Spitzer repère une rupture dans le bras de la Voie Lactée

A new study shows evidence for a previously unseen structure in our own Milky Way galactic neighborhood.

A break seen in the Milky Way’s inner arm. Credit: NASA/JPL-Caltech

Sometimes, it’s hard to see the forest through the trees. This is just the situation we’re in when we try to discern just what our Milky Way Galaxy might look like from the outside. Now, a new study looking at galactic structure noticed a gap in the Sagittarius spiral arm of the Milky Way that went previously unnoticed.

The study combined data from NASA’s Spitzer infrared telescope to peer through layers of gas and dust obscuring the view towards the galactic center, and measurements from the European Space Agency’s Gaia astrometry mission, which looks at the true distances of stellar populations.

Located about 2,000 light-years distant, the Sagittarius Arm is the next spiral arm in from our own home address in the small Orion Spur, located between the Perseus and Sagittarius Arm. The gap itself is about 3,000 light-years wide, a good portion of the overall 25,000 light-year distance from our solar system to the galactic core. This structure feathers out from the arm, similar to the feathery appendages seen in remote flocculent galaxies. In fact, this is the first such major break in galactic structure seen in the Milky Way itself. The key to uncovering the elusive structure is what’s known as pitch angle, or how far the winds of the Milky Way’s spiral arms deviate from circular (a pitch angle of 0). On average, 12 degrees is an expected pitch angle; however, the structure identified in the study has a noticeably high pitch angle of 60 degrees.

Spitzer versus Gaia

Retired in early 2020, the Spitzer Space Telescope compiled an infrared study of the sky during its 16 year-plus tenure. Astronomers in the study looked at newborn stars nestled in nebulous dust clouds hidden from view in visible light, but apparent in Spitzer’s view. The survey data used was from GLIMPSE, the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire. Then, to get a true 3D view, this structure was mapped using Gaia survey data. Launched in 2013, Gaia uses parallax measurements to determine stellar distances, in the most accurate astrometric catalog yet. The second full data release (the Gaia DR2 catalog) gives distance measurements for over 1.3 billion stars.

“Distances are among the most difficult things to measure in astronomy,” says co-author on the study Alberto Krone-Martins (University of California) in a recent press release. “It is only the recent, direct distance measurements from Gaia that make the geometry of this new structure so apparent.”

The history of the knowledge of our place in the Milky Way was hard won. William Herschel completed the first rough stellar population survey in 1785, outlining the first jagged-looking profile for what was to become our view of our home galaxy. But it was the key insight of astronomer Jacobus Kapteyn a little over a century ago in 1904 that caught evidence for galactic rotation in proper motion surveys.

Today, we know that we reside in a mature, barred spiral galaxy with four major arms, and that our Sun is located 25.000 light-years from the core, which it orbits once every quarter of a billion years.

Outstanding Objects in ‘The Gap’

But here’s what’s really weird: This 3,000 light-year wide gap actually lies in plain view. The plane of the Milky Way is a highlighted feature on summer evenings, and still spans the sky from the southwest to the northeast on October evenings. In fact, the feather structure composing ‘The Gap’ contains four famous deep-sky objects from Messier’s catalog: the Lagoon Nebula (M8), the Trifid Nebula (M20), the Omega Nebula (M17) and the Eagle Nebula (M16), which contains the iconic Pillars of Creation complex. Together, this spans a gap through the constellations Sagittarius, Serpens Cauda, and Scutum in the night sky.

Turns out, we don’t know our own galactic neighborhood as well as we thought. What other strange surprises exist waiting to be discovered, in our very own Milky Way Galaxy?