Galaxy Evolution in the Local Group
Probing Galaxy Evolution with Structures, Chemo-dynamics and Star Formation Histories of resolved galaxies

We also use both photometric (optical and near-infrared) and spectroscopic observations to constrain the properties of bright stellar pulsators such as RR Lyrae stars and Cepheids. The combination with parallaxes from the ESA Gaia mission will allow us to provide new relations to derive the distance for these classes of stars (Gilligan et al. 2021; Crestani et al. 2021; Mullen et al. 2022).

The overarching goal of ChronoGal is to unveil the sequence of events and physical processes that have shaped the Milky Way from its early assembly to the present, by providing the holy grail of Galactic Archaeology: precise age information. Our Galaxy is key to study disk galaxy formation and evolution, as it can be analyzed in exceptional detail using individual stars carrying the fossil record of its entire evolution. The importance of Galactic Archaeology A is underscored by the vast resources invested in it: Gaia parallaxes, homogeneous photometry, proper motions and velocities have transformed the field, aided by a huge effort from ambitious ground-based spectroscopic surveys.

Despite spectacular recent progress, Galactic Archaeology is nearing an impasse due to the lack of precise ages for large, unbiased stellar samples, which are critical for determining our Galaxy’s chronology. ChronoGal is introducing color-magnitude diagram fitting in the field’s toolbox. Previously limited to studying external Local Group galaxies, thanks to the accurate Gaia-based distances, this technique can finally now be applied to our Galaxy, providing unprecedentedly precise and homogeneous age-metallicity distributions for hundredths of millions of stars across all Milky Way components.

ChonoGal’s ages will provide clear answers to persistent questions on MW evolution. Crucially, ChronoGal will precisely date the disk emergence, determine the duration of the enigmatic thick disk phase, resolve the debate on the presence of intermediate-age stars in the bulge, reconstruct the major halo building blocks and pinpoint the accretion time of these fossils of high-redshift dwarf galaxies. This will clarify the impact of mergers on shaping the stellar content and morphology of disk galaxies. This ambitious and urgent project, with Gaia DR4 in 2026, will be transformative, providing the reliable and precise ages needed to advance our MW understanding.

Results:

Early work has allowed us to date the early merger of Gaia-Enceladus with the Milky Way and to identify an associated burst of star formation in the thick disc (Gallart et al. 2019), as well as to uncover a number of star formation burst in the Milky Way disc at intermediate ages, possible induced by pericentric passages of the Sagittarius dSph galaxy (Ruiz-Lara et al. 2020). (Press releases: Gaia-Enceladus , Sagittarius ).

In Gallart et al. (2024) we present CMD ft.Gaia, a new package to derive SFHs from CMD fitting tailored for Gaia data, and we use it to analyse the Gaia CMD of the stars currently within 100 pc of the Sun, providing an unprecedentedly detailed view of the age and metallicity distributions of the solar neighbourhood. We show that, with high-precision photometric and distance data, CMD ft.Gaia can achieve a precision better than 10% and an accuracy better than 6% in the dating of stellar populations, even at old ages. A comparison with independent spectroscopic metallicity information shows that metallicity distributions are also determined with high precision, without imposing any a priori metallicity information in the fitting process.

Caption: "Dynamically evolved star formation history of the stars within 100 pc of the Sun (Gallart et al. 2024)."

We use stellar chemo-dynamical properties to identify signatures of past accretion events which are direct traces of the hierarchical formation of our Galaxy (see the recent works by Thomas et al. 2022, Malhan et al. 2022). The inferred properties are used as constraints of models that allow to reconstruct the characteristics of the systems that merged with the Milky Way and are now part of our Galaxy (see Fernández-Alvar et al. 2018 where the SFH of the Gaia Enceladus satellite was determined from the first time based on APOGEE observations and chemical evolution models). The periods of enhanced star formation that can be identified in the Milky Way SFH provide insight on when these accretion events occurred (see Gallart et al. 2019; Ruiz-Lara et al. 2020; Ruiz-Lara et al., 2022).

Stellar structures, such as stellar streams are useful tracers of dark matter. We use results obtained from large scale observational surveys to constrain N-body models of the formation of these structures (see Jensen, Thomas et al. 2021, for the constraints set by the NGC 5466 stream or Mairtin et al. (inc GT) 2022 and Errani et al. (inc GT) for the discovery of the most metal-poor stellar streams ever discovered which can be originated from a non-dark matter free globular cluster). This allows us to probe the 3D distribution of the dark matter in the Milky Way up to large distances, as well as to quantify its level of clumpiness.

We compare the observed characteristics of Milky Way stellar populations with high-resolution state-of-the-art cosmological simulations to put constraints on the mechanisms that led to the current stellar populations and chemo-dynamical configurations of the Milky Way.

We study how dynamical phenomena drive observed galactic structure and help constrain the assembly/accretion history of the Galaxy. This has broader implications for the formation and evolution of Milky-Way-mass systems in general, and for our understanding of the nature of dark matter. Papers: Oria et al. 2021; Jensen et al. 2021; Grand et al. 2020.