Implications of the continuous radio-loudness distribution among AGNs in the local Universe

TL;DR

This study tests the long-standing claim of bimodality in AGN radio-loudness by exploiting a large, homogeneous local sample drawn from ROGUE I/II galaxies in the SDSS MGS. By combining optical, mid-infrared, and radio data, the authors derive bolometric luminosities, Eddington ratios, black-hole masses, and radio loudness for subsamples of radio Seyferts, OPARGs, and OPIRGs, with careful star-formation corrections. They find no evidence for bimodality in the radio-loudness distribution; instead, R shows a strong anti-correlation with the Eddington ratio λ, and the scatter at fixed λ can be explained by variations in the dimensionless magnetic flux φ and black-hole spin a within MAD/BZ frameworks. The work derives scaling relations linking jet power and radio output to accretion and host properties, showing that a continuous distribution of spin and φ, rather than distinct populations, underpins the observed diversity. This implies a common evolutionary path for radio AGN in the local Universe, with jets regulated by smoothly varying magnetic and spin parameters and modulated by jet duty cycles in extended sources.

Abstract

Aims. We investigate the radio loudness ($\mathcal{R}$) distribution in a large, homogeneous sample of radio galaxies. Methods. The sample is composed of galaxies from the ROGUE I/II catalogue belonging to the SDSS MGS and is divided into optically inactive radio galaxies (OPIRGs), optically active ones (OPARGs) and radio Seyferts. We use optical, mid-infrared and radio data to calculate the AGN bolometric luminosities, accretion rate ($λ$), black-hole mass ($M_{BH}$) and $\mathcal{R}$. Results. Contrary to some previous studies based on restricted samples, using our complete sample of objects with redshifts $z < 0.4$, we find no evidence of bimodality in $\mathcal{R}$. The highest $\mathcal{R}$ values are associated with extended radio structures. $\mathcal{R}$ is anti-correlated with $λ$, and spans about 2 dex at fixed $λ$. Radio Seyferts, OPARGs and OPIRGs form a sequence of increasing $M_{BH}$ with substantial overlap. Radio Seyferts show no $\mathcal{R}$-$M_{BH}$ correlation, whereas OPARGs and OPIRGs show a weak positive trend. From theoretical considerations, the observed two-dex spread in radio luminosity and $\mathcal{R}$ can be reproduced by a four-fold variation in the dimensionless magnetic flux $\varphi$ assuming realistic black-hole spins. Conclusions. The smooth distribution of radio loudness supports a common evolutionary path for all radio sources, with black-hole spin and magnetic field varying continuously. The radio loudness depends on black-hole mass and accretion rate, while moderate variations in $\varphi$ may account for the observed scatter in this relation.

Figures (8)

• Figure 1: Flowchart detailing our sample selection. Retired galaxies are galaxies that have stopped forming stars and are ionized by their populations of hot low-mass evolved stars (HOLMES), see Stasinska.etal.2008a. Similarly to the star-forming galaxies, they are not relevant for the present study. The parent 'ROGUE I & II SDSS MGS' sample already incorporates redshift range limits, optical spectral quality cuts, and removal of sources whose radio morphology is blended or non-detected. The number of sources in each subsample is given in the panels. Numbers in brackets show number counts before applying limits to the $S/N$ of O iii] [O iii] or $W3$ luminosities, to emission line detection and to the star-formation-corrected radio luminosity values.
• Figure 2: Diagrams showing the loci of our radio Seyferts (green), OPARGs (blue) and OPIRGs (red) subsamples in relation to our parent samples: ROGUE I & II SDSS MGS ('ROGUE-MGS') for panel (a), SDSS MGS ('MGS') for panels (b) and (c) (grey points). (a) $D_n(4000)$ versus $L_{1.4}/M_\star$ (DLM) plane. The dashed line by KozielWierzbowska.etal.2021a separates radio emission dominated (top) or not (bottom) by AGN. (b) [O iii]$\lambda$5007/Hβ H$\beta$ versus [N ii]$\lambda$6584/Hα H$\alpha$ (BPT) plane. The dashed line by Stasinska.etal.2006a separates galaxies ionized only by star formation (left) from those ionized by a hard ionizing radiation field (right). OPIRGs are not shown since they have not been selected based on this diagram. (c) (Hα) $W(\mathrm{H}\alpha)$ versus [N ii]$\lambda$6584/Hα H$\alpha$ (WHAN) plane. The dashed line shows the $W\mathrm{H}\alpha)$W(Hα)$\xspace = 3$ Å criterion by CidFernandes.etal.2011a to separate ionization only by HOLMES (bottom) from ionization by other sources (top). OPIRGs lacking N ii] [N ii] detection are not shown on this plane.
• Figure 3: (a)--(d) Total radio luminosity, (e)--(h) absolute magnitude in the $r$-band, and (i)--(l) mid-infrared $W3$ luminosity as a function of redshift. Colours indicate the samples: radio Seyferts are in green, OPARGs in blue, and OPIRGs in red. The grey points indicate the parent samples: ROGUE I & II SDSS MGS ('ROGUE-MGS') for the first row, SDSS MGS ('MGS') for the second and third rows. Numbers on the panel show the count of objects. Subsamples are shown overplotted on the parent sample on the first three columns; parent samples plus all subsamples are show together on the last column for comparison.
• Figure 4: Two-dimensional histograms of radio loudness ($\mathcal{R}$) versus Eddington ratio ($\lambda$) for (a) radio Seyfert and OPARG objects, and (b) radio Seyfert, OPARG, and OPIRG objects combined. Bin colours represent the logarithm of the number of objects, as indicated by the colour bars. Marginal 1D histograms of radio loudness are shown on the right hand side of each panel. Values with and without a prime indicate _bol $L_\mathrm{bol}$ derived solely from O iii] [O iii] or from a mix of O iii] [O iii] (for OPARG and Seyfert) and $W3$ (for OPIRG) luminosities, respectively. The number of objects is indicated in each panel. The dashed line corresponds to the classical radio-loudness threshold of Kellermann.etal.1989a, and the dotted line to the division from Gupta2018; both are shown using our convention. No bimodality is seen in either the 1D or 2D histograms.
• Figure 5: Scatter plots of radio loudness ($\mathcal{R}$) versus Eddington ratio ($\lambda$) for different subsamples: (a) radio Seyfert and OPARG; (b) radio Seyfert and extended sources; (c) radio Seyfert, OPARG, and OPIRG; and (d) by optical morphology (spiral, lenticular, elliptical). Extended sources are those classified with radio morphologies FRI, FRII, FRI/II, OI, OII, Z, X, DD, WAT, NAT, or HT in the ROGUE catalogues. Subsample sizes are indicated in each panel. Marginal 1D histograms of radio loudness for each subsample are shown on the right hand side of each panel. Reference lines and primed and unprimed quantities are as in Fig. \ref{['fig:Rlambda2D']}.