The Evolutionary history of the Milky Way discs and halo from Gaia DR2 colour-magnitude diagram fitting
Creators: name: Gallart, Carme
name: Ruiz-Lara, Tomás
name: Bernard, Edouard J.
name: Brook, Chris B.
name: Cassisi, Santi
name: Hill, Vanessa
name: Monelli, Matteo
Publisher: name: Zenodo
Year: 2019
Subjects: star formation history
Milky Way
Gaia DR2

description: Abstract: The technique of color-magnitude diagram (CMD) fitting is a well recognized method that has been used for over 20 years to determine detailed star formation histories (SFH) for Local Group galaxies (Gallart et al. 2005; Tolstoy et al. 2009).  The accurate distances for large stellar samples provided in Gaia DR2, together with the precise, homogeneous Gaia photometry, is enabling for the first time to apply the CMD-fitting technique to the different morphological components of our own Galaxy and to robustly derive their SFHs and age distributions using only the Gaia photometric information of millions of stars. This provides answers to long lasting questions regarding the formation and evolution of the disk and halo components of the Milky Way, for which the determination of stellar ages was necessary.  In this presentation we discuss two examples resulting from our ongoing work.

1. Thanks to the CMD fitting method, a SFH profile across the Galactic disk has been obtained for the first time: we have constructed CMDs and computed SFHs for a number of layers parallel to the Milky Way plane, at different heights up to Z=2 Kpc above and below the plane, and close to the solar radius. We call these SFHs 'dynamically evolved SFHs' because the stellar sample present in each volume is potentially affected by stellar migration and blurring. Several epochs of enhanced star formation (~10, 6 and 2 ago) can be observed close to the plane, up to |Z| ~ 400 pc. At larger distances from the plane, the younger star forming epochs gradually diminish their relative strength. This results in a smooth change in the SFR(t) as a function of height |Z|, with mean age increasing with |Z|. The distinct epoch of star formation between 13 and 8 Gyr ago may correspond to the formation of the thick disk.

2. The CMD of the kinematically selected Milky Way halo population published from Gaia DR2 data (Babusiaux et al. 2018) showed two enigmatic sequences running parallel in color from the main sequence to the sub-giant and red-giant branch, indicating the presence of two distinct subpopulations. The blue sequence has been associated with a major accretion event experienced by the Milky Way early in its history (Helmi et al. 2018; Haywood et al. 2018), while the red sequence has been associated with the thick disk (Haywood et al. 2018) by the similarity in chemical composition. We have derived age distributions for the two halo sequences and for a thick disk stellar sample. The age distributions clearly demonstrate that the two sequences in the halo CMD are composed by stars that are coeval and formed at the earliest possible times in the life of the Universe. The difference in color is a result of the red and blue sequence stars having different metallicities. The inferred age distributions also clearly show that the thick disc population includes a stellar population that is younger than the two halo populations.  These results, allow us to tightly constrain that the merger occurred 10 Gyr ago, and to conclude that the red sequence is composed by stars formed in the Milky Way during its first ~ 3 Gyr of evolution, right before the merger with Gaia-Enceladus took place, and heated to halo-like kinematics by the impact. The associated infalling gas would have contributed to maintain and stoke star formation in the early disk up to a second epoch of maximum inferred intensity 9.5 Gyr ago.
description: CG, CBB, MM and TRL  acknowledge financial support through the grants (AEI/FEDER, UE) AYA2017-89076-P, AYA2016-77237-C3-1-P and AYA2015-63810-P, as well as by the Ministerio de Ciencia, Innovación y Universidades (MCIU), through the State Budget and by the Consejería de Economía, Industria, Comercio y Conocimiento of the Canary Islands Autonomous Community, through the Regional Budget. TRL is supported by a MCIU Juan de la Cierva - Formación grant (FJCI-2016-30342). CBB is supported by a MCIU Ramón y Cajal Fellowship (RYC 2013-12784). SC acknowledges support from Premiale INAF "MITIC" and has been supported by INFN (Iniziativa specifica TAsP). VH was supported by the Programme National Cosmology et Galaxies (PNCG) of CNRS/INSU with INP and IN2P3, co-funded by CEA and CNES. We used data from the European Space Agency mission Gaia (\url{http://www.cosmos.esa.int/gaia}), processed by the Gaia Data Processing and Analysis Consortium (DPAC; see \url{http://www.cosmos.esa.int/web/gaia/dpac/consortium}).