Tunable filter imaging: structure forming around a quasar at z=0.9

Preliminary results are described for emission-line imaging of the field of a quasar at z=0.9 with a unique new instrument --- the TAURUS Tunable Filter (TTF). At least fourteen [O II]-emitting galaxy candidates are found within +/- 750 km/s of the quasar redshift with a significance of >3 sigma. Another eight candidates (>3 sigma) are also found with relative velocities +/- 750-1500 km/s from the quasar, indicative of a large velocity dispersion in the field. Together with the existence of a group of red galaxies in the field identified through broad-band imaging, we suggest that the TTF data are probing cluster merging which is ongoing at z=0.9.