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Exploiting the synergies between Gaia and spectroscopic stellar surveys to constrain next-generation Milky-Way models

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Exciting new data upend our view of the galactic bar and star clusters in the Milky Way

EU-funded researchers offered an extremely detailed 3D census of the galactic bar and the beautiful open clusters comprising dozens to hundreds of stars distributed in the Milky Way disc. Studying the stellar population of the Milky Way could reveal a lot about our home and the Universe as a whole.

Fundamental Research
Space

Galaxies are complex systems governed by competing physical processes that happen on vastly different space, time and energy scales. Major observational campaigns such as ESA’s flagship mission Gaia and ground-based spectroscopic surveys are dedicated to gathering kinematic, chemical and age information for billions of stars. “The complexity and richness of the newly released data pose great challenges to state-of-the-art analysis methods. We already begin to see where current galactic models can be further improved, but many details are still hidden by selection effects that could not be fully accounted for earlier,” notes Friedrich Anders, coordinator of GaiaSpectroSynergy, a project funded by the Marie Skłodowska-Curie Actions programme. Team members leveraged the synergies between Gaia’s richest star catalogue to date and complementary spectroscopic survey data to constrain galactic models and provide researchers with legacy tools that should greatly facilitate future research in the field. The project has produced and published several catalogues of precise photometric parameters, ages, distances and extinctions for more than 265 million stars.

Revisiting the galactic open-cluster population

Gaia's second data release published in April 2018 had already revealed that previous galactic open cluster catalogues were both incomplete and significantly contaminated by asterisms. These star aggregations may fool the eye into seeing them as part of the clusters. “We successfully discovered hundreds of new open clusters and determined homogeneous physical parameters and precise ages for more than 1 800 of them. Study results changed our view on the recent chemo-dynamical history of the galactic disc,” notes Anders. In particular, the team demonstrated that certain key observables of the galactic disc including the cluster age distribution had to be revised. “This affects previous estimates of star cluster formation and destruction rates. We can deduce that only around 16 % of all stars born in the solar neighbourhood form in clustered environments,” adds Anders. Shifting the focus from a separate analysis of field stars (randomly situated stars that contaminate study results) and open clusters towards a joint understanding of stellar evolution in the Milky Way will probably remain a key challenge over the next decade. “The arrival of the Gaia data in 2018 led us to a drastic revision of the galactic open-cluster census,” explains Anders.

Mapping our Milky Way’s bar

The team combined the second Gaia data release with several infrared surveys using a computer code called StarHorse. The code compared the observations with stellar models to determine star surface temperatures, extinctions and distances. “We determined distances of millions of stars with very high accuracy – in certain cases, the improvement (achieved by using multi-wavelength data) was up to 20 %,” says Anders. “What could have been a computational disaster turned out to work extremely well: our results improved the accuracy of the Gaia’s second data release and revealed the presence of the galactic bar in the Gaia data much more directly than before.” “We have known for decades that the Milky Way has a bar, but so far we only had indirect indications from the star motions 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,” concludes Anders.

Keywords

GaiaSpectroSynergy, Gaia, star, Milky Way, open cluster, galactic bar, spectroscopic survey, galactic disc, StarHorse

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