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The Origin of the Radio Emission in Active Galactic Nuclei (AGN)
Radio-quiet AGN represent the majority (~ 90%) of the AGN population. The radio emission in radio-quiet AGN can be produced by a variety of mechanisms, including the host galaxy star formation, thermal free-free emission from AGN photoionized gas, an AGN-driven wind interacting with the interstellar medium, a low-power jet, and the accretion disk coronal activity. Radio observations are a powerful tool to study the wide variety of physical mechanisms and to investigate the magnetic field and the synchrotron radiation of the relativistic electrons from the scale close to the supermassive black hole to the host galaxy scale.
My current researches focus on distinguishing a low-power jet, an AGN-driven wind, and the accretion disk corona based on the 1--24 GHz parsec-scale radio emission (Chen et al. 2023, MNRAS, 525, 164; Chen et al. 2025, ApJ, 979, 241).
The figure on the left illustrates that the various observed radio spectra are produced by a superposition of multiple synchrotron components, and the turnover frequency in each component increases with a decreasing radio source radius. Therefore, the turnover frequency can be used to constrain the size of the radio-emitting region, and thus to distinguish the radio emission mechanisms.
In the future, I will expand the study of the radio spectra to cover larger scales (from 100 pc to 5 kpc) and wider frequencies (0.3--300 GHz), to investigate the synchrotron radiation from parsec to kiloparsec scales.
The AGN Winds from the Multiwavelength Perspectives
How does the radio emission relate to the multiwavelength properties? Specifically, high-ionization optical, UV, and X-ray emission and absorption lines often show a blueshifted component with respect to the galaxy rest-frame, which indicates that the outflowing gas is at velocities of hundreds to thousands of km/s and forms an AGN wind. The ejected material can also have an impact on the surrounding gas and the host galaxy environment, a process called AGN feedback. How are the AGN winds related to the mass accretion rate and the emission mechanism? Do the AGN winds launch from the accretion disk, and expand to the broad-line region (~ 0.1 pc), the narrow-line region (~ pc--kpc), and the host galaxy (~ kpc)? Do the AGN winds interact with the surrounding gas and produce feedback on the host galaxy?
I found that high Eddington ratio objects tend to show a wind signature in both the radio band indicated by a steep radio slope and the UV band indicated by a blueshifted component in the C IV emission line, as shown in the figure on the left (Chen et al. 2024, ApJ, 975, 35).
The study suggests that the AGN wind is likely radiation-pressure-driven, and it extends from the broad-line region to parsec scales and produces radio emission.
In the future, I will expand the study of the AGN winds to the optical and X-ray bands, to investigate if the winds expand from the accretion disk, to the broad-line region, the narrow-line region, and the host galaxy, and if the winds interact with the ambient medium.
The Largest Jet in a Radio-Quiet AGN so far
Narrow-line Seyfert 1 (NLS1) galaxies are a unique subclass of AGN characterized by low black hole mass and high Eddington ratio. Many studies suggest that they are in an earlier phase of the AGN evolution. Therefore, studies of NLS1 galaxies will help to improve our understanding on the AGN evolution.
I created a new NLS1 sample in the southern hemisphere (Chen et al. 2018, A&A, 615, A167). Their kiloparsec-scale radio emission is likely related to an AGN-driven wind, which is more prevalent in high Eddington ratio objects (Chen et al. 2020, MNRAS, 498, 1278; Chen et al. 2022, MNRAS, 512, 471).
I found a peculiar NLS1 galaxy, which is the only known radio-quiet AGN with a 100-kpc two-sided radio jet, as shown in the figure on the left. A misalignment between the parsec-scale and kiloparsec-scale jets is found (Chen et al. 2024, ApJ, 963, 32).
The study challenges our understanding on how such a large jet forms in a radio-quiet AGN. It raises open questions worthy of future studies, for instance, whether and how the parsec-scale and kiloparsec-scale jets connect to each other, whether the jet moves at a lower speed in radio-quiet AGN compared to radio-loud AGN, and how common such large-scale jets are in radio-quiet AGN.
In the future, I will expand the study of the high Eddington ratio and low black hole mass AGN to high redshifts, to investigate their radio emission in the past and current universe.
Pearson overestimates the correlation of the red-noise light curves
The Pearson correlation is a common statistical method used in the cross-correlation analysis of multi-band light curves. However, is the actual correlation as significant as the Pearson correlation reported?
By a cross-correlation analysis of the quasi-simultaneous radio and X-ray light curves of a radio-quiet Seyfert galaxy, I found that the actual probability p(r) to get a Pearson coefficient r can deviate from the Pearson probability p(r) by a factor of up to ~ 1000, as shown in the figure on the left (Chen et al. 2022, MNRAS, 515, 1723).
The study reveals that the actual correlation is not as significant as the Pearson correlation reported. This occurs since the Pearson correlation assumes white noise, while both the radio and X-ray light curves follow red noise, which dramatically increases the chance to get a high r value and a low p value in uncorrelated data sets.
Future studies of radio variability require, firstly, high-resolution observations in order to avoid the contamination from the host galaxy and detect the AGN variability, and secondly, a long-period and high-sampling-rate monitoring in order to reliably detect a delay and search for a physical connection between the radio and the X-ray emission.
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