Early formation and recent starburst activity in the nuclear disk of the Milky Way

Most stars in the Galactic nuclear disk formed at least 8 Gyr ago, with a starburst event about 1 Gyr ago that formed roughly 5% of its mass. This long quiescence has implications on when the Galactic bar was formed and its gas transport efficiency. The nuclear disk is a dense stellar structure at the centre of the Milky Way, with a radius of ~150 pc (ref. (1)). It has been a place of intense star formation in the past several tens of millions of years(1-3), but its overall formation history has remained unknown(2). Here, we report that the bulk of its stars formed at least 8 Gyr ago. After a long period of quiescence, a starburst event followed about 1 Gyr ago that formed roughly 5% of its mass within ~100 Myr, in what may arguably have been one of the most energetic events in the history of the Milky Way. Star formation continued subsequently on a lower level, creating a few per cent of the stellar mass in the past ~500 Myr, with an increased rate up to ~30 Myr ago. Our findings contradict the previously accepted paradigm of quasi-continuous star formation at the Galactic Centre(4). The long quiescent phase agrees with the overall quiescent history of the Milky Way(2,5) and suggests that our Galaxy's bar may not have existed until recently, or that gas transport through the bar was extremely inefficient during a long stretch of the Milky Way's life. Consequently, the central black hole may have acquired most of its mass already in the early days of the Milky Way.