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

TLDR: In this paper, the authors studied 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.

Abstract: Aims 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 Methods We use ultradeep imaging at 24� m with the MIPS camera onboard Spitzer to determine the contribution of obscured light to the SFR of galaxies over the redshift range 08≤ z ≤12 Accurate galaxy densities are measured thanks to the large sample of ∼1200 spectroscopic redshifts with high (∼70 %) spectroscopic completeness Morphology and stellar masses are derived from deep HST-ACS imaging, supplemented by ground based imaging programs and photometry from the IRAC camera onboard Spitzer Results 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 for mation models (simulated lightcones from the Millennium model) predicted such a reversal to occur only at earlier epochs (z>2) and at a lower level We present a remarkable structure at z∼ 1016, containing X-ray traced galaxy concentrations, which will eventually merge into a Virgo-like cluster This structure illustrates how the ind ividual 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 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 i mplies 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 o f major mergers is only marginally stronger than that of spirals Conclusions These findings constrain the influence of the growth of large- scale structures on the star formation history of galaxies 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