Colours, Star formation Rates, and Environments of Star forming and
  Quiescent Galaxies at the Cosmic Noon
  We analyse the star formation rates (SFRs), colours, and dust extinctions of
galaxies in massive (10^12.5-10^13.5 Msun) haloes at z~2 in high-resolution,
cosmological zoom-in simulations as part of the Feedback in Realistic
Environments (FIRE) project. The simulations do not model feedback from active
galactic nuclei (AGN) but reproduce well the observed relations between stellar
and halo mass and between stellar mass and SFR. About half (a third) of the
simulated massive galaxies (massive central galaxies) at z~2 have broad-band
colours classifying them as 'quiescent', and the fraction of quiescent centrals
is steeply decreasing towards higher redshift, in agreement with observations.
The progenitors of z~2 quiescent central galaxies are, on average, more
massive, have lower specific SFRs, and reside in more massive haloes than the
progenitors of similarly massive star forming centrals. The simulations further
predict a morphological mix of galaxies that includes disk-dominated,
irregular, and early-type galaxies. However, our simulations do not reproduce
the reddest of the quiescent galaxies observed at z~2. We also do not find
evidence for a colour bimodality, but are limited by our modest sample size. In
our simulations, the star formation activity of central galaxies of moderate
mass (Mstar~10^10-10^11 Msun) is affected by a combination of two distinct
physical processes. Outflows powered by stellar feedback result in a
short-lived (<100 Myr), but almost complete, suppression of star formation
activity after which many galaxies quickly recover and continue to form stars
at normal rates. In addition, galaxies residing in slowly growing haloes tend
to experience a moderate reduction of their SFRs ('cosmological starvation').
The relative importance of these processes and AGN feedback is uncertain and
will be explored in future work.
