Subaru High-z Exploration of Low-luminosity Quasars (SHELLQs). XXI. Candidate Red Quasars at z ≥ 6 from Near-infrared Photometry

We present an analysis of near-infrared photometry of 59 quasars at 5.83 < z < 7.07, obtained with the Gemini North Telescope and the Gran Telescopio CANARIAS. The sample consists of low-luminosity quasars discovered in the Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs) project. With the near-infrared magnitudes combined with the optical photometric and spectroscopic data from the previous Hyper Suprime-Cam and SHELLQs observations, we searched for red (dust-reddened) quasars, characterized by the color excess of E(B − V) > 0.1. We identified five red quasar candidates. The low fraction (8%) of red quasars in our sample suggests that the low luminosity of the SHELLQs quasars is mostly due to the nature of the quasars, not due to extinction by dust. The candidates tend to be found on the low-luminosity side (measured with the rest-ultraviolet absolute magnitude at 1450 Å of M₁₄₅₀ > −24 mag) of the sample. This luminosity trend may support the idea that red quasars represent an evolutionary stage preceding unobscured quasars, often invoked in the merger-driven galaxy evolution scenario.