VLBI observations reveal the nuclear jet properties of Fanaroff-Riley type 0 radio galaxies
by Xiaopeng Cheng (Shanghai Astronomical Observatory, Chinese Academy of Sciences, China and Korea Astronomy and Space Science Institute, Republic of Korea), Tao An (Shanghai Astronomical Observatory, Chinese Academy of Sciences and Key Laboratory of Radio Astronomy, Chinese Academy of Sciences, China), Bong Won Sohn (Korea Astronomy and Space Science Institute and Department of Astronomy and Space Science, University of Science and Technology, Republic of Korea), Xiaoyu Hong (Shanghai Astronomical Observatory, Chinese Academy of Sciences and Key Laboratory of Radio Astronomy, Chinese Academy of Sciences and University of Chinese Academy of Sciences, China), Ailing Wang (Shanghai Astronomical Observatory, Chinese Academy of Sciences and University of Chinese Academy of Sciences, China)
Radio galaxies are divided into two groups according to their morphology, namely Fanaroff-Riley type I (FRI) and type II (FRII). FRIIs generally have higher jet power than FRIs. The evolutionary models of FRII and FRI galaxies have been extensively studies [1,2]: each radio source starts as a compact symmetric object (CSO), grows gradually through a medium-sized symmetric object (MSO) phase, and then a fraction of MSOs successfully evolve into large symmetric objects (LSOs), including FRII and FRI galaxies. The practical radio galaxy evolution of individual FRII or FRI is very complex and depends on several factors such as the initial jet power, the duration of the nuclear activity, the recurrence of the radio activity, and the loss of jet kinetic energy due to the jet-ISM interactions.
A class of radio sources, which have a compact morphology in arcsec-scale resolution images and lack extended jets (larger than 5 kpc) , was recently discovered and cannot be directly classified as FRIs or FRIIs, and is therefore treated as a separate class named FR0 radio galaxies. The number density of FR0s in the local Universe (z < 0.05) far exceeds that of FRIs and FRIIs, making this class important for a complete understanding of the radiative properties of radio galaxies and the radio feedback to the host galaxy. In terms of galactic nuclei and host properties, FR0s are not significantly different from FRIs, indicating that both classes have the same type of central engines and that the jet provides the main contribution to the overall radio power. Based on these commonality between FR0s and FRIs, it is thought that FR0s may be early-stage FRIs, but it is still not clear whether FR0s can eventually evolve into FRIs. Although FR0s lack extended jets of tens of kpc, sub-arcsec resolution imaging observations show the presence of kpc-scale jets in at least a fraction of FR0s. In addition, some FR0s (and candidates) have been detected in the gamma-ray band and have been suggested as possible sources of cosmic neutrinos and high-energy cosmic rays, indicating the presence of relativistic jets in FR0s. One of the direct ways to detect the relativistic jet in FR0s and to explore the connection of FR0s with other FR galaxies is to study the pc-scale radio emission with very long baseline interferometric (VLBI) observations.
A team of astronomers (X. Cheng & T. An) selected 14 FR0s with highest radio flux densities to study their radio properties on pc scales based on Very Long Baseline Array (VLBA) observations . Pc-scale jets were detected in 80% of the sample sources (at higher resolutions, this percentage may reach 100%). The jets show a diverse characteristic: lower bright temperatures than typical radio-loud quasars, slow structure and flux density variations, compact structures with two-sided jets or one-sided jet, steep spectra, and moderate jet velocities, implying that these FR0s may be mixed with CSOs and MSOs . The team continued to observe 8 out of the 14 FR0s using the European VLBI Network (EVN) and VLBA at 5 and 8 GHz (Figure 1), determining the jet kinematics and radio spectral properties, and found that the jets are mildly relativistic with velocities between -0.08c and 0.51c, and that the jets have low bulk Lorentz factors and large viewing angles. Detection of mildly relativistic jets in FR0s provides important information for discerning the nature of FR0s.
The above VLBI research is focused on brightest FR0s. In a follow-up work made by Baldi, Giovannini & Capetti  selected a sample of 15 FR0s with lower radio luminosities and carried out observations with the EVN and eMERLIN. Parsec-scale jets were detected in 11 sources (73%), suggesting that although FR0s lack extended emission, pc-scale jets are prevalent. The low brightness of the pc-scale FR0 jets requires high-sensitivity VLBI imaging observations.
The jet luminosity detected by VLBI is more representative of the jet power because the pc-scale radio emission region is closer to the jet launching zone. By investigating the correlation between radio jet power, optical spectroscopic luminosity, and black hole mass, a disc-jet link in FR0s similar to FRIs is explored to establish a homogeneous class with a common accretion-jet coupling mode, revealing a continuous population distribution from the low-luminosity radio-loud AGN to the powerful FRIs.
The similarities and differences between FR0s and FRIs can be explained in the framework of radio galaxy evolution. FR0s reside in moderately host environments, powered by low-spinning BHs and low accretion rates. This scenario predicts that FR0s are characterised by low jet velocities, consistent with the observations. Most FR0s have a jet structure in which the outer mildly relativistic spine is more easily disrupted by the gas in the host galaxies, leading to their inability to develop extended jets like FRIs.
Future high-resolution and high-sensitivity radio facilities, such as the SKA and ngVLA, offer promising opportunities to discover large numbers of FR0s, and follow-up VLBI observations enable to place tighter constraints on the jet velocities, thus improve the understanding of this rich population of compact radio galaxies.
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