Colors, Star formation Rates, and Environments of Star forming and Quiescent Galaxies at the Cosmic Noon

Feldmann et al., available on arXiv.

Abstract: We analyze the SFRs, stellar masses, galaxy colors, and dust extinctions of galaxies in massive (10^12.5-10^13.5 M_sun) halos 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 AGN but reproduce well the observed relations between stellar and halo mass and between stellar mass and SFR. About half of the simulated massive galaxies at z~2 have broad-band colors classifying them as `quiescent’, and the fraction of quiescent centrals is steeply decreasing towards higher redshift, in agreement with observations. However, our simulations do not reproduce the reddest of the quiescent galaxies observed at z~2. While simulated quiescent galaxies are less dusty than star forming galaxies, their broad band colors are often affected by moderate levels of interstellar dust. The star formation histories of the progenitors of z~2 star forming and quiescent galaxies are typically bursty, especially at early times. The progenitors of z~2 quiescent central galaxies are, on average, more massive, have lower specific SFRs, and reside in more massive halos than the progenitors of similarly massive star forming centrals. In our simulations, the suppression of SFR in moderately massive central galaxies at high z can be achieved – at least temporarily – by a combination of two distinct physical processes. Outflows powered by stellar feedback often 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 halos 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.