A Wide and Deep Exploration of Radio Galaxies with Subaru HSC (WERGS). IV. Rapidly Growing (Super-)Massive Black Holes in Extremely Radio-Loud Galaxies

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 $g_{\mathrm{AB}}\sim 26$, opening a new parameter space of extremely radio-loud galaxies (ERGs) with radio-loudness parameter of $\log {\mathcal{R}}_{\mathrm{rest}}=\log (f_{1.4\mathrm{GHz},\mathrm{rest}}/f_{g,\mathrm{rest}})>4$. Because of their optical faintness and small number density of $\sim 1$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 $\log (M_{\star }/M_{\odot })<10$. (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.

20 pages, 10 figures, accepted for publication in ApJ

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