[Paper Review] The HST view of FR I radio galaxies: evidence for non-thermal nuclear sources

M. Chiaberge, A. Capetti+1 more|arXiv (Cornell University)|Jul 6, 1999

Radio Astronomy Observations and Technology17 citations

TL;DR

This paper presents Hubble Space Telescope observations of 33 FR I radio galaxies, detecting unresolved nuclear sources (Central Compact Cores, CCCs) in 85% of cases. The CCCs show a tight linear correlation between optical and radio core luminosities, indicating non-thermal synchrotron emission from misaligned relativistic jets, supporting the unification of FR I radio galaxies with BL Lac objects and implying minimal thermal accretion or obscuring tori.

ABSTRACT

Unresolved nuclear sources are detected by the Hubble Space Telescope in the great majority of a complete sample of 33 FR I radio galaxies belonging to the 3CR catalogue. The optical flux of these Central Compact Cores (CCC) shows a striking linear correlation with the radio core one over four decades, arguing for a non--thermal synchrotron origin of the CCC radiation. We also find evidence that this emission is anisotropic, which leads us to identify CCCs with the misoriented relativistic jet component which dominates in BL Lac objects. This interpretation is also supported by the similarity in the radio-to-optical and optical spectral indices. The high rate of CCC detection (85 %) suggests that a `standard' pc scale, geometrically thick torus is not present in low luminosity radio-galaxies. Thus the lack of broad lines in FR I cannot be attributed to obscuration. CCC fluxes also represent upper limits to any thermal/disc emission. For a $10^9 M_{\sun}$ black hole, typical of FR I sources, these limits translate into a fraction as small as less than $10^{-7}-10^{-5}$ of the Eddington luminosity.

Motivation & Objective

• To investigate the nature of unresolved nuclear sources in FR I radio galaxies using high-resolution Hubble Space Telescope imaging.
• To determine whether the optical emission from these nuclei is thermal or non-thermal, and whether it is consistent with unification schemes involving relativistic jets.
• To test the presence of obscuring tori in low-luminosity radio galaxies by analyzing nuclear emission and its correlation with radio properties.
• To constrain the accretion efficiency in FR I nuclei by setting upper limits on thermal emission from the central region.
• To assess whether the lack of broad emission lines in FR I galaxies is due to obscuration or intrinsic low ionizing luminosity.

Proposed method

• Acquired high-resolution Hubble Space Telescope (HST) images of a complete sample of 33 FR I radio galaxies from the 3CR catalogue.
• Measured the flux densities of unresolved nuclear sources (Central Compact Cores, CCCs) in the optical band using HST data.
• Correlated the optical core luminosities (CCC) with radio core luminosities at 178 MHz to assess spectral energy distribution and emission mechanism.
• Analyzed the anisotropy of the CCC emission by comparing with known beamed sources like BL Lacs, using spectral index comparisons.
• Used the observed optical fluxes as upper limits to any thermal disc or accretion disc emission, assuming no significant contamination.
• Estimated the Eddington luminosity fraction for a typical $10^9 M_{\sun}$ black hole to constrain accretion efficiency.

Experimental results

Research questions

• RQ1 Are the unresolved nuclear sources in FR I radio galaxies consistent with non-thermal synchrotron emission from relativistic jets?
• RQ2 Does the observed linear correlation between optical and radio core luminosities support a jet-based origin for the nuclear emission?
• RQ3 Is the emission from these nuclei anisotropic, as expected for a relativistic jet viewed at a large angle?
• RQ4 Can the lack of broad emission lines in FR I galaxies be attributed to obscuration by a torus, or is it due to low ionizing luminosity?
• RQ5 What is the upper limit on thermal accretion luminosity in FR I nuclei, and how does it compare to the Eddington limit?

Key findings

• An unresolved Central Compact Core (CCC) was detected in 28 out of 33 FR I radio galaxies (85% detection rate), indicating a common nuclear component.
• The optical core luminosity of CCCs shows a tight linear correlation with the radio core luminosity over four decades in luminosity, strongly supporting a non-thermal synchrotron origin.
• The similarity in radio-to-optical and optical spectral indices between CCCs and BL Lac objects supports the interpretation that CCCs are misaligned relativistic jet components.
• The anisotropic nature of the emission is consistent with relativistic beaming, reinforcing the unification of FR I radio galaxies with BL Lacs.
• For a $10^9 M_{\sun}$ black hole, the observed CCC fluxes set an upper limit of $\sim 10^{-7}$ to $10^{-5}$ of the Eddington luminosity for any thermal emission, indicating extremely low accretion efficiency.
• The absence of a detectable thermal disc component and the lack of evidence for obscuring tori in the majority of sources challenge the presence of a standard geometrically thick torus in low-luminosity radio galaxies.