Tunable-filter imaging of quasar fields at z ∼ 1.: I.: A cluster around MRC B0450-221

Using a combination of multicolor broadband and narrowband imaging techniques and follow-up spectroscopy, we have detected an overdensity of galaxies in the field of quasar MRC B0450-221, whose properties are consistent with a cluster at the quasar redshift z = 0.9. An excess of red galaxies (V - I > 2.2, I-K' greater than or equal to3.8) is evident within 1' of the quasar, with the colors expected for galaxies at z = 0.9 that have evolved passively for 3 Gyr or more. A number of line-emitting galaxies (nine candidates with equivalent widths W-lambda > 70 Angstrom )are also detected in the field using the TAURUS Tunable Filter (TTF). Three have been confirmed spectroscopically to indeed lie at z = 0.9. The TTF candidates with the strongest [O II] line emission cluster in a group which lies 200-700 kpc away from the quasar and the red galaxy excess, and therefore most likely on the outskirts of the cluster. These observations are the first in a series probing quasar environments at z similar to 1 with TTF.