A Wide and Deep Exploration of Radio Galaxies with Subaru HSC (WERGS). IV. Rapidly Growing (Super)Massive Black Holes in Extremely Radio-loud Galaxies
Lee, Chien-Hsiu; Schramm, Malte; Ueda, Yoshihiro; Kawamuro, Taiki; Kajisawa, Masaru; Toba, Yoshiki; Matsuoka, Yoshiki; Tanaka, Masayuki; Uchiyama, Hisakazu; Nagao, Tohru; Inayoshi, Kohei; Yamashita, Takuji; Chen, Xiaoyang; Akiyama, Masayuki; Ichikawa, Kohei; Wagner, Alexander Y.; Charisi, Maria; Suh, Hyewon; Fukuchi, Hikaru; He, Wanqiu; Vijarnwannaluk, Bovornpratch; Pflugradt, Janek
Japan, Germany, Taiwan, China, United States
Abstract
We present the optical and infrared properties of 39 extremely radio-loud galaxies discovered by cross-matching the Subaru/Hyper Suprime-Cam (HSC) deep optical imaging survey and VLA/FIRST 1.4 GHz radio survey. The recent Subaru/HSC strategic survey revealed optically faint radio galaxies (RG) down to gAB ~ 26, opening a new parameter space of extremely radio-loud galaxies (ERGs) with radio-loudness parameter of $\mathrm{log}{{ \mathcal R }}_{\mathrm{rest}}\,=\mathrm{log}({f}_{1.4\mathrm{GHz},\mathrm{rest}}/{f}_{g,\mathrm{rest}})\gt 4$ . Because of their optical faintness and small number density of ~1 deg-2, such ERGs were difficult to find in the previous wide but shallow or deep but small area optical surveys. ERGs show intriguing properties that are different from the conventional RGs: (1) most ERGs reside above or on the star-forming main-sequence and some of them might be low-mass galaxies with $\mathrm{log}({M}_{\star }/{M}_{\odot })\lt 10$ . (2) ERGs exhibit a high specific black hole accretion rate, reaching the order of the Eddington limit. The intrinsic radio loudness ( ${{ \mathcal R }}_{\mathrm{int}}$ ), defined by the ratio of jet power over bolometric radiation luminosity, is one order of magnitude higher than that of radio quasars. This suggests that ERGs harbor a unique type of active galactic nuclei (AGN) that show both powerful radiations and jets. Therefore, ERGs are prominent candidates of very rapidly growing black holes reaching Eddington-limited accretion just before the onset of intensive AGN feedback.