Observed structure of the Milky Way's spiral arms

The Sagittarius Arm (also known as Sagittarius-Carina Arm; labeled "-I") is generally thought to be one of the spiral arms of our home galaxy, the Milky Way. Each spiral arm is a long, diffuse curving streamer of stars that radiates out from the galactic center. These gigantic structures are often composed of billions of stars, and Sagittarius is one of the largest arms in our galaxy.

The Milky Way is a barred spiral galaxy, consisting of a central crossbar from which several spiral arms radiate outwards. The Sagittarius Arm's innermost end connects to one of the ends of this central bar, making it one of the two major spiral arms of the galaxy. The other large arm is the Cygnus Arm.

The dense, inner arm of Sagittarius is located between the Scutum-Crux Arm and the Orion Arm (the Orion arm is marked as the Local Spur on the galactic map, and Orion also contains our sun). It is named for its proximity to the Sagittarius constellation as seen in the night sky from Earth, in the direction of the galactic center.

The Sagittarius Arm is divided into two parts. Curving outward from the galaxy's central bar is the Sagittarius Arm (Sagittarius bar), which further outward becomes the Carina arm.

Does the arm exist at all?

Recently it has been suggested that the Sagittarius Arm does not exist at all and is simply a concentration of gas and newly formed stars http://space.newscientist.com/article/dn14057-two-of-the-milky-ways-spiral-arms-may-be-demoted.html?feedId=online-news_rss20.