Blanc SIMI 5 - Sciences de l'information, de la matière et de l'ingénierie : Sciences de l’univers

Dissecting the past and present mechanisms of the inner Milky Way – GALHIS


Dissecting the past and present mechanisms of the <br />inner Milky Way

Study of the ancient stellar populations of the Milky Way, in particular the thin disc, thick disc, bar, bulge, as tracers of the formation of the Galaxy

With the up-coming of large scale spectroscopic surveys, the analysis of the spatial distributions of chemical abundances on the one hand, and of stellar velocities of the other hand, brings new constraints for our understanding of Galactic evolution. Combining observational approaches, collecting wide and accurate data sets, with the development of population synthesis models and of numerical simulations, convey means to understand the physical processes that explain Galactic evolution, and allow to test scenarios that can be inferred from the data analysis. Our project allowed to participate in the definition and the build up of the Gaia-ESO spectroscopic survey; to analyse the data in terms of structure, chemical abundances, kinematics of the thick disc and the bar of the Galaxy; to study by numerical simulations the evolution of the bulge and disc populations, especially the radial migration; to improve the Besançon Galaxy Model. The results have sensitively improved our knowledge about galactic evolution, on the point of view of star formation history and chemical evolution. Data, simulations, and the new methods developed during the project, will be used in the near future to analyse the data coming from the Gaia satellite.<br /><br />

New techniques have been developed and qualified for automatic classification of stellar spectra, determination of fundamental parameters and the chemical abundances of stars. These various approaches have been key techniques for the success of the large survey Gaia-ESO and have given us the opportunity to take the lead and coordinate the reduction and analysis of the spectra in the consortium. Moreover, a code for computing chemical evolution in the Milky Way has been developed. It has been used to interpret chemical abundances measured in the solar neighborhood and has led to propose a new scheme for chemical evolution of our Galaxy. This code has then been coupled with the SPH code to be able to treat correctly in SPH simulations the chemical evolution and recycling of the gas. A probabilistic approach of star formation and recycling has been implemented and tested. The comparison of observational data with predictions of the population synthesis model has benefited from the use of multivariate data analysis and of fitting process such as the Markov Chain Monte Carlo methods.

Measurement of the history of star formation in the local thick disc (Snaith et al. 2014). Analysis of process of radial migration in discs of spiral galaxies (Di Matteo et al. 2013) and in the solar neighbourhood (Haywood et al. 2013). Determination of the structure and evolution of the thick disc on large scales (size, shape, chemical composition, kinematics) (Robin et al. 2014). Link between chemical composition and kinematics in the thick disc and thin disc beyond the solar neighborhood (Recio-Blanco et al. 2014, Mikolaitis et al. 2014). Characterization of the bar of the Milky Way (Robin et al. 2012, Babusiaux et al. 2013). Link between the thick disc and the bulge region, analyse of the contribution of the thick disc in the inner Galaxy (Di Matteo et al. 2014ab, Robin et al, 2014, Rojas et al. 2014). Proposal of a new scheme for stellar populations in the Milky Way (Haywood et al. 2013). Analysis of the contribution of thick discs in the formation of galactic discs in general, using the exemple of the Milky Way (Lehnert et al. 2014).

An outstanding result obtained thanks to this ANR project is the new definition of stellar populations proposed by Haywood et al (2013), combined with new constraints on the bulge evolution (Di Matteo et al 2014ab). Thanks to these results and to the development of the chemical evolution model, we are engaged in several complementary studies. The first one concern the analyse of the relation between ages and abundances in alpha elements. The second aimed at generalising the chemical evolution code to the whole Galaxy. The third project will use the SPH code combined with the chemical evolution scheme to understand the formation of thick disc in galaxies in general. It will allow to confront this scenario with the up-coming Gaia mission data.

Another important result is the constrain on the dynamical evolution of the thick disc with the description of its contraction at early ages (Robin et al 2014). This advanced outcome will be studied in detail using the combined analysis of the Gaia-ESO and APOGEE surveys, and the soon coming Gaia data. The scenario will be confronted to these new data thanks to the optimised version of the Besancon Galaxy model. This renewed model will be rapidly put on line in order for potential users to take advantage of this simulation method, via the Virtual Observatory tools.

14 publications in major reviews, 38 communications in international meetings, 8 communications in national meetings. Organisation of 2 international meetings.

This project aims at understanding the formation of the Milky Way from observations of the stellar populations which bear
the imprint of early Galactic evolution in their chemical abundances, kinematics, and spatial distribution. We plan to study
the inner regions of the Milky Way, where most of the stellar mass resides, in order to understand the physical process of formation of the bulge, either by early accretions, by gravitational collapse or by disc instability (bar or pseudo-bulge). Using our data sets, we will study the role of radial mixing in the evolution of the disc, the links and interaction between the bulge, the inner disc and the thick disc populations. We shall analyse about 30 fields distributed in the central regions and around, several millions of stars with photometry and astrometry (proper motions), several thousands stars with spectroscopy (elemental abundances and radial velocities), complemented by observations in the visible and near-infrared from public data. We shall analyse these data in several steps. First a detailed field by field analysis will allow to characterize the mean properties of each population. Second a global analysis of the various fields will be performed to understand the overall characteristics of the populations (mass distribution, mean metallicity, rotation and velocity ellipsoid, various spatial gradients) and the correlations between the parameters which are the tracers of the evolution (ages, chemical abundances, kinematics). We shall then be able to identify the characteristics which allow to distinguish different scenarios of formation of these populations (determination of the epoch of formation, history of star formation, identify tracers of merging events in different populations, role of the dynamical interaction between those populations, role of migrations). In parallel we shall integrate the results obtained from our analysis in a population synthesis model which will permit to ensure the consistency of parameters obtained with an overall scheme. This will also test the robustness of the results against possible bias and if necessary to find the method for correcting these bias. Gobal fitting of the model parameters (density laws, star formation rate, velocity ellipsoids, chemical and kinematical gradients, etc.) will also be performed using efficient model fitting method like the Monte Carlo Markov Chain approach in order to optimize the results and to deduce a global and consistent population model. We thus be able to test the most probable scenario explaining our observations. Dynamical tests will be done in order to ensure that the mass distribution deduced from the various observables is consistent with the kinematics observed. Two approaches will be used, one based on the Syer & Tremaine approach called "Made-to-Measure" which adapts the orbits of the Schwarzschild method to the observables, the other based on N-body and Tree SPH simulations from the approach of Semelin & Combes. Starting from initial conditions provided by LCDM scenarios it will simulate the (chemo-dynamical) evolution of a typical galaxy, of the same morphological type and mass as the Galaxy, which will be called by analogy a “ready-to-wear” model.

Finally, we shalll be able to provide simulations in the form of mock catalogues, usable for the preparation of future projects, and in the framework of the Virtual Observatory. Our experience in large data sets analysis and model developement will be valuable in future analysis of large scale surveys in particular Gaia.

Project coordinator


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.



Help of the ANR 400,000 euros
Beginning and duration of the scientific project: - 36 Months

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