An Optical/Near‐Infrared Study of Radio‐loud Quasar Environments. II. Imaging Results

We use optical and near-IR imaging to examine the properties of thesignificant excess population of K>=19 galaxies found in the fields of 31 z=1-2radio-loud quasars by Hall, Green & Cohen (1998). The excess occurs on twospatial scales: a component at <40'' from the quasars significant compared tothe galaxy surface density at >40'' in the same fields, and a component roughlyuniform to ~100'' significant compared to the galaxy surface density seen inrandom-field surveys in the literature. The r-K color distributions of theexcess galaxy populations are indistinguishable and are significantly redderthan the color distribution of the field population. The excess galaxies are consistent with being predominantly early-typegalaxies at the quasar redshifts, and there is no evidence that they areassociated with intervening MgII absorption systems. The average excess within0.5 Mpc (~65'') of the quasars corresponds to Abell richness class ~0 comparedto the galaxy surface density at >0.5 Mpc from the quasars, and to Abellrichness class ~1.5 compared to that from the literature. We discuss the spectral energy distributions (SEDs) of galaxies in fieldswith data in several passbands. Most candidate quasar-associated galaxies areconsistent with being 2-3 Gyr old early-types at the quasar redshifts of z~1.5.However, some objects have SEDs consistent with being 4-5 Gyr old at z~1.5, anda number of others are consistent with ~2 Gyr old but dust-reddened galaxies atthe quasar redshifts. These potentially different galaxy types suggest theremay be considerable dispersion in the properties of early-type cluster galaxiesat z~1.5. There is also a population of galaxies whose SEDs are best modelledby background galaxies at z>2.5.