[Paper Review] The host galaxy/AGN connection in nearby early-type galaxies. A new view of the origin of the radio-quiet/radio-loud dichotomy?

A. Capetti, B. Balmaverde|ArXiv.org|Mar 14, 2006

Galaxies: Formation, Evolution, Phenomena46 references52 citations

TL;DR

This study links the radio-loudness of AGN in nearby early-type galaxies to their host galaxy's nuclear brightness profile: core galaxies (with shallow central profiles) host only radio-loud AGN, while power-law galaxies (with steep central profiles) host only radio-quiet AGN. The results suggest that the merger history shaping the host's structure—via black hole dynamics—determines AGN radio-loudness, offering a unified origin for the radio-loud/radio-quiet dichotomy.

ABSTRACT

[ABRIDGED] This is the third in a series of three papers exploring the connection between the multiwavelength properties of AGN in nearby early-type galaxies and the characteristics of their hosts. We selected 116 AGN candidates requiring a radio flux of 1 mJy. We classified the objects with HST images into ``core'' and ``power-law'' galaxies, on the basis of the nuclear slope of their brightness profiles. We used HST and Chandra data to isolate their nuclear emission to study the multiwavelength behaviour of their nuclei. The properties of the nuclei hosted by the 29 core galaxies were presented in Paper II. Core galaxies invariably host a radio-loud nucleus, with a median radio-loudness of Log R = 3.6 and an X-ray based radio loudness parameter of Log R,X = -1.3. Here we discuss the properties of the nuclei of the 22 ``power-law'' galaxies. They show a substantial excess of optical and X-ray emission with respect to core galaxies at the same level of radio luminosity. Conversely, their radio-loudness parameters, Log R ~ 1.6 and Log R,X ~ -3.3, are similar to those measured in Seyfert galaxies. Thus the radio-loudness of AGN hosted by early-type galaxies appears to be univocally related to the host's brightness profile: radio-loud AGN are only hosted by core galaxies, while radio-quiet AGN are found only in power-law galaxies. The brightness profile is determined by the galaxy's evolution, through its merger history; our results suggest that the same process sets the AGN flavour.

Motivation & Objective

• To investigate the connection between AGN properties and host galaxy structure in nearby early-type galaxies.
• To resolve the long-standing radio-loud/radio-quiet AGN dichotomy by linking it to host galaxy morphology.
• To test whether the nuclear brightness profile (core vs. power-law) predicts AGN radio-loudness.
• To explore the role of galaxy merger history in shaping AGN properties through black hole dynamics.
• To provide a unified framework for understanding the co-evolution of supermassive black holes and their hosts.

Proposed method

• Classified 116 AGN candidates from VLA 5 GHz surveys into 'core' (γ ≤ 0.3) and 'power-law' (γ ≥ 0.5) galaxies using archival HST images.
• Isolated nuclear emission in optical (HST) and X-ray (Chandra) bands to measure intrinsic AGN luminosities independent of host contamination.
• Calculated radio-loudness parameters R = log(L_5GHz / L_B) and R_X = log(L_5GHz / L_X) to compare AGN activity across host types.
• Used multiwavelength diagnostic planes (radio–optical–X-ray) to compare AGN behavior in core vs. power-law hosts.
• Analyzed the impact of black hole merger dynamics on accretion rates and jet production, linking structural profiles to AGN mode.
• Evaluated limitations of existing data (e.g., VLA resolution, Chandra sensitivity) and proposed future observations (VLBI, deeper X-ray, NIR HST).

Experimental results

Research questions

• RQ1 Is the radio-loudness of AGN in early-type galaxies correlated with the nuclear brightness profile of the host galaxy?
• RQ2 Do core galaxies (with shallow central profiles) exclusively host radio-loud AGN, while power-law galaxies host only radio-quiet AGN?
• RQ3 Can the merger history of a galaxy—reflected in its brightness profile—explain the origin of the radio-loud/radio-quiet dichotomy?
• RQ4 What is the role of black hole spin, mass ratio, or separation in determining AGN radio-loudness?
• RQ5 Can the observed differences in accretion rates and jet activity be explained by the same physical mechanism linked to host galaxy structure?

Key findings

• Core galaxies host only radio-loud AGN, with a median radio-loudness of log R = 3.6 and log R_X = -1.3, indicating a strong X-ray deficit at the same radio luminosity.
• Power-law galaxies host only radio-quiet AGN, with log R ≈ 1.6 and log R_X ≈ -3.3, consistent with Seyfert-like AGN.
• The nuclear luminosities of core galaxies are ~2 orders of magnitude fainter than those of power-law galaxies at the same radio luminosity, suggesting lower accretion rates.
• The observed luminosity difference is likely due to a common mechanism—possibly a relativistic outflow—reducing accretion efficiency in core galaxies despite jet presence.
• The association between host profile and AGN radio-loudness holds across a wide dynamic range, from low-luminosity AGN to radio galaxies with L_r ~ 10^42 erg s^-1.
• X-ray data provide a cleaner separation between core and power-law galaxies than optical data, due to reduced bias and less obscuration.