Efficient satellite quenching at z~1 from the GEEC2 spectroscopic survey of galaxy groups

We present deep GMOS-S spectroscopy for 11 galaxy groups at 0.8<z<1.0, for galaxies with r_AB<24.75. Our sample is highly complete (>66%) for eight of the eleven groups. Using an optical-NIR colour-colour diagram, the galaxies in the sample were separated with a dust insensitive method into three categories: passive (red), star-forming (blue), and intermediate (green). The strongest environmental dependence is observed in the fraction of passive galaxies, which make up only ~20 per cent of the field in the mass range 10^10.3<M_star/M_odot<10^11.0 but are the dominant component of groups. If we assume that the properties of the field are similar to those of the `pre-accreted' population, the environment quenching efficiency (ε_ρ) is defined as the fraction of field galaxies required to be quenched in order to match the observed red fraction inside groups. The efficiency obtained is ~0.4, similar to its value in intermediate-density environments locally. While green (intermediate) galaxies represent ~20 per cent of the star-forming population in both the group and field, at all stellar masses, the average sSFR of the group population is lower by a factor of ~3. The green population does not show strong H-delta absorption that is characteristic of starburst galaxies. Finally, the high fraction of passive galaxies in groups, when combined with satellite accretion models, require that most accreted galaxies have been affected by their environment. Thus, any delay between accretion and the onset of truncation of star formation (τ) must be <2 Gyr, shorter than the 3-7 Gyr required to fit data at z=0. The relatively small fraction of intermediate galaxies requires that the actual quenching process occurs quickly, with an exponential decay timescale of τ_q<1 Gyr.