HD1

• 1. Introduction

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  On the 7th of April 2022, astronomers at the University of Tokyo reported finding HD1. They described it as potentially one of the earliest and most distant galaxies ever identified in the observable universe. The light they detected had been traveling for 13.463 billion years before reaching Earth. Because the universe has expanded continuously since that light left its source, HD1's present proper distance is calculated at 33.288 billion light-years. Its estimated redshift stood at approximately z = 13.27. That figure placed HD1's observed state at roughly 324 million years after the Big Bang. Scientists date that event to approximately 13.787 billion years ago. Alongside HD1, the same team identified a companion object called HD2. What produced such an object at such an early moment in cosmic history? Why did HD1 radiate with such unusual brightness? And could the galaxy even be what its discoverers first believed?

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• 2. H Band Dropout

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  The method behind HD1's detection came down to a subtle contrast in infrared brightness. Astronomers searched two patches of sky for objects with a specific spectral signature. The target was any source shining far more intensely in the K band of the infrared spectrum than in the H band, which sits at around 1.6 microns. A galaxy far brighter in K than in H could indicate a Lyman-break galaxy at an extreme distance from Earth. One patch of sky came from the Cosmic Evolution Survey. The other was the Subaru/XMM-Newton Deep Survey Field, imaged by the Subaru Telescope. Within these two regions, the team found not one candidate but two. HD1 appeared in the Sextans constellation; HD2 was located in the Cetus constellation. The name applied to both was the same: HD for "H band dropout." The designation has nothing to do with the stars called HD 1 and HD 2 in the Henry Draper Catalog. HD2 was found to sit nearly as far from Earth as HD1, giving the team two extraordinary candidates from a single search.

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• 3. The Ultraviolet Riddle

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  HD1's ultraviolet emission was far brighter than expected for a galaxy at its proposed distance. Similar galaxies in the same redshift range did not shine with anything close to HD1's luminosity. At least four explanations were put forward. One possibility was that HD1 was an active Lyman-break galaxy, energized by unusually intense processes within it. A second scenario described HD1 as an extreme starburst galaxy, producing new stars at a rate beyond anything previously observed. A third possibility involved Population III stars. These are a hypothetical early class of stars thought to have formed in the universe's first hundreds of millions of years. They would be far more massive and far more luminous than the stars that make up present-day galaxies. The fourth hypothesis placed a supermassive black hole at HD1's center, making it a quasar. A black hole active so early in cosmic history would constrain existing models of how black holes form and grow. The previous record-holder for distant galaxies, GN-z11, had been found in 2015 with a redshift of 11. Its observed light corresponded to roughly 420 million years after the Big Bang. Yet GN-z11 did not show the same puzzling brightness that made HD1 so difficult to explain. A resolution, the discoverers noted, would likely require the James Webb Space Telescope.

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• 4. A Revised Reading

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  Spectroscopic measurements published in 2024 assigned HD1 a redshift of z = 4.0. That figure placed the galaxy much closer to Earth in cosmic time than the original reading had implied. At z = 4.0, HD1 is classified as a passive galaxy: an object that has largely stopped forming new stars. The original reading had suggested a window into the universe's first few hundred million years. The revised reading places HD1 in a considerably later, calmer epoch. The gap between the two measurements is vast, and the debate is not closed. Only purpose-built space missions can settle it.

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• 5. What Telescopes May Find

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  The researchers who discovered HD1 and HD2 described the two objects as a potential "remarkable laboratory to study the Universe at previously inaccessible redshifts." That assessment rested on the condition that the high-redshift interpretation would be spectroscopically confirmed. Three space missions are positioned to provide that confirmation. They are the James Webb Space Telescope, the Nancy Grace Roman Space Telescope, and the GREX-PLUS mission. Together, they should be able to determine whether HD1 and HD2 are galaxies, quasars, or black holes. HD1's proposed extreme age also made it a prime candidate for revealing the first visible Population III stars. The same missions could also enable the discovery of more than 10,000 galaxies from this early epoch of the universe. The researchers saw the detection of HD1 and HD2 as the first step toward that much broader survey.

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