Brazos espirales de la Vía Láctea: los brazos Perseo, Exterior y Scutum-Centaurus exterior de nuestra galaxia natal.

The graphic illustrates a new result that indicates the outer spiral arms in the Milky Way galaxy may reach wider than previously thought, according to data from NASA’s Chandra X-ray Observatory and ESA’s XMM-Newton. This finding could lead astronomers to adjust their understanding of our home galaxy’s structure and is described in our latest press release.

The sequence begins with an artist’s concept showing the Milky Way galaxy as seen from above with the estimated positions of spiral arms based on previously-obtained data from various telescopes. The second artist’s concept shows new positions of the two spiral arms most distant from the center of the galaxy, that have been adjusted based on X-ray data from Chandra and XMM-Newton.

A team of researchers determined the distances to these spiral arms by studying rings around gamma-ray bursts (GRBs), some of the brightest bursts of light in the universe. GRBs happen when massive stars collapse or neutron stars merge, and they are located at enormous distances — well beyond the confines of our galaxy. The distance measurement technique capitalized on the phenomenon of light echoes, where the light from the GRB bounced off intervening dust clouds in the spiral arms. The diameters of the rings in X-rays give the distances to Earth, with larger rings being generated by dust clouds closer to us.

A composite image shows one set of light echoes used in the new study to determine the distance to the Milky Way’s spiral arms. This image combines X-ray data from Chandra (blue) and optical data from Pan-STARRS (red, green and blue) showing X-ray rings generated by the GRB. The GRB is located at the center of the circles defining the rings, to the left of the X-ray data outlined by the white square.

The researchers used three different GRBs to determine the distances to three spiral arms in the Milky Way. In order of increasing distances from the Galactic Center, they are the Perseus, the Outer, and the Outer Scutum-Centaurus arms. Along the direction to one of the GRBs they found that both the Outer and Outer Scutum-Centaurus arms are about 10% more distant than astronomers previously thought. The differences in the positions of these spiral arms based on the new study are depicted in another artist’s illustration where the updated positions of outermost spiral arms are shown in red and bordered by dashed lines.

Although this technique is a major improvement, it may be difficult to use it for further measurements because bright GRBs that are visible through the plane of the galaxy are rare.

A paper describing these results, led by Beatrice Vaia of Scuola Universitaria Superiore IUSS Pavia and University of Trento in Italy, has been recently published by the Astronomy & Astrophysics journal and is available here. NASA's Marshall Space Flight Center manages the Chandra program. The Smithsonian Astrophysical Observatory's Chandra X-ray Center controls science operations from Cambridge, Massachusetts, and flight operations from Burlington, Massachusetts.

This release features a short video and a series of images, all related to an updated understanding of our home galaxy's structure. By studying rings of X-ray light echoes, researchers now believe that two of the Milky Way's spiral arms may be more distant from the center of the galaxy than previously thought.

The updated understanding of the structure of the Milky Way is highlighted in a short video, which compares two artist concept images. In both images, our spiral Milky Way galaxy is shown face-on. It has a bright white core with several arms that spiral out from the center, like long thin clouds corkscrewing counterclockwise. The two longest arms make a full rotation of the spiral galaxy, and curve all the way around to the upper right of the images.

The first image in the video shows the previous understanding of the Milky Way. Here, the two longest arms are curled around the core in a fairly tight spiral. In the second image, which represents the updated understanding, the two longest arms are more loosely spiraled. Visually, this means there is more open space between the curving arms, which are further away from the bright galaxy core. The video fades back and forth between the two artist concept images to illustrate the structural differences between the two understandings.

These findings are further shown by a static image which overlays the new understanding on top of the earlier understanding. In this artist's concept illustration, dotted lines and different colors are used to differentiate between the two.

A team of astronomers made this discovery by studying gamma-ray bursts that bounce off of dust clouds in the galaxy's spiral arms. The resulting rings of X-rays, known as light echoes, were detected and mapped by NASA's Chandra X-ray Observatory and ESA's XMM-Newton. In a supplemental data image, the light echoes resemble concentric arches of neon blue dots trailing across a speckled sky.

Identifying the position of the Milky Way's spiral arms through X-ray light echoes has allowed astronomers to use geometry, rather than assumptions about galaxy rotation, to better understand the structure of our galaxy.