Evidence for Mature Bulges and an Inside-out Quenching Phase 3 Billion
  Years After the Big Bang
  Most present-day galaxies with stellar masses $\geq10^{11}$ solar masses show
no ongoing star formation and are dense spheroids. Ten billion years ago,
similarly massive galaxies were typically forming stars at rates of hundreds
solar masses per year. It is debated how star formation ceased, on which
timescales, and how this "quenching" relates to the emergence of dense
spheroids. We measured stellar mass and star-formation rate surface density
distributions in star-forming galaxies at redshift 2.2 with $\sim1$ kiloparsec
resolution. We find that, in the most massive galaxies, star formation is
quenched from the inside out, on timescales less than 1 billion years in the
inner regions, up to a few billion years in the outer disks. These galaxies
sustain high star-formation activity at large radii, while hosting fully grown
and already quenched bulges in their cores.
