An optical-infrared study of radio-loud quasar environments

Abstract

I present the data for an optical/near-infrared study of radio-loud quasar environments from z = 0.6-2.0, and the analysis of the data from z = 1.0-2.0. I thoroughly discuss the sample selection, observing, data reduction, and object cataloging. Even accounting for possible systematic offsets, I find a significant excess of K ≳ 19 galaxies in the fields of z = 1-2 RLQs, on two spatial scales. One component is 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 across the fields (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 thus consistent with being predominantly early-type galaxies at the quasar redshifts. The average excess within 0.5h⁻¹₇₅ Mpc $(∼65'') of the quasars corresponds to Abell richness class ∼0 compared to the galaxy surface density at >0.5h⁻¹₇₅ Mpc from the quasars, and to Abell richness class ∼1 compared to that from the literature. I estimate -0.65⁺⁰·⁴¹₋₀.₅₅ magnitudes of evolution in M*(K) to z̄ = 1.67 by assuming the excess galaxies are at the quasar redshifts. I 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 similar to extremely late-type stars; others have SEDs consistent with being 4-5 Gyr old at z∼1.5 and others are consistent with 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 modelled by background galaxies at z≳2.5, Many of these are dusty or have composite stellar populations, or both, and some may be ≳2 Gyr old at z≳ 2.5. Confirmation of old galaxies at high redshift would constrain the cosmology by requiring a relatively old universe at large lookback times.