3C 84, the central galaxy of the Perseus cluster, is one of the nearest and brightest radio active galactic nucleis (AGNs) in the sky. Recently, researchers have utilized the extremely faint counterjet in 3C 84 as a tomographic probe, discovering that the dense ionized gas surrounding the central supermassive black hole is lumpy and inhomogeneous.

The study, led by researchers from the Shanghai Astronomical Observatory (SHAO) of the Chinese Academy of Sciences, treats this dim jet as a moving backlight to probe the black hole's hidden surroundings. The findings have been published in The Astrophysical Journal.

According to the researchers, this study establishes 3C 84 as a vital benchmark for studying the interaction between jets and clumpy circumnuclear gas in AGN.

Artist's impression: The clumpy ionized gas surrounding the central black hole in 3C 84 obscures the northern counterjet. The southern jet is brighter, while the northern counterjet serves as a backlight to probe the circumnuclear absorbing medium. (Image by SHAO)

In the study, the researchers identified compelling evidence of an external free-free absorption screen by conducting a dual-frequency spectral index analysis. While the approaching southern jet hotspot maintained an optically thin synchrotron spectrum, the northern counterjet components, identified as N1 and N2, exhibited strongly inverted radio spectra between 15 and 43 GHz.

They estimated that the free-free absorption optical depth at 15 GHz decreased from approximately 3.0 to 1.9 in N1, and from 3.7 to 2.4 in N2, yielding an electron number density of 10⁴ to 10⁵ cm^-3 under a reasonable absorption path length, consistent with typical AGN values.

The team also revealed that time-domain evolution of the counterjet components demonstrated that the absorbing material is not a smooth, uniform mist, but rather a highly structured medium. The observation showed that as N1's spectrum transitioned from strongly to moderately inverted, it was accompanied by a rapid flux density increase at 43 GHz, whereas N2 exhibited stronger overall absorption and a weaker, delayed brightening.

According to the researchers, these distinct behaviors indicate that as the compact bright spots moved outward, they sequentially crossed different lines of sight, revealing that the obscuring structure, likely a dusty torus or its driven outflows, is clumpy or filamentary in both radial and transverse directions.

In addition, kinematic measurements of the jet features provided geometric constraints on the parsec-scale structure of 3C 84. Assuming intrinsic symmetry of the two-sided jet, the researchers determined a viewing angle of approximately 20 degrees and an apparent pattern speed of 0.45 to 0.52 times the speed of light for the radio knots.

The researchers also noted a potential slow modulation of 7-11 years in the transverse position of the counterjet, though currently treated as a candidate signal rather than a confirmed periodicity.

The first author of this paper is LIU Chuanzeng Liu, a PhD student jointly affiliated with SHAO and ShanghaiTech University. The co-authors include his supervisors Prof. HONG Xiaoyu and Prof. AN Tao.

This research was supported by the National SKA Program of China and the Shanghai Oriental Talents Program. The study also made use of the computing resources from the China SKA Regional Centre prototype, along with public data from the MOJAVE project and the VLBA-BU Blazar Monitoring Program.