  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) predicted such a reversal to occur only at only at earlier epochs (z>2)
and at a lower level. 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 at the ~1-2 Mpc scale. The SFR of z~1 galaxies
is found to correlate with stellar mass suggesting that mass plays a role in
the observed star formation-density trend. However the specific SFR (=SFR/M*)
decreases with stellar mass while it increases with galaxy density, which
implies that the environment does directly affect the star formation activity
of galaxies. Major mergers do not appear to be the unique or even major cause
for this effect since nearly half (46%) of the luminous infrared galaxies
(LIRGs) at z~1 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 irregular (14%) galaxies. Moreover, the specific SFR of
major mergers is only marginally stronger than that of spirals. Reproducing the
SFR-density relation at z ~ 1 is a new challenge for models, requiring a
correct balance between mergers and in-situ star formation at early epochs.
