Gaia starts mapping our Milky Way’s bar

“We knew the Milky Way had a bar, like other barred spiral galaxies. But we only had indirect indications from the motions of stars and gas. This is the first time we see the galactic bar in 3D space, based on geometric measurements of stellar distances.”

Since its second data release in 2018, the Gaia satellite of the European Space Agency (ESA) has been revolutionizing the way we see our home galaxy, the Milky Way. On July 16, 2019, astronomers mining Gaia data in combination with infrared and optical surveys performed from ground and space – looking specifically at the distribution of 150 million Milky Way stars, of the billion tracked so far by Gaia – announced the first direct measurement of the bar-shaped collection of stars at our galaxy’s center. In the video above, you can see an artist’s concept of this bar, a large and elongated feature made of stars. In the video, orange/yellow hues indicate a greater density of stars (mostly red giants). Our sun is represented by the larger orange/yellow blob in the lower part of the image (in reality, the sun is nowhere near this bright or prominent). Astronomers called this new work:

… the first geometric indication of the galactic bar.

Because Gaia, of course, is all about geometry, about the measurements of points and lines. More specifically, in the language of astronomy, Gaia is about astrometry. The satellite is equipped to measure the positions and brightnesses of Milky Way stars and other objects, over and over, as these objects move in space around the center of our galaxy. In this way, astronomers can obtain exact distances for these objects via parallax. The ultimate goal is to construct a precision 3D map of our Milky Way. Gaia’s measurements are complicated by the fact that our galaxy is a very dusty place, and the dust obscures distant stars. In this study of the galactic bar, astronomers refined their analysis of Gaia data – giving consideration to this dust – via a computer code called StarHorse, developed by co-author Anna Queiroz and collaborators.

Friedrich Anders from University of Barcelona, Spain (@frediferente on Twitter) is lead author of the new study. He said in a statement:

We looked in particular at two of the stellar parameters contained in the Gaia data: the surface temperature of stars and the ‘extinction,’ which is basically a measure of how much dust there is between us and the stars, obscuring their light and making it appear redder.

These two parameters are interconnected, but we can estimate them independently by adding extra information obtained by peering through the dust with infrared observations.

Team member Cristina Chiappini from Leibniz Institute for Astrophysics Potsdam, Germany, added:

With the second Gaia data release, we could probe a radius around the sun of about 6,500 light-years, but with our new catalog, we can extend this ‘Gaia sphere’ by three or four times, reaching out to the center of the Milky Way.

There, these astronomers said – at the center of our galaxy – the data clearly reveal a large, elongated feature in the three-dimensional distribution of stars: the galactic bar. Anders said:

We know the Milky Way has a bar, like other barred spiral galaxies, but so far we only had indirect indications from the motions of stars and gas, or from star counts in infrared surveys. This is the first time that we see the galactic bar in 3D space, based on geometric measurements of stellar distances.

Chiappini added:

Ultimately, we are interested in galactic archaeology: we want to reconstruct how the Milky Way formed and evolved, and to do so we have to understand the history of each and every one of its components.

It is still unclear how the bar – a large amount of stars and gas rotating rigidly around the center of the galaxy – formed, but with Gaia and other upcoming surveys in the next years we are certainly on the right path to figure it out.

The team said it is looking forward to the next data release from the Apache Point Observatory Galaxy Evolution Experiment (APOGEE-2), as well as upcoming facilities such as the 4-metre Multi-Object Survey Telescope (4MOST) at the European Southern Observatory in Chile and the WEAVE (WHT Enhanced Area Velocity Explorer) survey at the William Herschel Telescope (WHT) in La Palma, Canary Islands. Their statement added:

The third Gaia data release, currently planned for 2021, will include greatly improved distance determinations for a much larger number of stars, and is expected to enable progress in our understanding of the complex region at the center of the Milky Way.

Face-on spiral galaxy with two major spiral arms and a central yellow bar with a bulge in the middle.

Artist’s concept of our Milky Way galaxy and its central bar. The larger orange/yellow blob in the lower part of the image is a glorified representation of our sun, showing its approximate location with respect to the bar. Image via ESA.

Bottom line: Data from European Space Agency’s Gaia satellite, combined with infrared and optical surveys performed from ground and space, have enabled astronomers to obtain the first direct measurements of our Milky Way’s central bar.

Via ESA

Deborah Byrd