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

Periodic Reporting for period 1 - GaiaSpectroSynergy (Exploiting the synergies between Gaia and spectroscopic stellar surveys to constrain next-generation Milky-Way models)

Reporting period: 2019-02-01 to 2021-01-31

Galaxies are complex systems governed by competing physical processes that happen on very different space, time and energy scales, such as mergers, internal secular evolution, gas flows, star formation, supernova feedback, gas cooling etc. The field of Galactic archaeology aims at disentangling these processes, eventually unfolding the detailed formation history of the Milky Way by resolving all Galactic components and substructures and studying them in greatest possible detail.

The main challenge of Galactic archaeology today is to unravel the Milky Way’s assembly and evolution history by determining ages, chemical compositions, and kinematics of millions of stars covering all parts of the Milky Way. Major observing campaigns of the last decade, and the success of the Gaia mission, have assured that this is in principle possible: precise radial velocities as well as basic chemical information for millions of stars have already been obtained by surveys such as RAVE, SEGUE, APOGEE, LAMOST, GALAH, or the Gaia-ESO survey. In the near future, even bigger projects like 4MOST, WEAVE, and MOONS will allow us to look even deeper into the stellar components of the Milky Way. Gaia, on the other hand, allows us to measure parallaxes and transverse kinematics for more than a billion stars for the first time with unprecedented precision; the second Gaia data release (DR2), published in April 2018, represented a huge leap for precision astrometry. The combination of Gaia with public spectroscopic surveys is a goldmine for stellar and Galactic astrophysicists – and the ideal testbench for present-day and next-generation Milky-Way models.

We undertake a major analysis project exploiting Gaia and complementary spectroscopic survey data to constrain Galactic models, and to simultaneously provide the community with legacy tools and datasets that will greatly facilitate such studies in the future. The project has produced several catalogues of precise stellar parameters, distances, and extinctions for more millions of Milky Way stars. All data products are made publically available.
"In Anders et al. (2019), we applied StarHorse for the first time on the huge amount of Gaia data combined with multi-wavelength data from photometric surveys - without spectroscopic measurements. What could have been a computational disaster turned out to work extremely well: our results improved the accuracy of the Gaia DR2 astrophysical parameters and geometric distances and revealed the presence of the Galactic bar in the Gaia data in a much more direct manner than before. This culminated in a press release by the European Space Agency ( which had some echo in the media. By the end of the project, our 2019 StarHorse paper has already been cited 85 times - and the published data are being used actively throughout the astronomical community.

StarHorse also serves to determine distances, extinctions, and fundamental parameters for spectroscopic surveys. In our latest StarHorse paper led by my former student Anna Queiroz (Queiroz, Anders, et al. 2020), we obtain an extensive chemical map of the Galactic plane using data from APOGEE DR16, with unprecedented coverage of the disc close to the Galactic mid-plane from the Galactic centre out to the edge of the disc. The improvements in statistics as well as distance and extinction uncertainties confirmed the presence of Galactic the bar in stellar density and revealed a striking chemical duality in the innermost regions of the disc, which now clearly extends to the inner bulge. In the accompanying catalogues, we provide distances and extinctions for 6 million stars observed by spectroscopic surveys. These results are being widely used by the Galactic astrophysics community too.

Open clusters are groups of a dozen to several thousands of stars that were born together from the same parent molecular cloud, and remain bound by gravity. Their distances and ages can be estimated more easily than for individual stars (and as such they provide important quality tests for codes like StarHorse), and they can be used to trace the structure of our Galaxy. Gaia DR2 provides us with precise measurements of parallaxes and proper motions that can be used to identify compact groups of stars traveling together through the Galaxy. Most of these newly discovered clusters cannot be seen in static images of the sky, but can easily be spotted using the Gaia measurements of distance and velocity (see e.g. this Gaia image of the week: In collaboration with the rest of the GaiaUB group I have worked on producing the largest and most complete homogeneous catalogue of Galactic open clusters, including precise ages, distances, and interstellar extinctions (Cantat-Gaudin, Anders, et al. 2020). I have also been involved in solving the mystery of the many ""poorly studied"" clusters that Gaia DR2 has proven to be false positives (Cantat-Gaudin & Anders 2020), analysing new cluster candidates (Castro-Ginard et al. 2020), as well as determining the completeness of the cluster catalogue and measuring the cluster age function (Anders et al. 2021)."
"During the Fellowship, the Galactic Astrophysics community has advanced a lot: precision maps of Milky Way stellar populations beyond the solar vicinity have now become a reality. This is mainly thanks to the second data release of Gaia, but our StarHorse catalogues and our work on the Galactic open-cluster census have pushed the state of the art even beyond that. With the very recent Gaia Early Data Release 3, the work on improving these catalogues and the resulting maps of the Milky Way continues.

Also our work on open cluster science, in some ways complementary to the field star research for which StarHorse was developed, is at the forefont of the current state of the art in the field. With the strong collaboration in the GaiaUB group we were able to substantially increase the number of confirmed Galactic star clusters, and unequivocally weed out dubious objects that had been hidden in the open-cluster literature for decades. Since open clusters provide the prime benchmarks for stellar evolution theory and stellar atmospheric parameters, this work is also of prime importance for future runs of the StarHorse code.

Within the European COST framework MW-Gaia, I taught at the Winter School ""The Milky Way As A Galaxy"", and gave lectures on Nucleosynthesis & Chemical Evolution in the past two Astrophysisc MSc courses at the University of Barcelona (UB). I am co-organising a Special Session at the European Astronomical Society Meeting 2021 called ""The Gaia revolution in Milky Way modelling"", covering topics closely linked to the scope of the Fellowship. During my time as an MSCA Fellow I supervised one ERASMUS+ MSc thesis, one MSc at UB, and three TFG (BSc) theses at UB. Galactic Astronomy is fascinating a very broad audience. Our outreach video on the direct detection of the Galactic bar in the StarHorse data has been viewed more than 10,000 times already: The video about the discovery of new open clusters with Gaia DR2 ( has been translated to several European languages. I am involved in regular scientific outreach via my twitter account. I was also guest in a German radio show in September 2020."
Galaxy coverage with spectroscopic surveys, using StarHorse distances (Queiroz et al. 2020)
Density distribution of Gaia DR2 field stars, using StarHorse data (Anders et al. 2019)