[Paper Review] The host galaxy/AGN connection in nearby early-type galaxies. Is there a miniature radio-galaxy in every "core" galaxy?

B. Balmaverde, A. Capetti|ArXiv.org|Oct 10, 2005

Galaxies: Formation, Evolution, Phenomena77 references58 citations

TL;DR

This study investigates the connection between radio-loud active galactic nuclei (AGN) and their host galaxies in nearby early-type 'core' galaxies, using Hubble and Chandra data to isolate nuclear emission. It finds that these galaxies host miniature radio galaxies with jet-dominated emission, extending the Luminous Low-Luminosity Radio Galaxy (LLRG) correlation down to 10^36 erg s⁻¹, supporting a common non-thermal origin and implying relativistic jets even in quiescent systems.

ABSTRACT

This is the second of 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. In Capetti et al. (2005) we presented a study of the surface brightness profiles for the 65 objects with available archival HST images out of the 116 radio-detected galaxies. We classified early-type galaxies into ``core'' and ``power-law'' galaxies, discriminating on the basis of the slope of their nuclear brightness profiles. Here we focus on the 29 core galaxies (hereafter CoreG). We used HST and Chandra data to isolate their nuclear emission. The CoreG invariably host radio-loud nuclei, with an average radio-loudness parameter of Log R = 3.6. The optical and X-ray nuclear luminosities correlate with the radio-core power, smoothly extending the analogous correlations already found for low luminosity radio-galaxies. This supports the interpretation of a common non-thermal origin of the nuclear emission also for CoreG. The luminosities of the nuclear sources, most likely dominated by jet emission, set firm upper limits, as low as L/L_Edd = 10^{-9} in both the optical and X-ray band, on any emission from the accretion process. The similarity of CoreG and LLRG indicates that they are drawn from the same population of early-type galaxies. LLRG represent only the tip of the iceberg associated with (relatively) high activity levels, with CoreG forming the bulk of the population. A minimum black hole mass of M_{BH} = 10^8 M_sun is apparently associated with the radio-loud nuclei in both CoreG and LLRG, but this effect must be tested on a sample of less luminous galaxies, likely to host smaller black holes. ABRIDGED.

Motivation & Objective

• To investigate the multiwavelength connection between AGN and host galaxies in nearby early-type galaxies, focusing on 'core' galaxies with shallow nuclear brightness profiles.
• To determine whether core galaxies host radio-loud nuclei and whether their nuclear emission shares a common origin with low-luminosity radio galaxies (LLRG).
• To test the hypothesis that core galaxies represent a low-luminosity extension of the LLRG population, possibly forming the bulk of the population of radio-loud AGN.
• To constrain the accretion efficiency and black hole mass threshold for radio-loud activity by analyzing optical and X-ray nuclear luminosities relative to Eddington luminosity.
• To explore the implications for unification models by comparing CoreG with BL Lac objects and radio galaxies at low radio flux limits.

Proposed method

• Selected 29 core galaxies from a larger sample of 116 radio-detected early-type galaxies with 5 GHz VLA fluxes down to 1 mJy.
• Used Hubble Space Telescope (HST) imaging to classify galaxies into 'core' (γ ≤ 0.3) and 'power-law' (γ ≥ 0.5) types based on Nuker scheme surface brightness profiles.
• Combined HST and Chandra X-ray data to isolate and measure optical and X-ray nuclear luminosities, correcting for host galaxy contribution.
• Calculated radio-loudness parameter R = L₅GHz / L_B and compared it with optical and X-ray luminosities to assess emission origin.
• Computed Eddington ratio L/L_Edd for optical and X-ray bands to estimate accretion efficiency and constrain accretion flow models.
• Analyzed correlations between radio power, host galaxy luminosity (M_K), black hole mass (M_BH), and nuclear luminosities to test for scaling relations.

Experimental results

Research questions

• RQ1 Do core galaxies host radio-loud AGN, and is their nuclear emission consistent with a jet-dominated origin?
• RQ2 Is there a continuous correlation between radio, optical, and X-ray luminosities in core galaxies, extending the LLRG correlation to lower luminosities?
• RQ3 What is the upper limit on accretion luminosity in core galaxies, and does this imply low accretion efficiency?
• RQ4 Is there a minimum black hole mass threshold for radio-loud activity in core galaxies?
• RQ5 Can core galaxies be considered miniature radio galaxies, and do they represent the misaligned counterparts of low-luminosity BL Lac objects?

Key findings

• Core galaxies invariably host radio-loud nuclei with a median radio-loudness parameter log R ≈ 3.6, indicating strong non-thermal emission.
• Optical and X-ray nuclear luminosities correlate smoothly with radio-core power across a 6-order-of-magnitude dynamic range, supporting a common non-thermal (jet-dominated) origin.
• Nuclear luminosities are extremely low in Eddington units, with L/L_Edd ≈ 10⁻⁹ in both optical and X-ray bands, setting firm upper limits on accretion-powered emission.
• No significant correlation is found between radio power and black hole mass, suggesting other parameters dominate radio luminosity beyond M_BH.
• A minimum black hole mass of ~10⁸ M☉ appears associated with radio-loud nuclei, though this may be biased by sample selection and requires testing on less luminous galaxies.
• Core galaxies resemble miniature radio galaxies and may represent the misaligned counterparts of low-luminosity BL Lac objects, implying relativistic jets are present even in quiescent systems.