Tunable-Filter Imaging of Quasar Fields at [CLC]z[/CLC] ∼ 1. I. A Cluster around MRC B0450−221

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Abstract
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' ≥ 3.8) 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 Wλ > 70 Å) 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 ~ 1 with TTF.
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