CE31 - Physique subatomique et astrophysique

Getting the record from the disc: A study of the Milky Way disc with WEAVE and Gaia – MWDisc

Getting the record from the disc: A study of the Milky Way disc using WEAVE and Gaia

The Milky Way is a test-bed to study in details the mechanisms that shape galaxies. The synergy between Gaia and WEAVE gives access, for the 1st time, to more than 30 tracers of the past of our Galaxy for a 10^6 stars of the Solar neighborhood, and to a dozen of tracers for another 2 million stars outside of it. Our project concerns the study of the Galactic disc, a structure that encodes both internal and external mechanisms that come into play in the chemo-dynamical evolution of our Galaxy.

Difficulties met with WEAVE

The WEAVE (WHT Enhanced Area Velocity Explorer) survey, is a five- to seven-year project of spectroscopic observations to be carried out from the William Hershel Telescope at Roque de los Muchachos Observatory (Canary Islands, Spain). The Galactic Archaeology (GA) part of this project, from which this project originates, aims to collect spectra for ~3 million stars from the Galactic stellar populations, including a large fraction from the disc.<br />WEAVE, after having been delayed by the combination of the pandemic and administrative issues to Brexit, was further delayed by the eruption of the volcano in La Palma in 2021. The project met hardware difficulties during integration (damage to the positionner during transport) and commissioning (spectrograph out of focus, positionner not meeting the specifications etc.). In principle, these problems are tractable, once their origin is understood. However, the latter is still not completely achieved to date. For this reason, it has been almost a year during which the survey is on the verge to start, but not actually starting. This uncertainty made the organization and re-focus of the project rather complicated. However, this has been done, as described in the summaries of the nodes, in the attached document.

Machine learning techniques for the label transferring research in Nice, together with spectral analysis and isochrone fitting for age determination. Numerical codes for the modelling in Strasbourg, and python scripts for data mining in the Paris node.

In the first year of F. Gran (ANR Postdoc in Nice), significant progress was made towards homogenizing astronomical catalogs for the upcoming WEAVE survey. The main goal was to ensure consistent stellar measurements for stars observed with different instruments, enhancing Milky Way evolution studies.

Preparations were made for the main survey by researching data homogenization techniques. Public spectroscopic catalogs like APOGEE and GALAH, with resolutions similar to WEAVE, served as testing grounds for machine learning algorithms. These algorithms successfully translated observations from one scale to another using stellar parameters and key chemical elements. This effort resulted in a catalog containing 50% more stars than originally, a significant milestone for Milky Way research.

Additionally, 3 projects led to publications: optimizing age determination for FGK stars using various data, investigating spectral lines for accurate stellar abundance derivations, and characterizing 5 newly discovered Galactic globular clusters using optical and near-infrared data.

The Strasbourg node is developing numerical codes to model perturbed action space distribution functions for the Milky Way disc's mono-age populations, with an eye towards interpreting Gaia+WEAVE data. Their goal includes creating a perturbative Milky Way disc model in angle-action coordinates, considering bar and spiral arm perturbations, and addressing resonance regions. Initial results are in Al Kazwini et al. (2022), with ongoing work for multiple populations and resonance handling.

In the absence of WEAVE data, a particle test approach was used to study time-varying perturbations on the Gaia phase space spiral. They demonstrate that a decreasing pattern speed Galactic bar can replicate key features of Gaia's phase space spiral in the RVS sample (Li et al. 2023).

In Paris, V. Cerqui's PhD began in October 2021, with an aim to establish age-chemical abundance relationships to understand Milky Way disc star formation. While waiting for WEAVE data, she optimized age determination for star samples with available chemical data, incorporating spectroscopic and photometric data from surveys like 2MASS and Gaia. She also adapted her code for Gaia photometry, obtaining new age-metallicity and age-alpha relations.

In her second year, V. Cerqui investigated young alpha-rich (YAR) stars alongside her age analysis. Her findings, published in August 2023, indicated that YAR stars, despite being alpha-enhanced, appear younger than typical high-alpha old thick disc stars. She studied their kinematics and chemical properties, suggesting they are rejuvenated thick disc objects, possibly evolved blue stragglers, with shifts in certain elemental abundances.

All of the project members are waiting for WEAVE to start. Current prospect is for the end of 2023. Until that moment, here is a list of the foreseen steps in the different nodes:
The immediate objective is to apply the same process to other surveys when WEAVE observations are available. This aims to access both ends of the Gaia catalog, providing comprehensive dynamical and chemical information.

Currently, V. Cerqui is using age determinations to study Milky Way disc evolution, expanding her sample with data from APOGEE DR17 and the AstroNN Value Added Catalogue. She's working on her second article, aiming to complete it by the end of 2023, with the goal of understanding whether age-chemistry relations in the solar neighborhood reflect isolated or broader evolutionary processes.

Strasbourg :
In Rozier et al. (2022) a code has been developed for the time-varying linear response of a self-gravitating disc under linear perturbations. However, for a galactic disk, numerical complexities are being addressed, together with fine-tuning potential-density pairs, and optimizing the Fourier transform algorithm in angle space.

Published papers:
Al Kazwini et al., A&A 658, A50 (2022)
Cerqui et al., A&A 676, A108 (2023)
Kordopatis et al., A&A 669, A104 (2023)
Kordopatis et al. A&A 674, A104 (2023)
Li et al. in print to MNRAS, doi:10.1093/mnras/stad2199
Gran et al. accepted to A&A

In a hierarchically-formed Universe, the Milky Way is a test-bed to study in details the mechanisms that shape galaxies. The synergy between the Gaia space satellite and the ground-based spectroscopic survey WEAVE gives access, for the first time, to more than thirty tracers of the past of our Galaxy for a million stars of the extended Solar neighbourhood, and to a dozen of tracers for another two million stars outside of it. Our project concerns the study of the Galactic disc, a structure that encodes both internal (e.g. stellar radial migration) and external (e.g. accretion events) mechanisms that come into play in the chemo-dynamical evolution of our Galaxy.

We have built a versatile team with nodes in Nice, Paris and Strasbourg, including experts in Galaxy evolution, simulations and modelling. The team members are heavily involved in both WEAVE and Gaia in order to extract the maximum of information available in those combined catalogues. Over the course of the four-year MWDisc ANR project, and alongside to the accumulation of the WEAVE data (starting in Q1 2021), we aim to produce added-value catalogues for the WEAVE stellar targets (containing homogeneous stellar chemical abundances, ages, orbits and extinctions) and models associated to the diffusion of mono-age populations by time-varying perturbations and superposition of perturbations (associated to the spiral arms and the Galactic bar). These models and catalogues will allow us, in turn, to evaluate the star formation history in various regions of the disc, put constraints on the merger tree of the Milky Way (including the analysis of existing simulations of ours), to link the geometrical properties of the thin and thick disc with their chemical counterparts and finally to characterize the efficiency of radial migration throughout the disc.

Project coordination

Georges Kordopatis (Laboratoire J-L. Lagrange (OCA/CNRS/UCA))

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.


GEPI Galaxies, Etoiles, Physique, Instrumentation
LAGRANGE (OCA/CNRS/UCA) Laboratoire J-L. Lagrange (OCA/CNRS/UCA)
ObAS Observatoire astronomique de Strasbourg (UMR 7550)

Help of the ANR 569,368 euros
Beginning and duration of the scientific project: - 48 Months

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