Early formation and recent starburst activity in the nuclear disc of the
  Milky Way
  The nuclear disc is a dense stellar structure at the centre of the Milky Way,
with a radius of $\sim$150 pc. It has been a place of intense star formation in
the past several tens of millions of years but its overall formation history
has remained unknown up to now. Here we report the first detailed star
formation history of this region. The bulk of its stars formed at least eight
billion years ago. This initial activity was followed by a long period of
quiescence that was ended by an outstanding event about 1 Gyr ago, during which
roughly 5% of its mass formed in a time window $\sim$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
percent of the stellar mass in the past $\sim$500 Myr, with an increased rate
up to $\sim$30 Myr ago. Our findings contradict the previously accepted
paradigm of quasi-continuous star formation at the centre of the Milky Way. The
long quiescent phase agrees with the overall quiescent history of the Milky Way
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, and that the central black hole may have acquired most
of its mass already in the early days of the Milky Way.
