While stellar-mass and supermassive black holes are well-known, intermediate-mass black holes (IMBHs, ~10²–10⁶ M☉) that bridge the gap remain elusive. They are key to unraveling black hole mass growth and their co-evolution with galaxies, yet fully confirmed IMBHs are extremely scarce. 

Dwarf galaxies, characterized by their low luminosity and mass, serve as prime hunting grounds for detecting these hidden objects.

Now, a team from the Xinjiang Astronomical Observatory (XAO) of the Chinese Academy of Sciences (CAS) has used the European VLBI Network (EVN) to examine the nearby dwarf galaxy merger system SDSS J101747.09+393207.7 (RGG66) and detected a milliarcsecond-scale, compact non-thermal radio component. 

By analyzing historical data alongside these new observations, they identified it as a rare, fading compact radio jet powered by an IMBH —offering fresh insight into how galaxy mergers can trigger black hole accretion and drive mass growth.

Led by XAO Ph.D. student LI Chao, his supervisor Prof. CUI Lang, and an international team of collaborators, the findings were published in the journal Astronomy & Astrophysics.

The radio component shows a total flux density of 0.41 mJy and a brightness temperature of about 6 × 10⁶ K. By stitching together archival data from surveys such as LoTSS, RACS, VLASS, NVSS, and FIRST, the team reconstructed its activity over the last 30 years. Invisible in 1990s surveys, the source first popped up in the 2015 LoTSS data and has been steadily fading ever since. Modeling the multi-band data gives a spectral index of α ≈ −1.15 and a fading timescale of roughly 8 years. At this rate, its 5 GHz flux is projected to drop below 0.1 mJy within about 12 years.

Given its steep spectrum, steady fading, and three‑decade evolutionary track, the team concludes that this milliarcsecond‑scale component is a young, short‑lived jet —born from unstable accretion onto the central black hole—which will likely disappear completely in the coming decades.

Previously, VLBI had only caught six IMBH candidates with parsec‑scale jets, and none of those host galaxies showed merger signs. Remarkably, RGG66 boasts an X‑ray to Eddington luminosity ratio of about 0.1—the highest among all known VLBI‑detected IMBH accretion systems —while its compact morphology points to an extremely nascent stage of jet activity.

This work was supported by the National Natural Science Foundation of China and the National Key R&D Program of China.

Left: HST imaging of RGG66 with the F110W infrared filter. Right: EVN image of RGG66 at 4.926 GHz. The black cross marks the optical position from Gaia DR3; the semi-major and semi-minor axes of the yellow ellipse represent the total 1σ uncertainties. (Image by XAO)