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We use the deep wide‐field optical imaging data of the Subaru/XMM–Newton Deep Survey to discuss the luminosity‐ (mass‐)dependent galaxy colours down to z′= 25.0 (5 × 109h−270 M⊙) for z∼ 1 galaxies in colour‐selected high‐density regions. We find an apparent absence of galaxies on the red colour–magnitude sequence below z′∼ 24.2, corresponding to ∼M*+ 2 (∼ 1010 M⊙) with respect to passively evolving galaxies at z∼ 1. Galaxies brighter than M*− 0.5 (8 × 1010 M⊙), however, are predominantly red passively evolving systems, with few blue star‐forming galaxies at these magnitudes. This apparent age gradient, where massive galaxies are dominated by old stellar populations while less massive galaxies have more extended star formation histories, supports the ‘down‐sizing’ idea where the mass of galaxies hosting star formation decreases as the Universe ages. Combined with the lack of evolution in the shape of the stellar mass function for massive galaxies since at least z∼ 1, it appears that galaxy formation processes (both star formation and mass assembly) should have occurred in an accelerated way in massive systems in high‐density regions, while these processes should have been slower in smaller systems. This result provides an interesting challenge for modern cold dark matter based galaxy formation theories which predict later formation epochs of massive systems, commonly referred to as ‘bottom‐up’.

Fibre Multi-Object Spectrograph (FMOS) is the first near-infrared instrument with a wide field of view capable of acquiring spectra simultaneously from up to 400 objects. It has been developed as a common-use instrument for the F$/$2 prime-focus of the Subaru Telescope. The field coverage of 30$^\prime$ diameter is achieved using a new 3-element corrector optimized in the near-infrared (0.9–1.8$\ \mu$m) wavelength range. Due to limited space at the prime-focus, we have had to develop a novel fibre positioner, called “Echidna”, together with two OH-airglow suppressed spectrographs. FMOS consists of three subsystems: the prime focus unit for IR, the fibre positioning system/connector units, and the two spectrographs. After full systems integration, FMOS was installed on the telescope in late 2007. Many aspects of the performance were checked through various test and engineering observations. In this paper, we present the optical and mechanical components of FMOS, and show the results of our on-sky engineering observations to date.