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 } ) \sim 9.5 ) 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 } ) \sim 11 ) 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 .