Abstract
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 AB ∼ 26, opening a new parameter space of extremely radio-loud galaxies (ERGs) with radio-loudness parameter of log rest = log ( f 1.4 GHz , rest / f g , rest ) > 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 log ( M ⋆ / M ⊙ ) < 10 . (2) ERGs exhibit a high specific black hole accretion rate, reaching the order of the Eddington limit. The intrinsic radio loudness ( 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.
Highlights
The formation of supermassive black holes (SMBHs) and their growth across the cosmic time are fundamental questions in modern astronomy
Since most extremely radio-loud galaxies (ERGs) are fainter than the SDSS limit, Subaru/Hyper Suprime-Cam (HSC) has enabled us to investigate this unique population for the first time
We investigate the SMBH properties through the specific black hole accretion rate, which may be considered a proxy for the Eddington ratio
Summary
The formation of supermassive black holes (SMBHs) and their growth across the cosmic time are fundamental questions in modern astronomy. Data covering the optical, IR, and radio band for the WERGS sample and derived physical parameters of host galaxies such as stellar mass and star formation rate as well as the AGN luminosity estimated from the mid-IR bands. By using this data set, we will explore the AGN and host-galaxy properties of the ERGs. Throughout this paper, we adopt the same cosmological parameters as Yamashita et al (2018) and Toba et al (2019); H0 = 70 km s−1 Mpc−1, ΩM = 0.27, and ΩΛ = 0.73
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