81’s Halo

  1. Ronny S

    From what I gather in the various papers dealing with Arp’s Loop, it is by no means certain that it is related to M81 at all. It’s highly probable that it is part of the foreground system of cirrus clouds in our own Milky Way, seen via reflected light of M81.

  2. Conrad

    If you would like to see a wide view of M-81 showing this feature I would recommend Tony Hallas’ deep-image of M-81
    found on his site here:http://astrophoto.com/M81LRGBDEEP.htm. It won’t have the resolution of a 8.2 meter telescope, but it’s still a pretty amazing image.

    To add to what was mentioned above, in the Hallas image you can see the feature more as a loop and you can trace what look like dark nebula filaments tht are seen in front of M-81. There is also a dwarf irregular galaxy nearby.

    Maybe the folks at Subaru should also take a peek at the Hallas image. Just a thought 🙂

  3. Grant Martin

    After viewing Tony Hallas’ fantastic image of M-81, it sure seems that Arp’s Loop is indeed local. In fact, it appears that way in the Subaru image as well. The wispy structure should not occur on the galactic scale. The dark nebula filaments brought out by Hallas’ masking would certainly seem to indicate that the Loop is in the near foreground. Tony Hallas, if you are reading this, my hat’s off to you. Your work is truly in a class of its own!

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