Spiral galaxy M83

The galaxy M83 has well-defined spiral arms that trace regions of active star formation. But purely gaseous arms extend beyond the visible disk and are visible only at radio wavelengths. The Milky Way's newly discovered spiral arm may be analogous to these 'invisible' arms.

Courtesy Adam Block / NOAO / AURA / NSF.

If Earth were positioned far above the plane of the Milky Way, we could look down on the glorious spectacle of the galactic disk, where four main spiral arms delineate the structure of our galaxy. Alas, it is not to be. We're buried deep within the disk itself, which muddles our view of galactic structure.

Despite this handicap, radio astronomers have made great strides over the past few decades mapping the Milky Way, identifying four main spiral arms that all originate near the galactic center. In an ongoing radio survey of the southern sky, a team led by Naomi McClure-Griffiths (Commonwealth Science and Industrial Research Organization, Australia) recently identified what could be a new spiral arm in the outer reaches of the disk.

In the team's survey, which includes data from the Australia Compact Telescope Array and the 64-meter Parkes radio telescope, the arm showed up as a region of enhanced density of neutral hydrogen gas. Starting at about 60,000 light-years from the galactic center, the structure curls outward to a distance of 80,000 light-years, putting it beyond the Milky Way's visible disk of stars and gas.

Milky Way's spiral structure

The newly discovered spiral arm lies well beyond the visible disk of the Milky Way galaxy. It may be an extension of the Outer Arm, which probably connects to the Norma Arm in the inner part of the galaxy.

S&T diagram by Steven Simpson. Source: James Cordes / Joseph Lazio / Naomi McClure-Griffiths.

Astronomers do not know yet if this arm is forming stars. Gas clouds in the four main spiral arms continuously churn out newborn stars. But in galaxies such as M81 in Ursa Major and M83 in Hydra, gaseous spiral arms extend well beyond the visible disk and contain few if any stars. The southern radio survey can only detect hydrogen, but recent radio observations by Masanori Nakagawa (Nagoya University, Japan) and his colleagues found carbon monoxide, a tracer of dense gas clouds, from the region of this new spiral arm. "This result may point to star formation, but it's inconclusive," says McClure-Griffiths.

Because there are gaps in the radio survey's sky coverage, the possibility exists that the new arm connects to one of the four main spiral arms: the Outer Arm (which probably attaches to the Norma Arm in the inner galaxy). If all three features connect, it would be the first Milky Way spiral feature known to wrap more than 360° around the galactic center.

The gravitational tugs of satellite galaxies could have disrupted the outer disk of the Milky Way, forming the newly discovered arm. "Maybe spiral structure in this region of the galaxy is affected by the Magellanic Clouds," says McClure-Griffiths. Dark matter presumably provides the gravitational glue that prevents the arm from disintegrating.

The team's discovery paper will be published in the June 1st Astrophysical Journal Letters.


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