Modelling the Milky Way in the Gaia era – MOD4Gaia

The uniqueness of our approach consists in aiming to address all these questions by using different and complementary numerical methodologies: test particle methods, orbits reconstruction, chemical evolution, N-body simulations. From test particle methods, where the motion of “test particles” is integrated in Milky Way-like, barred potentials, we expect to have information about the level of complexity of the Galaxy. Can we describe it today «simply« as a disc galaxy evolved secularly in the last 8-9 Gyr under the effect of stellar asymmetries, and its main resonances? Or rather does the comparison with data available with the first Gaia releases indicate some complexity that these simple models are not able to reproduce? The reconstruction of the orbits of some ten million stars should help to understand the origin of the complexity possibly not captured by test particle methods, by quantifying the level of discontinuity in the orbital properties of the different galactic stellar populations. Chemical evolution models will reinforce our understanding, by providing: age-chemistry-kinematics relations, identifying the different chemical patterns and their possible in-situ, ex-situ origin; the star formation history of the Galaxy, and the mass of stars locked in the different stellar populations. All these ingredients will provide the basis for setting the scene for new N-body simulations, that will fully implement dissipative processes and detailed chemical enrichment. With them we aim at reconstructing possible evolutionary paths for the Milky Way in the last 8-9 Gyr, describing the chemo-dynamical links between the inner disc, the bar, and the bulge, and exploring scenarios for the accretion history of the Galaxy. Each of these methodologies, separately, allows to reconstruct some pieces of the puzzle of the chemo-dynamical processes experienced by the Milky Way. All of them, together, should allow us to build a robust and coherent picture of its evolution.

In Pouliasis et al 2017 (A&A), we have developed two new mass models for the Galaxy, which include the contribution oa stellar thin disc and of a thick disc, as massive as the thin counterpart but with a shorter scale-length. Both models satisfy a number of observational constraints. We numerically integrated into these new models the motion of all Galactic globular clusters for which distances, proper motions, and radial velocities are available, the orbits of about one thousand stars in the solar vicinity, and more recently (thanks to the work of A. de La Llave) the orbits of all stars in the TGAS release, which have radial velocities in the APOGEE and RAVE surveys. We have been working on the stellar populations of the Galactic bulge, and a number of works have been published or are in preparation on this topic. In particular we are exploring the scenario proposed in Di Matteo et al 2014, 2015 (see Di Matteo 2016 for a review) according to which the MW bulge is simply the result of the mapping of the Galactic thin and thick discs into the inner regions, through the intermediate of the bar. In Fragkoudi et al (A&A submitted), we show in particular that in the bulge thick disc stars can appear to rotate faster than thin disc stars: an unexpected result! but easily understandable in terms of the redistribution of angular momentum in the inner regions of a MW-type disc. Because the bulge is now recognized as being mainly a boxy peanut-shaped bar, disc stars should be its main constituents, and therefore also stars with ages significantly younger than 10 Gyr. In Haywood et al 2016 (A&A), we show that given the range of metallicities observed in the bulge, a uniformly old population would be reflected in a significant spread in color at the turn-off, which is not observed. The correlation between age and metallicity expected to hold for the inner disc would instead conspire to form a color-magnitude diagram with a remarkably narrow spread in color, as observed.

The first step to understand the Galaxy consists into “summarizing« its main characteristics in dynamical models, that is in equilibrium phase models which are constrained by our current knowledge of the sizes and masses of the stellar populations of the MW, by its rotation curve, etc. These models do not intend to describe the chain of physical processes that drove the Galaxy to its current state. Rather, they represent “snapshots” of the Galaxy today, and are intended to capture some of their main current characteristics. For each of these models, we will analyze the characteristics of the spatial distribution of disc stars and of the velocity fields. This will constitute a first step to understand to what extent the Galaxy can be described by a smooth sequence and continuous transition of stellar populations, and/or how much such simple models fail to describe the observed complexity. By reconstructing the orbits of stars in the TGAS and future Gaia DR2 release, we will study the characteristics and links between Galactic stellar populations for an unprecedented number of stars, by reconstructing the correlation, if any, between their orbital and chemical properties. Moreover, the high resolution age-scale provided by Gaia for the best parallaxes within a few hundred parsecs of the Sun will give us access to the measurement of a detailed SFH of the MW, including at early epochs, thanks to the methodology already used in Snaith et al. 2014, 2015. Very rich data on kpc-scales will also allow us to explore the chemical build-up of our Galaxy and complement the measurement of its SFH at much larger distances. Finally, dissipationless and dissipative N-body simulations will allow us to explore a number of questions related to the evolution of our Galaxy, the evolutionary link between its stellar populations, and the process responsible of the quenching of its star formation (Haywood et al 2016; Khoperskov et al, in prep).

