An article published in the journal “Astronomy & Astrophysics” reports the discovery of an anomalous structure in the Milky Way’s Sagittarius Arm. A team of researchers used observations made with NASA’s Spitzer Space Telescope and data collected by ESA’s Gaia space probe to locate a structure about 3,000 light-years long that has an orientation that’s very different from the rest of the Sagittarius Arm. Such structures have been identified in other galaxies but it’s the first time that one of them has been identified within the Milky Way. That’s not a surprise, as it’s difficult to study such large structures of the galaxy from the inside.
Astronomers have only roughly mapped the Milky Way due to difficulties in measuring distances to stars accurately from within the galaxy. Increasingly sophisticated instruments are helping to build an ever more precise map of the Milky Way but it’s a very long job.
In examining the Sagittarius Arm, a team of astronomers searched for newborn stars using observations conducted with the Spitzer Space Telescope, whose mission ended in January 2020, as part of the GLIMPSE (Galactic Legacy Infrared Mid-Plane Survey Extraordinaire) project. The choice is due to the fact that those stars are still surrounded by large amounts of gas and dust that block many electromagnetic emissions but infrareds pass through those materials and Spitzer detected them. Other data were obtained thanks to the Gaia space probe, a special space telescope that for some years has been mapping stars and many other objects also to accurately measure their distance from Earth.
The top image (NASA/JPL-Caltech) shows a reproduction of the likely Milky Way’s shape and in the inset, there are details of the Sagittarius Arm with its “splinter”. The orange stars indicate star formation regions.
Caltech astrophysicist Michael Kuhn, the lead author of this study, explained that a key property of spiral arms is how tightly they wind around the galaxy. This parameter is measured by what is called in jargon the pitch angle, which increases as a spiral becomes more open. According to the best measurements made so far, the Sagittarius Arm forms a spiral with a pitch angle of 12 degrees. The structure composed of the young stars observed with the Spitzer Space Telescope revealed a pitch angle of nearly 60 degrees, a remarkable difference.
The structure discovered has been likened to a splinter, and the ones discovered in the arms of other spiral galaxies are sometimes called spurs or feathers. Astronomers have been wondering for decades if the Milky Way had structures of that type as well and finally this study located the structure in the Sagittarius Arm.
The first measurements of distances to objects conducted in the 1950s were approximate but helped to deduce the existence of the Sagittarius Arm. It includes some of the most beautiful nebulae known: the Eagle Nebula, which includes the famous Pillars of Creation, the Omega Nebula, the Trifid Nebula, and the Lagoon Nebula, shown in the bottom image (NASA/JPL-Caltech).
Astronomers are still trying to fully understand the mechanisms behind the formation of the spiral arms of galaxies such as the Milky Way. Understanding the formation of anomalies such as the “splinter” in the Sagittarius Arm can help shed light on the processes taking place in this type of galaxy.
