Gigantic X-shaped structure throws (infrared) light on Galactic history

19. Juli 2016

Two astronomers have produced the first direct images of a gigantic X-shaped distribution of stars in the center of the Milky Way. The collaboration shows the value of open science: it began when Dustin Lang (University of Toronto) tweeted an image he had recently created. From the tweet, Melissa Ness (Max Planck Institute for Astronomy) recognized the image's significance for reconstructing the history of our home galaxy. The X-shaped distribution indicates that the bulge of stars surrounding the center of the galactic disk was formed through dynamical interactions of stars, not by the merger of smaller galaxies with our own.

Sometimes, new science starts with a single tweet. When Dustin Lang of the Dunlap Institute for Astronomy & Astrophysics at the University of Toronto tweeted an image of the Milky Way in May 2015, he was glad to have finished a complex project ("I don’t want to admit how long it took to summarize 150 gigapixels into this WISE [image]," his comment read). He didn't realize the tweet would kick off a new study altogether.

But when Melissa Ness of the Max Planck Institute for Astronomy saw the tweet, she recognized traces of a structure astronomers had long sought for, but never seen directly in an image: an X-shaped distribution of stars in the central region ("bulge") of the Milky Way.

Ness and Lang met in person a few weeks later at a conference and began to collaborate, processing the maps to obtain the first clear image of the giant X at the heart of our galaxy. The results have recently been published in the Astronomical Journal.

For astronomers like Ness, who are interested in the evolution of our home galaxy, the shape of the bulge is a key marker of Milky Way history. As Ness states, "If we understand the bulge we will understand the key processes that have formed and shaped our galaxy." In particular, the presence of the X shape indicates that the central bulge of stars in our Milky Way formed from dynamical interactions of our galaxy's stars and not, as an alternative model posits, from mergers of smaller galaxies with our Milky Way.

For Lang, the collaboration holds a more general lesson about open science. He says: "To me, this study is an example of the interesting, serendipitous science that can come from large data sets that are publicly available," he said. "I'm very pleased to see my WISE sky maps being used to answer questions that I didn't even know existed."

Additional information

A more complete explanation of the research and its context can be found on this background page.

Images for download

Background information: Gigantic X-shaped structure throws (infrared) light on Galactic history

[Back to main text]

In an improved map made using archival data from NASA’s infrared satellite WISE, the astronomers Melissa Ness (Max Planck Institute for Astronomy) and Dustin Lang (Dunlap Institute for Astronomy & Astrophysics at the University of Toronto) have revealed an X-shaped distribution of stars in the bulge, likely to help settle a key question of galaxy evolution: the origin of our Galaxy’s bulge, a group of mostly older stars, arranged in the shape similar to that of a peanut in the middle of the galactic disk.

Galaxies like our own Milky Way are gigantic disks of stars. But real-life structures are never entirely simple. Stars orbiting within a disk that is slightly irregular tend to lump together to form more complex structures. The most prominent features of spiral galaxies, the prominent spiral arms, are thought to have come about either in this way or when nearby dwarf galaxies disturb stellar motion. The bar, a longish structure made of stars in the center regions of our galaxy, is a direct example for an additional structure that arises from slight irregularities of stellar motion.

Forming the galactic bulge

Disk, spiral arms and a bar are three of the four characteristic large-scale structures of our Milky Way. The fourth major structure is the bulge. For the formation of the bulge, there are two competing models: The first links bulge formation to the formation of the galaxy as a whole. The bulge forms first in the innermost region via mergers with other galaxies. The alternative model sees the bulge as a dynamical structure that has formed via internal evolution, from the bar, which extends out of the plane in the inner region as the Galaxy evolves.

The dynamical model of bulges that form from the bar in the disk make specific predictions for the distribution of stars in the central region with respect to their movement around the galactic center, their movement orthogonal to the galactic plane, and a radial component (movement in and out). The orbits combine together to create pretzel-shaped morphologies around the bar (the three-dimensional version of what are known as Lissajous curves). Statistically, this creates a distribution of stars that, seen from the side, would make the bulge somewhat box-like or peanut-shaped. Depending on which orbital resonances dominate, there could even be a gigantic X-shaped distribution of stars crossing at the Galactic centre.

A number of previous studies had reported evidence for an X-shaped or at least box-shaped bulge in the Milky Way, suggesting that our home galaxy's bulge has indeed evolved out of a galactic bar. Some studies had come to this conclusion by counting the number of stars visible along various lines of sight in and near the central regions of the Milky Way. Images from the NASA satellite Cosmic Background Explorer (COBE) had suggested at least a boxy structure for the bulge, but fell short of revealing an X. A 2013 study by researchers at the Max Planck Institute for Extraterrestrial Physics, using data from the European Southern Observatory's VISTA telescope, yielded a 3D reconstruction of the Milky Way's bulge that included an X-shaped structure, but could not yield an actual image of the X.

One problem faced by all such studies is that, from the position of an observer on Earth, substantial parts of the bulge structures are hidden behind numerous layers of dust, requiring infrared observations at just the right wavelengths. Observations such as those made with NASA’s WISE space telescope in 2010-2011.

A serendipitous collaboration

While the original maps created with WISE were not enough to show the central X, the maps produced by Dustin Lang, an astronomer at the Dunlap Institute for Astronomy & Astrophysics at the University of Toronto, in 2014-2015 were a different story. When Lang re-analyzed the WISE infrared sky maps, his goal was much more general. The maps had been optimized to facilitate the detection of point-like objects. Lang, instead, wanted to combine WISE data with additional astronomical observations. For this, he chose a different way of combining WISE data into sky maps, which amounted to sharper images of most of the structures visible in the maps.

When Lang was done in May 2015, he tweeted an overview image of the new map, stating somewhat facetiously “I don’t want to admit how long it took to summarize 150 gigapixels into this WISE [image]”. Melissa Ness, a post-doctoral researcher at the Max Planck Institute for Astronomy, saw that tweet and the image, which shows a clearly box-like bulge for our galaxy, and realized the potential Lang’s new map could have for research into the history of our galaxy.

At a conference in Michigan a few weeks later, Ness and Lang met in person for the first time, and the two decided to collaborate on an analysis of the bulge, using Lang’s new map. By subtracting a smoothed-out model for the large-scale distribution of stars, they were able to make the giant X at the heart of our galaxy clearly visible for the first time. The results have recently been published in the Astronomical Journal.

With these images, the presence of this structure is not any more in doubt. As Ness states: "We see the boxy shape, and the X within it, clearly in the WISE image, which demonstrates that internal formation processes have driven the bulge formation." It will be the task of future observations to analyze the dynamics and properties of the stars in our galaxy’s bulge, drawing conclusions for galactic history. The region these observations need to target is clearly marked with an X.

[Back to main text]

Zur Redakteursansicht