An international team of astronomers has identified what is believed to be the most distant astronomical object ever observed: a galaxy named HD1, located roughly 13.5 billion light-years away from Earth. This galaxy appears to have formed its first stars incredibly quickly, marking the birth of stellar generations not yet directly observed. HD1 may also host the earliest known supermassive black hole at its core, with a mass around 100 million solar masses. Two studies dedicated to this discovery, which can be accessed on preprint repositories, appear alongside articles in the Astrophysical Journal and Monthly Notices of the Royal Astronomical Society.
HD1 was uncovered after more than 1,200 hours of ground-based observations using the Subaru Telescope, the Visible and Infrared Survey Telescope for Astronomy (VISTA), the UK Infrared Telescope, and the Spitzer Space Telescope.
“Finding HD1 among more than 700,000 objects was extremely challenging,” notes a Japanese astronomer from the University of Tokyo, who led the study in the Astrophysical Journal. “The measured redshift for HD1 aligns with expectations for a galaxy this far away, and the moment of discovery sent a chill down the spine.”
To confirm the distance and age, a team of astronomers conducted multiple checks with the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile. The estimated distance to HD1 exceeds that of GN-z11, the previous distant galaxy record holder, by about 100 million light-years. In this context, z denotes redshift, with GN-z11 at z=11.1 and HD1 emerging with z values exceeding 12–13. Redshift tracks the Doppler shift of spectral lines as galaxies move away, enabling earthbound observers to estimate their distances.
Two main theories have been proposed to explain HD1’s unusually rapid birth. The first suggests massive stars formed from gas concentrations that merged and collapsed, eventually giving rise to a supermassive black hole through repeated interactions. The second proposes that stellar systems formed around a preexisting central supermassive black hole.
“Interpreting data from a source this distant can be tricky,” explains a theoretical astrophysicist involved in the MNRAS study and a co-author of the ApJ paper. “It’s like judging a ship’s nationality from a distance while the vessel fights through a storm and is partially shrouded in fog. You might distinguish color and shape of a flag, but a single view never yields a complete answer. Much analytical work remains to gradually rule out implausible scenarios.”
HD1 shines brightly in the ultraviolet range. This is interpreted as a sign of high-energy processes at work, potentially persisting for billions of years after the initial observations.
Initial analyses suggested HD1 resembled an ordinary galaxy with a sudden burst of star formation, yet the inferred star formation rate proved to be extraordinarily high. Estimates indicated more than a hundred new stars per year, an order of magnitude higher than typical for galaxies of this type. This discrepancy led researchers to consider that the stars in HD1 may belong to an early class known as Population III, which are expected to emit far more ultraviolet light than later generations and could explain HD1’s unusually intense ultraviolet luminosity.
The earliest giant stars in HD1 likely formed at least 300 million years after the Big Bang. Yet the intense emission could also be explained by a rapidly growing central black hole accreting vast amounts of surrounding gas, which would generate a bright accretion disk and high-energy photons. If the black hole hypothesis holds, HD1 would represent the oldest known example of a supermassive black hole, growing much faster and earlier than previous records suggested.
“The presence of ultraviolet light in the HD1 system is clear, but distinguishing whether it comes from newborn stars or a shrouded black hole remains an open question,” notes another expert involved in the research. “It may be that both processes are contributing, with the young stars later evolving into a system centered on a black hole.” The notion that the first stellar clusters were enormous, containing millions of solar masses, further supports this view—that those early clouds fragmented into large stellar associations before assembling into galaxies. The idea that the earliest stars formed with little to no heavy elements likely occurred within 100–200 million years after the Big Bang.”
Although the peak emission of the earliest stars lies in the ultraviolet, redshift corrections allow Earth-based observers to study HD1 in the infrared and submillimeter windows.
“The discovery of an ancient galaxy is a landmark achievement that deserves close attention,” comments another researcher. “The James Webb Space Telescope is just getting started and may uncover many similar objects, but this record will certainly precede additional discoveries.”
In upcoming work, the research team plans to observe HD1 with James Webb Space Telescope to refine distance measurements and explore the galaxy’s deeper properties. If current calculations hold, HD1 will be confirmed as the most distant and oldest galaxy yet documented, while the observations will help determine which formation scenario best explains the earliest galaxies in the universe. [attribution: contemporary astronomical collaborations and peer-reviewed publications]