A Wide and Deep Exploration of Radio Galaxies with Subaru HSC (WERGS). X. The Massive and Passive Nature of Radio Galaxies at z ∼ 4

Abstract

High-z radio galaxies (HzRGs) are considered important objects for understanding the formation and evolution of massive galaxies in the early Universe. However, until to date, detailed studies of the stellar population of HzRGs, such as the star formation history, have been scarce. Therefore, this study conducted a new survey to establish a less-biased sample of HzRGs and consequently investigate their properties. We utilized a sample of g-dropout Lyman break galaxies (LBGs) obtained from an optical wide and deep imaging survey made by Subaru Hyper Suprime-Cam. Based on the cross matching of this LBG sample with the Very Large Array Faint Images of the Radio Sky at Twenty cm radio survey data, we constructed a photometric sample of HzRGs at z ∼ 4 for a ∼560 deg2 survey field. Consequently, we identified 146 HzRG candidates. To analyze the characteristics of these candidates, we focus on objects exhibiting the near-infrared photometry of VIKING or UKIRT Infrared Deep Sky Survey and the mid-infrared photometry of unWISE (28 objects). The results indicate that seven objects exhibit spectral energy distributions (SEDs) consistent with galaxies at z ∼ 4. The HzRG candidates have very large stellar masses with ∼4.2 × 1011 M ⊙ on average. This stellar mass is similar to that of previously discovered ultra-steep-spectrum HzRGs at z ∼ 4, though our sample is affected by a sample selection bias that selects only HzRGs with M ⋆ > 1011 M ⊙. Further, the SEDs of those HzRG candidates suggest a past fast quenching with a rough timescale of ∼0.1 Gyr, as evidenced from the rest-frame UVJ diagram.