Euclid found more ancient quasars in a year than science had found in a decade

For the past decade, confirming a single quasar powered by a black hole that already weighed a billion solar masses when the universe was less than a billion years old required a coordinated effort across multiple telescopes and months of follow-up spectroscopy. The entire cumulative result of those efforts stood at roughly ten confirmed objects. In its first year of science operations, Euclid confirmed twelve.

That figure is the central result of a paper by Leiden University doctoral student Daming Yang and colleagues, published in Astronomy & Astrophysics as part of a 41-paper special issue drawing on Euclid’s first quarter of sky data. The full catalog contains 31 previously unknown quasars from the universe’s earliest epoch — ancient light sources, each burning with the output of roughly one trillion suns, powered by supermassive black holes already in place when the cosmos was a fraction of its current age.

The two most distant objects in the catalog, designated EUCL J172902.75+641018.1 and EUCL J125308.55+705432.3, carry redshifts of 7.77 and 7.69, placing them among the most distant objects individually resolved in any survey. Their light departed when the universe was approximately 670 million years old.

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How Euclid identifies objects that look like ordinary stars

Detecting ancient quasars is a problem of needle and haystack. At extreme distances, a quasar’s ultraviolet emission has been stretched by the expansion of the universe into the near-infrared, a shift that places its characteristic spectral lines at wavelengths most ground-based instruments struggle to reach efficiently. More practically, the resulting faint, red appearance makes these objects nearly indistinguishable from much closer, far more numerous M-dwarf stars in standard visible-light images. Most pre-Euclid detections depended on matching objects across several surveys of varying depth and filter coverage, then prioritizing candidates for expensive observing time on large telescopes.

Euclid addresses both problems at once. Its Near Infrared Spectrometer and Photometer (NISP) covers wavelengths from 0.95 to 2.0 microns, exactly where the redshifted Lyman-alpha emission from z≥7 quasars falls, while simultaneously capturing broadband photometry that enables initial candidate selection. The survey’s area, designed to eventually cover a third of the sky at depths unreachable from the ground, generates a statistical volume large enough to contain useful samples of the rarest objects. “Their primordial light is both faint and easy to confuse with that from stars lying closer to us,” said Antonio La Marca, an ESA research fellow on the Euclid team.

Yang’s team applied a photometric selection algorithm to the Q1 data, identified candidates consistent with quasars at z≥7, and confirmed detections using NISP’s spectroscopic mode without requiring a separate ground-based campaign. The efficiency gain over prior survey methods is the difference between a decade’s cumulative result and twelve confirmed objects in a year.

What the redshift-7 threshold actually means

Redshift quantifies how much the universe has expanded since a given photon was emitted. A redshift of z=7 corresponds to a universe that was roughly one-eighth of its current linear size, translating to a lookback time of about 13 billion years and a cosmic age of 670 million years after the Big Bang. At that moment, the universe was completing reionization, the transition in which ultraviolet output from the first luminous sources ionized the hydrogen gas that had kept the early cosmos opaque.

Quasars at z≥7 were among the main drivers of reionization, but they are also its paradox: they require supermassive black holes that grew fast enough to reach billions of solar masses at a point in cosmic history when, under standard models of structure formation, there had barely been time to form the first stars. The Milky Way’s central black hole, Sagittarius A*, weighs approximately four million solar masses and accumulated that mass over the full 13.8-billion-year age of the universe. The black holes powering the z≥7 quasars in the Euclid catalog weigh hundreds to thousands of times more, yet accumulated that mass in under 5% of the same timeframe.

“These monsters — weighing billions of times the mass of our sun — somehow already existed when the universe was in its infancy,” said Joseph Hennawi, Yang’s supervisor at UC Santa Barbara and a co-author on the paper. Finding more than a dozen of them in a single year’s data demonstrates they are not statistical anomalies: the sample is now large enough to treat as a population.

What the catalog does not resolve

Additional confirmed detections strengthen a quantitative case without yet discriminating between proposed formation mechanisms. The leading candidates include sustained super-Eddington accretion, in which gas falls into a seed black hole faster than the canonical radiation-pressure limit for periods long enough to build the observed masses; direct collapse of massive primordial gas clouds into seed black holes far heavier than any stellar remnant; and rapid merging of dense early star clusters before the first generation of supermassive black holes switched on. Each mechanism faces independent observational constraints, and the Euclid data do not yet include the host-galaxy characterizations needed to test them directly.

Yang’s paper notes that the 31-object catalog represents a bright subset of a larger underlying population, those luminous enough and at the right combination of redshift and sky position to emerge clearly from the Q1 data. Completeness models will require the full Euclid wide survey, which continues to observe. One practical caveat applies to all 31 objects: host-galaxy characterization, essential for testing formation models, demands deeper observations than the survey itself provides. Silvia Belladitta of the Max Planck Institute for Astronomy in Heidelberg conducted follow-up spectroscopy for the second-most distant object in the catalog; planned ground-based campaigns will address the full sample.

Common questions about Euclid’s ancient quasars

What exactly is a quasar, and why does its brightness matter?

A quasar is the intensely luminous core of a galaxy powered by a supermassive black hole actively accreting surrounding gas. As material heats up in the accretion disk, it radiates across the electromagnetic spectrum at a brightness capable of outshining every star in the host galaxy combined. At the distances reported here, only the central engine is detectable; the host galaxy is too faint and too compact to resolve. The extreme luminosity is what allows Euclid to detect objects from 13 billion light-years away.

Why are these objects described as a problem for cosmology?

Standard models of black hole growth set a natural ceiling on accretion rates, known as the Eddington limit. A stellar-mass seed, the largest black hole that a star can leave behind, accreting continuously at this rate cannot reach a billion solar masses in the time available between the Big Bang and the epoch these quasars inhabit. Finding more than a dozen in a single survey year means they are common enough that no exotic single event can explain them; the formation mechanism has to work at scale.

How does Euclid compare to previous surveys for this type of object?

The Euclid Wide Survey will eventually cover approximately 14,000 square degrees at near-infrared sensitivities ground-based surveys cannot match over comparable areas. The previous generation of surveys, including the Sloan Digital Sky Survey and the UKIRT Infrared Deep Sky Survey, identified most of the prior z≥7 quasar catalog over more than a decade of combined observations. Euclid’s NISP instrument performs the equivalent of initial selection and spectroscopic screening simultaneously, compressing what previously required separate campaigns into a single observing pass.

What happens next in this research program?

Ground-based follow-up spectroscopy is planned for the full 31-object sample to refine redshift measurements and characterize host galaxies. Additional Euclid data releases will expand the catalog as the wide survey accumulates sky area. The Q2 data release from Euclid, which covered the Milky Way’s galactic bulge with 60 million stars captured in 26 hours of observation, was published in late June; subsequent releases will add more extragalactic sky area relevant to high-redshift quasar searches. “By finding and studying them,” Yang wrote, “we can better understand how these enormous systems formed and grew so quickly.”

Reference: Yang et al., “Euclid: Discovery of 31 high-redshift quasars including two of the most distant quasars known,” Astronomy & Astrophysics, 2026. DOI: 10.1051/0004-6361/202658883

Tags: astronomy, supermassive black holes, early universe, esa, euclid, quasars

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