Did the Milky Way just light up? The recent star formation history of the Galactic disc

Abstract

We map the stellar age distribution (≲1 Gyr) across a 6 kpc × 6 kpc area of the Galactic disc in order to constrain our Galaxy’s recent star formation history. Our modelling draws on a sample that encompasses all presumed disc O-, B-, and A-type stars (∼500 000 sources) with G < 16. To be less sensitive to reddening, we did not forward-model the detailed CMD distribution of these stars; instead, we forward-modelled the K-band absolute magnitude distribution, n(MK), among stars with MK < 0 and Teff > 7000 K at certain positions x in the disc as a step function with five age bins, b(τ | x, α), logarithmically spaced in age from τ = 5 Myr to τ ∼ 1 Gyr. Given a set of isochrones and a Kroupa initial mass function, we sampled b(τ | x, α) to maximise the likelihood of the data n(MK | x, α), accounting for the selection function. This results in a set of mono-age stellar density maps across a sizeable portion of the Galactic disc. These maps show that some, but not all, spiral arms are reflected in overdensities of stars younger than 500 Myr. The maps of the youngest stars (< 10 Myr) trace major star-forming regions. The maps at all ages exhibit an outward density gradient and distinct spiral-like spatial structure, which is qualitatively similar on large scales among the five age bins. When summing over the maps’ areas and extrapolating to the whole disc, we find an effective star formation rate over the last 10 Myr of ≈ 3.3 M⊙ yr−1, higher than previously published estimates that had not accounted for unresolved binaries. Remarkably, our stellar age distribution implies that the star formation rate has been three times lower throughout most of the last Gyr, having risen distinctly only in the very recent past. Finally, we used TNG50 simulations to explore how justified the common identification of local age distribution with global star formation history is: we find that the global star formation rate at a given radius in Milky Way-like galaxies is approximated within a factor of ∼1.5 by the young age distribution within a 6 kpc × 6 kpc area near R⊙.