Refereed papers :
1.Jean-Baptiste, Di Matteo, Haywood et al, 2017, A&A in press ; arxiv.org/abs/1611.07193
2.Pouliasis, Di Matteo, Haywood 2017, A&A, 598, 66
3.Haywood, Di Matteo et al, 2016, A&A, 593, 82
4.Di Matteo, P. 2016, PASA, 33, 27 (invited review)
5. Fragkoudi et al, A&A soumis

Oral contributions to conferences (selection) :
1.“On the kinematic detection of accreted streams in the Gaia era: a cautionary tale”, P. Di Matteo, oral contribution to “The Milky Way and its environment” Paris, France, September 2016.
2. “Phylogeny of the inner disk and bulge stellar populations”, M. Haywood, oral contribution to “The Milky Way and its environment”, Paris, France, September 2016.
3. “Bars and b/p bulges in thin and thick discs”, F. Fragkoudi, oral contribution to EWASS 2016 SS6, Athens, Greece
4. “Mapping thin and thick discs into bars and b/p bulges”, F. Fragkoudi, contribution to “The Milky Way and its environment”, Paris, France, September 2016.
5. “Thin and thick discs in bars and boxy/peanut bulges” F. Fragkoudi, oral contribution to t ““Beyond the Solar Neighbourhood: Entering into the Gaia Era”, January 2017, Sesto, Italy.
6. “Star formation quenching in barred galaxies”, S. Khoperskov, oral contribution to ““Beyond the Solar Neighbourhood: Entering into the Gaia Era”, January 2017, Sesto, Italy.
7. Haywood, M.,”Phylogeny of the inner disk and bulge of the Milky Way”,  Janvier 2017, Sesto, Beyond the solar neighborhood: entering into the Gaia Era. , 23-27 January, 2017 in Sesto, Italy. 
8. Haywood, M.,”The thick disk in the HR and LR (disk, halo) WEAVE surveys”, Week of Weave, 28 Nov- 2 Dec, 2016 Leyden
9. Haywood, M.,  “Phylogeny of the inner disk and bulge stellar populations”, The Milky Way and its environment«, 19-23 septembre, IAP, 2016 Paris

Galactic research has entered a thrilling epoch. Our knowledge of Galactic stellar populations, until few years ago mostly confined to stars at the solar vicinity, is rapidly extending to large regions of the disc and bulge of our Galaxy. Large spectroscopic surveys are acquiring an unprecedented amount of data, with radial velocities and chemical composition for hundred thousands stars, from the innermost regions to the periphery of the Milky Way disc, up to ~ 15 kpc from the Galactic center. This unique, because unprecedented, cartography of our Galaxy will acquire all its potential with the publication of the data from Gaia, the European astrometric mission, which will deliver positions and proper motions for 1 billion objects, and radial velocities for about one tenth of them. Without waiting for the final catalogue of the mission (planned for ~ 2022), with the second release of the Gaia data, scheduled by early 2017, the astrometric solution for most the sky will be made public, together with radial velocities for some ten millions stars. In less than two years from now, we will thus be able to reconstruct the orbits of several millions stars in the Galaxy, to have detailed chemical abundances for some hundred thousands and ages for several thousands. The tremendous amount of data that the mission will deliver will need efficient tools for their analysis but also sophisticated models for their interpretation. We are interested to answer to some of the simplest but still unraveled questions of Galactic studies : What are the characteristics of the different Milky Way stellar populations? How were they shaped over time? What is the evolutionary link between them? Which of them is the result of in-situ star formation or rather the deposit of structures accreted over time? The uniqueness of our approach consists in aiming to address them by using different and complementary numerical methodologies, that we have been developing in the last years and that are usually used independently one of the others: test particle methods, orbits reconstruction, chemical evolution, N-body simulations. From test particle methods, where the motion of “test particles” is integrated in a gravitational potential made of a thin and a thick stellar disc, an optional classical bulge, a rotating stellar bar, and a dark matter halo, we expect to have information about the level of complexity of the Galaxy. Can we describe it today "simply" as a disc galaxy evolved secularly in the last 8-9 Gyr under the effect of stellar asymmetries, and its main resonances? Or rather does the comparison with data available with the first Gaia releases indicate some complexity that these simple models are not able to reproduce? The reconstruction of the orbits of some ten million stars should help to understand the origin of the complexity possibly not captured by test particle methods, by quantifying the level of discontinuity in the orbital properties of the different galactic stellar populations. Chemical evolution models will reinforce our understanding, by providing: age-chemistry-kinematics relations, identifying the different chemical patterns and their possible in-situ, ex-situ origin; the star formation history of the Galaxy, and the mass of stars locked in the different stellar populations. All these ingredients will provide the basis for setting the scene for new N-body simulations, that will fully implement dissipative processes and detailed chemical enrichment. With them we aim at reconstructing possible evolutionary paths for the Milky Way in the last 8-9 Gyr, describing the chemo-dynamical links between the inner disc, the bar, and the bulge, and exploring scenarios for the accretion history of the Galaxy. Each of these methodologies, separately, allows to reconstruct some pieces of the puzzle of the chemo-dynamical processes experienced by the Milky Way. All of them, together, should allow us to build a robust and coherent picture of its evolution.