Astronomers using the James Webb Space Telescope and ALMA have identified two supermassive black holes just 4,900 light-years apart in a galaxy merger 1.3 billion years after the Big Bang, offering new insight into rapid black hole growth
Astronomers have directly identified the closest-known pair of supermassive black holes in the early universe, using the James Webb Space Telescope (JWST) and the Atacama Large Millimeter/submillimeter Array (ALMA). The system, designated LID-1166, is observed as it existed only 1.3 billion years after the Big Bang, providing a rare look at a critical phase in the assembly of massive black holes and galaxies.
Discovery in a Merging Galaxy
LID-1166 was first flagged as a strong X-ray source in the Chandra COSMOS Legacy Survey, but remained invisible in even the deepest Hubble Space Telescope images. The system's X-ray brightness suggested intense accretion activity, prompting follow-up observations with JWST's Near-Infrared Spectrograph (NIRSpec). These data revealed two compact, luminous sources separated by just 4,900 light-years (1.5 kiloparsecs) within a single merging galaxy-much closer than previously confirmed dual black hole systems at similar cosmic epochs.
While dual black holes are expected to form frequently during galaxy mergers, especially in the early universe, direct confirmation has been challenging. Most candidates have been separated by tens of thousands of light-years, and dense gas and dust often obscure the view. The proximity of the LID-1166 pair, combined with their active accretion signatures, makes this system a uniquely valuable target for studying black hole growth mechanisms.
Evidence from JWST and ALMA
The JWST NIRSpec spectra showed both objects exhibiting broad emission lines and high-velocity gas, consistent with active galactic nuclei powered by supermassive black holes. The team used multiple methods to subtract the host galaxy's light, consistently recovering the signal from both sources. ALMA observations further detected large reservoirs of cold gas associated with each nucleus, supporting the interpretation that both are embedded in the centers of merging galaxies rather than being ejected or isolated black holes.
These findings were reported in a study accepted for publication in Nature Astronomy, with the preprint available on arXiv. The researchers note that while the evidence strongly favors two distinct black holes, further high-resolution follow-up will be needed to fully rule out alternative explanations, such as a single black hole with complex surrounding emission. The presence of cold gas around both nuclei argues against scenarios where one black hole has been stripped of its galactic environment.
Implications for Black Hole Growth
The discovery of such a close black hole pair at high redshift addresses a longstanding question in astrophysics: how did supermassive black holes in the early universe grow so rapidly? Models suggest that galaxy mergers can funnel large amounts of gas toward galactic centers, fueling black hole accretion. However, direct evidence for this process at early times has been limited. LID-1166 provides a rare observational anchor for these models, showing two actively accreting black holes on the verge of coalescence.
Understanding the frequency and properties of such systems is essential for reconstructing the growth history of black holes and galaxies. The ability of JWST to resolve faint, dust-obscured nuclei at high redshift marks a significant advance over previous instruments. As more data become available, astronomers expect to identify additional close black hole pairs, helping to clarify the role of mergers in shaping the early universe.
Context in Cosmic Surveys
The identification of LID-1166 builds on a growing body of work mapping energetic phenomena in the distant universe. Large-scale X-ray surveys, such as those conducted by Chandra and eROSITA, have cataloged millions of high-energy sources, including black holes and galaxy clusters. For example, the eROSITA X-ray survey recently expanded the known population of cosmic X-ray sources, providing a statistical foundation for targeted follow-up with next-generation observatories like JWST and ALMA.
As observational capabilities improve, astronomers are increasingly able to test theoretical predictions about black hole and galaxy evolution. The LID-1166 system demonstrates the power of combining multiwavelength data to resolve complex, dust-enshrouded environments that were previously inaccessible.
To interpret faint, distant objects like LID-1166, astronomers rely on spectroscopy-a technique that disperses light into its component wavelengths to reveal the physical properties of cosmic sources. Spectroscopy allows researchers to identify emission lines from specific elements, measure gas velocities, and distinguish between different types of active galactic nuclei. In this case, the JWST NIRSpec instrument provided the spectral resolution needed to separate the two nuclei and confirm their nature as actively accreting black holes. Such measurements are essential for disentangling overlapping sources and for building a detailed picture of galaxy and black hole assembly in the early universe.
