Léa Marques

Reconstructing the star formation history (SFH) of disk galaxies is central to understanding their growth and evolution, yet such estimates can be strongly biased by stellar radial migration over cosmic time. Using 186 Milky Way (MW) and Andromeda (M31) analogs from the TNG50 cosmological simulation, we compare star formation rates (SFRs) inferred from present-day stellar positions with those based on stellar birth radii to quantify the magnitude, spatial structure, and temporal evolution of migration-induced biases. We find that radial migration systematically produces artificial star formation in regions that had not yet formed stars. Notably, ~80% of galaxies exhibit outer-disk stars older than 10 Gyr, which we find to have formed at radii interior to the outer disk and to have reached their present locations via substantial outward migration. Similar effects appear in ~45% of galaxies at intermediate radii during early epochs, and in 30% of quenched inner disks within the past 4 Gyr. Migration also smooths SFHs, washing out localized bursts and suppressions by dispersing stars across neighboring radii. The strength and imprint of these distortions depend sensitively on galactic structure and evolutionary history: strong bars drive mean SFR overestimates of up to 75% in the inner disk and 150% in the outskirts; thinner, dynamically cold disks suffer average outer-disk biases up to 160%; while thick disks exhibit typical inner-disk biases up to 125%. Merger timing further modulates these patterns. Our results demonstrate that failing to account for stellar migration can lead to severe misinterpretations of when and where stars formed, with direct implications for the chemical and evolutionary histories of the MW and external galaxies.

Context . The Milky Way’s (MW’s) star formation history (SFH) offers insights into the chronology of its assembly and the mechanisms driving its structural development. Aims . In this study, we present an inference and analysis of the spatially resolved SFH and the MW disc growth. Methods . Our approach leverages both stellar birth radii estimates and the complete reconstruction of the MW stellar disc using a novel orbit superposition method from APOGEE data, allowing us to trace the orbit-mass weighted SFH based on formation sites, while taking into account stellar mass loss. Results . We find that the MW is a typical disc galaxy exhibiting inside-out formation: it was compact at z > 2 (R _eff ≈ 2 kpc), had a peak in its star formation rate (SFR) 9–10 Gyr ago, and grew to a present-day size of R _eff ≈ 4.3 kpc. A secondary peak in the SFR ~4 Gyr ago is responsible for the onset of the outer disc, which comprises the metal-poor, low- α population. We find that in situ star formation in the solar neighbourhood started 8–9 Gyr ago. The MW disc is characterised by a negative mean age gradient, as the result of the inside-out growth, with additional flattening induced by stellar radial migration. Conclusions . Our work showcases the importance of accounting for radial migration and the stellar sample selection function when inferring the SFH and build-up of the MW disc.

Knowledge of the spatially resolved star formation history (SFH) of disk galaxies provides crucial insight into disk assembly, quenching, and chemical evolution. However, most reconstructions, both for the Milky Way and for external galaxies, implicitly assume that stars formed at their present-day radii. Using a range of zoom-in cosmological simulations, we show that stellar radial migration introduces strong and systematic biases in such SFH estimates, and in a Milky Way–like case study we link these biases directly to the disk’s merger-driven, non-axisymmetric response. In the inner disk ( R ≲ h _d ), early star formation is typically underestimated by 25–50% and late star formation overestimated, giving the misleading impression of prolonged, moderate activity. An exception occurs in the most central bin considered (∼0.4 h _d ), which is consistently overestimated due to a net inflow of inward migrators. At intermediate radii and in the outer disk, migration drives the opposite trend: intermediate-age populations are overestimated by 100–200% as stars born in the inner disk migrate outward, whereas genuinely in situ populations are underestimated by ∼50% as they themselves continue to migrate. The net effect is that SFH peaks are suppressed and broadened, and the true rate of inside-out disk growth is systematically underestimated. These distortions affect all galaxies in our sample and have direct implications for interpreting spatially resolved SFHs from integral field unit surveys such as CALIFA and MaNGA, where present-day radii are often used as proxies for stellar birth sites. Correcting these biases will require accounting for the disk mass, bar presence, disk kinematics and morphology, and recent birth-radius estimation techniques for Milky Way stars offer a promising path forward.

