[Paper Review] The physics of the radio emission in the quiet side of the AGN population with the SKA

M. Orienti, F. D’Ammando+3 more|arXiv (Cornell University)|Dec 18, 2014

Radio Astronomy Observations and Technology3 citations

TL;DR

This paper investigates the origin of radio emission in radio-quiet active galactic nuclei (AGN) using the upcoming Square Kilometer Array (SKA), proposing that deep, high-resolution, polarimetric radio observations will disentangle non-thermal synchrotron emission from thermal processes like free-free or cyclo-synchrotron radiation. The SKA’s sub-arcsecond resolution and μJy-level sensitivity will enable the first statistical study of faint, high-redshift radio-quiet AGN, resolving the nature of their central engine emission and distinguishing it from star formation. The key contribution is a roadmap for using SKA data to resolve the long-standing debate on the physical mechanisms powering radio emission in the majority of AGN.

ABSTRACT

Despite targets of many multiwavelength campaigns, the main physical processes at work in AGN are still under debate. In particular the origin of the radio emission and the mechanisms involved are among the open questions in astrophysics. In the radio-loud AGN population the radio emission is linked to the presence of bipolar outflows of relativistic jets. However, the large majority of the AGN population do not form powerful highly-relativistic jets on kpc scales and are characterized by radio luminosity up to 10^23 W/Hz at 1.4 GHz, challenging our knowledge on the physical processes at the basis of the radio emission in radio-quiet objects. The main mechanisms proposed so far are synchrotron radiation from mildly relativistic mini-jets, thermal cyclo-synchrotron emission by low-efficiency accretion flow (like ADAF or ADIOS), or thermal free-free emission from the X-ray heated corona or wind. The difficulty in understanding the main mechanism involved is related to the weakness of these objects, which precludes the study of non-local radio-quiet AGN. Multifrequency, high-sensitivity radio observations are crucial to constrain the nature of the power engine, and they may help in distinguishing between the contribution from star formation and AGN activity. The advent of the SKA, with its sub-arcsecond resolution and unprecedented sensitivity will allow us to investigate these processes in radio-quiet AGN, even at high redshift for the first time. Both the broad-band radio spectrum and the polarization information will help us in disentangling between non-thermal and thermal origin of the radio emission. The jump in sensitivity of a few order of magnitudes at the (sub-)uJy level will enable us to detect radio emission from a large number of radio-quiet AGN at high redshift, providing a fundamental step in our understanding of their cosmological evolution. (Abridged)

Motivation & Objective

• To resolve the long-standing uncertainty in the origin of radio emission in radio-quiet AGN, which constitute the vast majority of the AGN population.
• To disentangle the contributions of star formation and AGN activity in the radio band, especially in systems where both processes coexist.
• To determine whether the radio emission arises from non-thermal synchrotron radiation (e.g., from mini-jets) or thermal processes (e.g., free-free or cyclo-synchrotron emission).
• To use the SKA’s unprecedented sensitivity and resolution to probe the physical conditions in the central engine and accretion flows of high-redshift radio-quiet AGN.
• To constrain the role of AGN feedback by characterizing the structure and spectral properties of radio emission on parsec to kpc scales.

Proposed method

• Leverage the Square Kilometer Array (SKA)’s sub-arcsecond angular resolution and sensitivity down to ~1–10 μJy to detect and resolve radio emission from high-redshift radio-quiet AGN.
• Use multi-frequency, high-spectral-resolution (100 MHz) observations to construct quasi-continuous radio spectra across bands 2 and 5, enabling spectral index analysis to distinguish between thermal and non-thermal emission mechanisms.
• Apply polarimetric observations to identify ordered magnetic fields and jet-like structures, as non-thermal synchrotron emission from jets is expected to be polarized, while diffuse star-forming regions are highly depolarized.
• Combine total intensity and polarization data to model the emission mechanisms: e.g., flat-spectrum, unpolarized core emission may indicate thermal bremsstrahlung, while low polarization with spectral turnover may suggest synchrotron self-absorption.
• Use circular polarization measurements to detect cyclo-synchrotron emission from low-efficiency accretion flows (e.g., ADAF/ADIOS), a unique signature of thermal emission in magnetized, hot coronal gas.
• Apply beam depolarization modeling to correct for instrumental and structural effects that may mask intrinsic polarization in unresolved or complex structures.

Experimental results

Research questions

• RQ1 What is the dominant physical mechanism (thermal vs. non-thermal) responsible for the radio emission in radio-quiet AGN?
• RQ2 To what extent can the radio emission in radio-quiet AGN be attributed to star formation versus AGN activity, and how can they be disentangled?
• RQ3 Can the SKA detect and resolve parsec-scale structures in high-redshift radio-quiet AGN, particularly evidence of mini-jets or accretion flow emission?
• RQ4 How do the spectral and polarimetric properties of radio emission vary across different AGN types and redshifts, and what do they reveal about the accretion mode?
• RQ5 Can circular polarization measurements help identify cyclo-synchrotron emission from low-efficiency accretion flows in the central regions of radio-quiet AGN?

Key findings

• The SKA will achieve a sensitivity of ~20 μJy at 100 MHz spectral resolution in ~1 hour with uniform weighting, enabling detection of radio-quiet AGN down to luminosities of ~10^19 W/Hz at z ≈ 0.01.
• With a maximum baseline of 2000 km, the full SKA will achieve milliarcsecond resolution at high frequencies, allowing direct imaging of parsec-scale structures in high-redshift AGN.
• The SKA1-MID early-science phase will achieve 5σ sensitivity of ≤60 μJy in ~1 hour, enabling detailed study of local AGN (z < 0.1) with luminosities as low as ~10^21 W/Hz.
• Polarization measurements will help distinguish between synchrotron emission (polarized, steep spectrum) and thermal emission (depolarized, flat spectrum), especially when combined with spectral index analysis.
• The detection of circular polarization may uniquely identify cyclo-synchrotron emission from hot coronal gas, providing direct evidence for low-efficiency accretion flows in radio-quiet AGN.
• The combination of broad-band radio spectra and polarization data will allow robust discrimination between thermal bremsstrahlung, free-free emission, and non-thermal synchrotron radiation in the nuclear regions of AGN.