The evolution of star formation histories of quiescent galaxies
  Although there has been much progress in understanding how galaxies evolve,
we still do not understand how and when they stop forming stars and become
quiescent. We address this by applying our galaxy spectral energy distribution
models, which incorporate physically motivated star formation histories (SFHs)
from cosmological simulations, to a sample of quiescent galaxies at
$0.2<z<2.1$. A total of 845 quiescent galaxies with multi-band photometry
spanning rest-frame ultraviolet through near-infrared wavelengths are selected
from the CANDELS dataset. We compute median SFHs of these galaxies in bins of
stellar mass and redshift. At all redshifts and stellar masses, the median SFHs
rise, reach a peak, and then decline to reach quiescence. At high redshift, we
find that the rise and decline are fast, as expected because the Universe is
young. At low redshift, the duration of these phases depends strongly on
stellar mass. Low-mass galaxies ($\log(M_{\ast}/M_{\odot})\sim9.5$) grow on
average slowly, take a long time to reach their peak of star formation
($\gtrsim 4$ Gyr), and the declining phase is fast ($\lesssim 2$ Gyr).
Conversely, high-mass galaxies ($\log(M_{\ast}/M_{\odot})\sim11$) grow on
average fast ($\lesssim 2$ Gyr), and, after reaching their peak, decrease the
star formation slowly ($\gtrsim 3$ Gyr). These findings are consistent with
galaxy stellar mass being a driving factor in determining how evolved galaxies
are, with high-mass galaxies being the most evolved at any time (i.e.,
downsizing). The different durations we observe in the declining phases also
suggest that low- and high-mass galaxies experience different quenching
mechanisms that operate on different timescales.
