We introduce the first two simulations of the IllustrisTNG project , a next generation of cosmological magnetohydrodynamical simulations , focusing on the optical colors of galaxies .
We explore TNG100 , a rerun of the original Illustris box , and TNG300 , which includes 2 \times 2500 ^ { 3 } resolution elements in a volume twenty times larger .
Here we present first results on the galaxy color bimodality at low redshift .
Accounting for the attenuation of stellar light by dust , we compare the simulated ( g-r ) colors of 10 ^ { 9 } < M _ { \star } /M _ { \odot } < 10 ^ { 12.5 } galaxies to the observed distribution from the Sloan Digital Sky Survey ( SDSS ) .
We find a striking improvement with respect to the original Illustris simulation , as well as excellent quantitative agreement with the observations , with a sharp transition in median color from blue to red at a characteristic M _ { \star } \sim 10 ^ { 10.5 } M _ { \odot } .
Investigating the build-up of the color-mass plane and the formation of the red sequence , we demonstrate that the primary driver of galaxy color transition is supermassive blackhole feedback in its low-accretion state .
Across the entire population the median color transition timescale \Delta t _ { green } is \sim 1.6 Gyr , a value which drops for increasingly massive galaxies .
We find signatures of the physical process of quenching : at fixed stellar mass , redder galaxies have lower SFRs , gas fractions , and gas metallicities ; their stellar populations are also older and their large-scale interstellar magnetic fields weaker than in bluer galaxies .
Finally , we measure the amount of stellar mass growth on the red sequence .
Galaxies with M _ { \star } > 10 ^ { 11 } M _ { \odot } which redden at z < 1 accumulate on average \sim 25 % of their final z = 0 mass post-reddening ; at the same time , \sim 18 % of such massive galaxies acquire half or more of their final stellar mass while on the red sequence .