The reversal of the star formation-density relation in the distant universe

We study the relationship between the local environment of galaxies and their star formation rate (SFR) in the Great Observatories Origins Deep Survey, GOODS, at z~1, from ultradeep imaging at 24 microns with the MIPS camera onboard Spitzer. We show that the star formation-density relation observed locally was reversed at z~1. The average SFR of an individual galaxy increased with local galaxy density when the universe was less than half its present age. Hierarchical galaxy formation models (simulated lightcones from the Millennium model) do expect a reversal but at a lower level and only at earlier epochs (z>2). We present a remarkable structure at z~1.016, containing X-ray traced galaxy concentrations, which will eventually merge into a Virgo-like cluster. This structure illustrates how the individual SFR of galaxies increases with density and shows that it is the ~1-2 Mpc scale that affects most the star formation in galaxies at z ~ 1. There is a correlation between the SFR and stellar mass of galaxies at z ~ 1 but the specific SFR (=SFR/M*) increases with galaxy density hence favoring the interpretation that the environment does directly affect the star formation activity of galaxies. Nearly half of the z ~ 1 LIRGs present the HST-ACS morphology of spirals, while only a third present a clear signature of major mergers, the remaining galaxies are divided into compact (9%) and irreguliar (14%) galaxies. The specific SFR of major mergers is only marginally stronger than that of spirals, hence major mergers appear not to be the major cause for the reversal of the star formation-density relation at z ~ 1. Reproducing the SFR-density relation at z ~ 1 is a new challenge for models, requiring a correct balance between mass assembly through mergers and in-situ star formation at early epochs