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

Abstract

[Abridged] We map the stellar age distribution ($\lesssim 1$ Gyr) across a 6kpc$\,\times\,$6kpc area of the Galactic disc to constrain our Galaxy's recent star-formation history. Our modelling draws on the sample of Zari et al. (2021) that encompasses all presumed disc OBA stars ($\sim 500,000$ sources) with $G<16$. To be less sensitive to reddening, we do not forward model the detailed CMD distribution of these stars, but instead the K-band absolute magnitude distribution, $n(M_K)$, among stars with $M_K<0$ and $T_{\mathrm{eff}} > 7000$K at a certain positions $\vec{x}$ in the disc as a step function with five age bins, $b(\tau~|~\vec{x}, \vec{\alpha})$, logarithmically-spaced in age from $\tau = 5$ Myr to $\tau = 1$ Gyr. Given a set of isochrones and a Kroupa (2001) initial mass function, we sample $b(\tau\,|\,\vec{x}, \vec{\alpha})$ to maximize the likelihood of the data, 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 all age intervals. When summing over the maps' area and extrapolating to the whole disc, we find an effective star-formation rate over the last 10 Myr of $\approx 3.3 \mathrm{M_{\odot}/yr}$, higher than previously published estimates. 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.