This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Astronomers have discovered the first confirmed dual active galactic nucleus in a Green Pea galaxy, revealing two supermassive black holes growing at the same time inside a compact, intensely star-forming system. X-ray observations with Chandra resolved two actively accreting black holes, while Keck spectroscopy confirmed that both belong to the same merging galaxy.
The paper outlining the research was submitted to the arXiv preprint server on July 20. Green Pea galaxies are considered local-universe analogs of the rapidly growing galaxies that populated the early universe. They have low metal content (elements beyond hydrogen and helium), are tiny in size, and have high star formation rates relative to their stellar mass.
They are an important class of galaxies because they serve as a unique laboratory for understanding how stars and massive black holes formed and evolved within the first few million to a billion years of the universe. In particular, finding close pairs of feeding black holes powering their hosts, also known as active galactic nuclei (AGNs), in local analogs is useful because they shed light on a crucial phase of galaxy evolution and supermassive black hole growth. Despite extensive investigation, researchers were unable to confirm a dual AGN system among Green Pea galaxies until now.
In a new study, Konstantinos Kouroumpatzakis of the Czech Academy of Sciences and colleagues studied a Green Pea galaxy, J1622+3521, well-known for its messy and irregular structure. Researchers using deep optical imaging found signs of a merger and the presence of two separate cores in this galaxy. To confirm whether these two components were separate, actively feeding black holes, they further analyzed the system using Chandra X-ray data and Keck spectroscopy.
The Keck data revealed classic signatures of an actively feeding black hole—strong emission lines, including rare "high-ionization" lines that only intense radiation from an AGN can produce. They measured each component's precise redshift and found a velocity difference of only about 7 km/s between them, within the uncertainty of the measurement. Researchers say this is consistent with two cores sharing a common systemic velocity, supporting the idea that this is an interacting system of galaxies caught mid-merger before their black holes have merged.
Using Chandra, they precisely located two separate X-ray sources that exactly matched the positions of the two optical nuclei, separated by about 27,000 light-years. The team also estimated each black hole's mass at roughly 19.5 million solar masses and 20.5 million solar masses, respectively. They also ran a standard diagnostic that separates light powered by black holes from light powered by ordinary star formation, which confirmed that this system falls into the AGN category.
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