Science & Technology

Mapping the Milky Way: Galactic bar measured by Gaia

The ESA's Gaia satellite has made the first measurement of the twirling bar of stars at the centre of the Milky Way.
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The ESA’s Gaia satellite has made the first measurement of the twirling bar of stars at the centre of the Milky Way.

A team of astronomers have combined data from the Gaia satellite with optical and infrared observations and a computer code–Starhorse–to better estimate the distance between Earth and the stars at the centre of the Milky Way, also obtaining more details about the bar of stars and gas which rotate rigidly around the centre of our galaxy.

The team, led by Friedrich Anders, were able to obtain a better determination of the distances to roughly 150 million stars at the centre of our galaxy. In the process, they have been able to trace the distribution of stars across the Milky Way better than ever before. Better too, than data from Gaia alone would allow.

3D Visualisation of the density of stars in the ESA/Gaia data release 2. The view rotates around the solar position. The colour encodes density: blue is low, yellow/orange are high. Due to the Gaia selection function, the highest stellar density is measured close to the Sun, but it is also possible to discern some nearby star clusters, and even the Galactic bar (mainly its orange-clump stars). ( StarHorse team. Visualisation: Arman Khalatyan Credit background image in the beginning of the video: NASA/Caltech/R. Hurt )

Co-author Cristina Chiappini from Leibniz Institute for Astrophysics Potsdam, Germany, where the project was coordinated, explains: “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.”

The research, published in the journal Astronomy & Astrophysics, clearly reveals a large, elongated feature in the three-dimensional distribution of stars–the galactic bar–in greater detail than before.

Anders says: “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.”

This colour 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. The distances to the stars shown in this chart, along with their surface temperature and extinction - a measure of how much dust there is between us and the stars - were estimated using the StarHorse computer code. (Data: ESA/Gaia/DPAC, A. Khalatyan(AIP) & StarHorse team; Galaxy map: NASA/JPL-Caltech/R. Hurt (SSC/Caltech))
This colour 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 the 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. The distances to the stars shown in this chart, along with their surface temperature and extinction – a measure of how much dust there is between us and the stars – were estimated using the StarHorse computer code. (Data: ESA/Gaia/DPAC, A. Khalatyan(AIP) & StarHorse team; Galaxy map: NASA/JPL-Caltech/R. Hurt (SSC/Caltech))

Chiappini explains that the team’s ultimate aim in galactic archaeology. She continues: “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.”

The team is still unclear how the galactic bar formed. They hope that further data from Gaia will shed more light on this puzzle over the coming years.

Gaia: the gift that keeps giving

This latest research is just another example of the wealth of data Gaia has been able to provide in a short space of time and as such, holds much promise for the future of astronomy.

The Gaia satellite mission is revolutionising astronomy. astounding consider only 22 months worth of data has been released thus far. (ESA)
The Gaia satellite mission is revolutionising astronomy. astounding consider only 22 months worth of data has been released thus far. (ESA)

Even though the European Space Agency’s satellite has been operating Gaia for five years, the data the satellite released in 2018–which has been revolutionising astronomy– comes from just its first 22 months of surveys.

Newly released data collected between now and the end of the mission in 2022 will steadily improve measurements of qualities like the surface temperature of stars, and the amount of extinction by gas and dust in the galactic plain which stops their light from reaching us.

As it happens, it was these two parameters which Anders and team used to gain the results documented above.

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

As such, the team is waiting with bated breath for the next data-dump from Gaia–scheduled for 2021. Co-author Anthony Brown, Leiden University, Netherlands, points out: “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.”

The team expects this data to be further strengthened by data releases 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).

Timo Prusti, a project scientist at the ESA, concludes: “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 .”

Original research: https://www.aanda.org/component/article?access=doi&doi=10.1051/0004-6361/201935765

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