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New Data Indicates Greater Density of Stars in the Milky Way

Complementary observations by space- and ground-based telescopes and data from the Gaia mission (European Space Agency, ESA) were integrated to make the first-ever direct measurement of the bar-shaped group of stars located at the middle of the Milky Way galaxy

This color chart, superimposed on an artistic representation of the galaxy, shows the distribution of 150 million stars in the Milky Way probed using data from the second release of ESA’s Gaia mission in combination with infrared and optical surveys, with orange/yellow hues indicating greater density of stars. Most of these stars are red giants. While the majority of charted stars are located closer to the Sun (the larger orange/yellow blob in the lower part of the image), a large and elongated feature populated by many stars is also visible in the central region of the galaxy: this is the first geometric indication of the galactic bar. (Image credit: ESA/Gaia/DPAC, A. Khalatyan (AIP) & StarHorse Team; mapa artístic de la Galaxia: NASA/JPL-Caltech/R. Hurt (SSC/Caltech)

Reported in Astronomy & Astrophysics, the study was headed by scientists from the Leibniz Institute for Astrophysics Potsdam (Germany) and from the Institute of Science Cosmos of the University of Barcelona.

Published in 2018, the second release of data from Gaia star-mapping satellite has been redefining several fields of astronomy. The unparalleled catalog includes the motions, distance indicators, positions, and brightness across the sky for over a billion stars present in the Milky Way galaxy, together with data about other celestial objects.

This is only the beginning. Although the second release of data is predicated on the first 22 months of Gaia’s surveys, the satellite has been monitoring the sky for five years, and will continue to do so at least until 2022.

The release of new data scheduled in the upcoming years would not only steadily  enhance the measurements but would also offer additional information that will help in charting the home galaxy and thoroughly investigate its history like never before.

In the meantime, several astronomers have integrated the newest Gaia data with optical and infrared observations carried out from space and ground to offer a preview of what upcoming releases of ESA’s stellar surveyor will disclose.

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.

Friedrich Anders, Study Lead Author and ICCUB Member, University of Barcelona

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

The researchers integrated the second release of data from Gaia satellite with a number of infrared surveys utilizing a computer code known as StarHorse. Co-author Anna Queiroz and other collaborators developed this code, which compares the observations made with stellar models so as to establish the stars’ surface temperature, the extinction, and also get an enhanced estimate of the distance to the stars.

Consequently, the astronomers were able to more accurately determine the distances to approximately 150 million stars – in certain cases, the enhancement is around 20% or more. This allowed the astronomers to track the stars’ distribution across the Milky Way galaxy to relatively greater distances than feasible using only the original data from the Gaia satellite.

With the second Gaia data release, we could probe a radius around the Sun of about 6500 light years, but with our new catalogue, we can extend this ‘Gaia sphere’ by three or four times, reaching out to the centre of the Milky Way.

Cristina Chiappini, Study Co-Author, Leibniz Institute for Astrophysics Potsdam, Germany

The project was coordinated at the Leibniz Institute for Astrophysics Potsdam. At the middle of the Milky Way galaxy, the data evidently shows a huge, elongated trait in the 3D distribution of stars—that is, the galactic bar.

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 three-dimensional space, based on geometric measurements of stellar distances,” stated Friedrich Anders.

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,” added Cristina Chiappini.

It is still unclear how the bar—a large amount of stars and gas rotating rigidly around the centre 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,” observed the researcher.

The researchers are looking ahead to the subsequent release of data from the Apache Point Observatory Galaxy Evolution Experiment (APOGEE-2), and also from upcoming centers, for example, the 4-m Multi-Object Survey Telescope (4MOST) deployed at the European Southern Observatory in Chile and the WHT Enhanced Area Velocity Explorer (WEAVE) survey at the William Herschel Telescope (WHT) deployed in La Palma (Canary Islands).

Presently scheduled for 2021, the third Gaia data release will include considerably enhanced determinations of distance for a relatively larger number of stars, and is anticipated to promote advances in gaining a better insight into the intricate region at the center of the Milky Way galaxy.

With this study, we can enjoy a taster of the improvements in our knowledge of the Milky Way that can be expected from Gaia measurements in the third data release.

Anthony Brown, Study Co-Author, Leiden University (the Netherlands)

We are revealing features in the Milky Way that we could not see otherwise: this is the power of Gaia, which is enhanced even further in combination with complementary surveys,” concluded ESA’s Gaia project scientist Timo Prusti.

Flyby around the StarHorse Gaia DR2 density distribution of Milky Way stars

Flyby around the StarHorse Gaia DR2 density distribution of Milky Way stars. (Video credit: Friedrich Anders on Vimeo)


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