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

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Abstract
We have previously reported a significant excess of K 19 galaxies in the fields of a sample of 31 z = 1-2 quasars (Hall, Green, & Cohen). Here we examine the properties of this excess galaxy population using optical and near-IR imaging. The excess occurs on two spatial scales. One component lies at θ < 40'' from the quasars and is significant compared to the galaxy surface density at θ > 40'' in the same fields. The other component appears roughly uniform to θ ~ 100'' and is significant compared to the galaxy surface density seen in random-field surveys in the literature. The r - K color distributions of the excess galaxy populations are indistinguishable and are significantly redder than the color distribution of the field population. The excess galaxy population is consistent with being predominantly early-type galaxies at the quasar redshifts, while there is no evidence that it is associated with intervening Mg II absorption systems. The average excess within 0.5 h−175 Mpc (~65'') of the quasars corresponds to Abell richness class ~0 compared to the galaxy surface density at >0.5 h−175 Mpc from the quasars and to Abell richness class ~1.5 compared to that from the literature. We estimate -0.65+ 0.41−0.55 mag of evolution in MK* to =1.67 by assuming the excess galaxies are at the quasar redshifts. We discuss the spectral energy distributions (SEDs) of galaxies in fields with data in several passbands. Most candidate quasar-associated galaxies are consistent 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, and a number of others are consistent with ~2 Gyr old but dust-reddened galaxies at the quasar redshifts. These potentially different galaxy types suggest there may be considerable dispersion in the properties of early-type cluster galaxies at z ~ 1.5. There is also a population of galaxies whose SEDs are best modeled by background galaxies at z 2.5.
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