Context. Central bars and spirals are known to impact significantly the evolution of their host galaxies, both in terms of dynamics and star formation. Their typically different pattern speeds cause them to regularly overlap, which induces fluctuations in bar parameters. Aims. In this paper, we analyze both numerical simulations of disk galaxies and observational data to study the effect of bar-spiral physical overlap on stellar radial migration and star formation in the bar vicinity, as a function of time and galactic azimuth. Methods. We studied three different numerical models, two of which are in a cosmological context, alongside APOGEE DR17 data and the WISE catalog of Galactic HII regions. Results. We find that periodic boosts in stellar radial migration occur when the bar and spiral structures overlap. This mechanism causes net inward migration along the bar leading side, while stars along the bar trailing side and minor axis are shifted outward. The signature of bar-spiral-induced migration is seen between the bar inner Lindbald resonance and well outside its corotation, beyond which other drivers take over. We also find that, in agreement with simulations, APOGEE DR17 stars born at the bar vicinity (which are mostly metal rich) can migrate out to the solar radius while remaining on cold orbits. For the Milky Way, 13% of stars in the solar vicinity with an eccentricity <0.5 were born inside the bar, compared to 5–20% in the simulations. Bar-spiral reconnections also result in periodic starbursts at the bar ends with an enhancement of up to a factor of 4, depending on the strength of the spiral structure. Similarly to the migration bursts, these do not always happen simultaneously at the two sides of the bar, which hints at the importance of odd spiral modes. Data from the WISE catalog suggest this phenomenon is also relevant in our own Galaxy.

The stellar disc is the dominant luminous component of the Milky Way (MW). Although our understanding of its structure is rapidly expanding due to advances in large-scale surveys of stellar populations across the Galaxy, our picture of the disc remains substantially obscured by selection functions and an incomplete spatial coverage of observational data. In this work, we present the comprehensive chrono-chemo-kinematic structure of the MW disc, recovered using a novel orbit superposition approach combined with data from APOGEE DR 17. We detected periodic azimuthal metallicity variations within 6–8 kpc with an amplitude of 0.05–0.1 dex peaking along the bar major axis. The radial metallicity profile of the MW also varies with azimuth, displaying a pattern typical among other disc galaxies, namely: a decline outside the solar radius and an almost flat profile in the inner region, attributed to the presence of old, metal-poor high-α populations, comprising ≈40% of the total stellar mass. The geometrically defined thick disc and the high-α populations have comparable masses, but with differences in their stellar population content, which we quantified using the reconstructed 3D MW structure. The well-known [α/Fe]-bimodality in the MW disc, once it has been weighted by the stellar mass, is less pronounced at a given metallicity for the whole galaxy but distinctly visible in a narrow range of galactic radii (5–9 kpc), explaining its relative lack of prominence in external galaxies and galaxy formation simulations. Analysing a more evident double age–abundance sequence, we constructed a scenario for the MW disc formation, advocating for an inner and outer disc dichotomy genetically linked to the MW’s evolutionary stages. In this picture, the extended solar vicinity is a transition zone that shares the chemical properties of both the inner (old age-metallicity sequence) and outer discs (young age-metallicity sequence).

Context. Recent works have identified a way to recover the time evolution of a galaxy’s disk metallicity gradient from the shape of its age–metallicity relation. However, the success of the method is dependent on how the width of the star-forming region evolves over time, which in turn is dependent on a galaxy’s present day bar strength. Aims. In this paper, we account for the time variation in the width of the star-forming region when deriving the interstellar medium (ISM) metallicity gradient evolution over time (∇[Fe/H](τ)), which provides more realistic birth radii estimates of Milky Way (MW) disk stars. Methods. Using MW/Andromeda analogs from the TNG50 simulation, we quantified the disk growth of newly born stars as a function of present day bar strength to provide a correction that improves recovery of ∇[Fe/H](τ). Results. In TNG50, we find that our correction reduces the median absolute error in recovering ∇[Fe/H](τ) by nearly 30%. To confirm its universality, we tested our correction on two galaxies from NIHAO-UHD and found the median absolute error is almost four times smaller even in the presence of observational uncertainties for the barred MW-like galaxy. Applying our correction to APOGEE DR17 red giant MW disk stars suggests the effects of merger events on ∇[Fe/H](τ) are less significant than originally found, and the corresponding estimated birth radii expose epochs when different migration mechanisms dominated. Conclusions. Our correction to account for the growth of the star-forming region in the disk allows for better recovery of the evolution of the MW disk’s ISM metallicity gradient and, thus, more meaningful stellar birth radii estimates. With our results, we are able to recover the evolution of the ISM gradient, providing estimates for the total stellar disk radial metallicity gradient and key constraints to select MW analogs across redshift.

We introduce a novel orbit superposition method designed to reconstruct the stellar density structure, kinematics, and chemical abundance distribution of the entire Milky Way by leveraging 6D phase-space information from its resolved stellar populations, limited by the spatial coverage of APOGEE DR17.