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

Eleonora Zari

Wednesday, Dec. 7th, 14:50CET

The star formation rate (SFR) describes the evolution of galaxies across cosmic time. To put the Milky Way (MW) in the context of the overall population of galaxies, we study the recent (< 1 Gyr) SFR of the MW, aiming at understanding how it had varied in space and time. To this end we determine the stellar age distribution of stars <1Gyr within ~3kpc of the Sun, using the Gaia EDR3-selected young massive star (OBA) sample presented in Zari et al. (2021). We describe the young stellar age distibution within ~100pc x 100pc patches of the disc with a simple non-parametric functional form. Given a set of isochrones and a Kroupa initial mass function (IMF) corrected for unresolved binaries, we predict the magnitude distribution of luminous, hot stars in each patch and maximize the data likelhood varying the model age distribution. The derived age distributions are able to reproduce the number and observed density distribution of OBA stars in the Galactic plane. We find strong spatial variations in the density of very young stars (<50 Myr), corresponding to the location of spiral arms and star forming regions. Using TNG50 simulations of Milky Way analog galaxies, we find that our survey volume is large enough that we can identify our derived age distribution with the global disc SFR to good approximation. The global SFR over the last 50 Myr is in general consistent with previous studies (~2 MSun/yr). Remarkably, we find that the Milky Way’s SFR in the last 50Myrs has been 3-5 times higher than it was 200-500 Myr ago. Whether this is related to the recent passage of the LMC would be interesting to explore.