J-PLUS Reconstructing the Milky Way Disc's star formation history with twelve-filter photometry

Wide-field, multi-filter photometric surveys enable the reconstruction of the Milky Way’s star formation history (SFH) on Galactic scales and offer new insights into disc assembly. The twelve-filter system of the Javalambre Photometric Local Universe Survey (J-PLUS) is particularly suitable, as its colours trace stellar chemical abundances and help alleviate the age-metallicity degeneracy in colour-magnitude diagram fitting. We aim to recover the SFH of the Galactic disc and separate its chemically distinct components by combining J-PLUS DR3 photometry with Gaia astrometry. We also evaluate the potential of isochrone fitting to estimate stellar ages and metallicities as proxies for evolutionary trends. We fit magnitudes and parallaxes of $1.38\times10^{6}$ stars using a Bayesian multiple isochrone method. The bright region of the colour-absolute-magnitude diagram ($M_{r}\leq4.2$ mag) constrains ages, while the faint region provides an empirical metallicity prior mitigating the age-metallicity degeneracy. Both PARSEC and BaSTI isochrones, in solar-scaled and $α$-enhanced forms, are adopted. The recovered SFH reveals two sequences: an $α$-enhanced population forming rapidly between $12.5$ and $8$ Gyr ago, enriching from [M/H]$\sim-0.6$ to $0.1$ dex; and a solar-scaled sequence emerging $\sim8$ Gyr ago, dominating after $\sim7$ Gyr with slower enrichment reaching solar metallicity by $3$ Gyr. Metal-rich ([M/H]$\gtrsim0$) stars are confined to $\vert z_{GC}\vert\lesssim1$ kpc, whereas metal-poor ([M/H]$\lesssim-0.5$) stars reach $\vert z_{GC}\vert\sim2$ kpc. Simultaneous fitting of both isochrone families reveals distinct formation epochs for the thin and thick discs. J-PLUS multi-filter photometry, combined with Gaia parallaxes, mitigates age-metallicity degeneracies and enables detailed mapping of the Milky Way’s temporal and chemical evolution.

💡 Research Summary

This paper presents a comprehensive reconstruction of the Milky Way disc’s star formation history (SFH) by synergistically combining twelve-filter photometry from the Javalambre Photometric Local Universe Survey (J-PLUS) Data Release 3 (DR3) with precise astrometry from the Gaia mission. The core objective is to overcome the long-standing age-metallicity degeneracy problem in stellar astrophysics and derive a detailed timeline of the Galactic disc’s assembly and chemical evolution.

The study leverages the unique strength of J-PLUS’s filter system, which includes five broad and seven narrow bands designed to trace specific spectral features (e.g., Hα, Ca II H & K, Mg b triplet). These filters provide indirect proxies for stellar chemical abundances, offering a powerful complement to broad-band photometry. The final stellar sample consists of approximately 1.38 million stars within 5 kpc, carefully selected from J-PLUS DR3 using quality flags and cross-matched with Gaia Early Data Release 3 (EDR3) based on stringent astrometric quality cuts (parallax error/parallax ≤ 0.3, RUWE < 1.4).

The methodological cornerstone is a Bayesian multiple-isochrone fitting technique. For each star, its 12-dimensional absolute magnitude vector (converted from photometry using Gaia parallaxes) is compared against extensive grids of theoretical stellar evolution models. Critically, the analysis employs both PARSEC and BaSTI isochrone sets, each in two chemical variants: solar-scaled and α-enhanced. This approach allows the model to distinguish between populations with different alpha-element abundances, a key signature of different formation timescales. The fitting process cleverly uses different regions of the color-absolute magnitude diagram (CAMD): the bright region (M_r ≤ 4.2 mag) primarily constrains stellar ages, while the faint region provides an empirical metallicity prior that helps break the age-metallicity degeneracy.

The recovered SFH reveals two distinct chemical sequences, corresponding to the canonical thick and thin discs of the Milky Way:

1. The α-enhanced sequence (Thick Disc): This population formed rapidly in the early Universe, between approximately 12.5 and 8 billion years ago. It experienced fast chemical enrichment, with metallicity (