What Is the Most Distant Quasar Ever Discovered?

What did the cosmos look like when it was barely 5% of its present age, with the very first galaxies only just flickering on? Welcome, curious reader — you have come to the right place. A single European telescope has just handed astronomers a fresh window onto that distant dawn, and the view is astonishing. Stay with us to the end, because the story of these 31 ancient beacons tells us as much about how we hunt the early Universe as it does about the objects themselves.

What did Euclid discover in its first eighteen months?

Euclid has discovered 31 new high-redshift quasars, and one of them is the most distant quasar ever recorded. The haul comes from roughly 3,000 square degrees of sky — about 7% of the whole celestial sphere — observed during the first year and a half of the Euclid Wide Survey. The results were published in Astronomy & Astrophysics by D. Yang of Leiden University, J. F. Hennawi and a large team from the Euclid Consortium.

All 31 lie in the redshift range 6.6 < z < 7.8, meaning we see them as they were when the Universe was between roughly 650 and 820 million years old. Twelve of them sit at redshift 7 or higher. That single survey slice more than doubles the count of z ≥ 7 quasars known to science — before this work, only nine had been confirmed since the first one turned up in 2011.

Why is EUCL J1729 the most distant quasar ever found?

EUCL J172902.75+641018.1 — EUCL J1729 for short — is the most distant quasar known because its light was emitted at redshift z ≈ 7.77, when the Universe held only about 662 million years on the clock. That edges past the previous record-holder by a redshift of roughly 0.13, which sounds tiny but corresponds to peering back a further 15 million years into cosmic history.

To pin down that redshift, the team gathered a deep spectrum of the object with the Large Binocular Telescope, exposing for 2.8 hours. The spectrum shows several emission lines — Lyman-alpha, nitrogen, oxygen and silicon — and a broad absorption feature that marks EUCL J1729 as a likely broad absorption line quasar, where fast winds strip material off the black hole’s surroundings.

Two of its siblings sit close behind. EUCL J1253 (z ≈ 7.69) and EUCL J1012 (z ≈ 7.61) are the second and third most distant in the sample, and they are also among the faintest quasars ever seen at these redshifts — nearly ten times fainter in ultraviolet light than the handful of luminous quasars known at similar distances before Euclid.

What exactly is a quasar?

A quasar is a short, blazing chapter in a galaxy’s life, when gas and dust spiral into its central supermassive black hole and pour out light across the whole electromagnetic spectrum. Astronomers think almost every big galaxy hides a supermassive black hole at its heart — a monster with the mass of millions to billions of Suns. Most of the time that monster sleeps.

When fresh material starts falling in, though, the black hole’s surroundings light up as an active galactic nucleus. A quasar is the most extreme version: the infall runs so fast that the glow outshines every star in the host galaxy, sometimes by thousands of times. That makes quasars some of the brightest steady sources in the Universe — we tell that story in full in our primer on what quasars are and how they shine.

Distance sharpens the point. The nearest quasar to us, Markarian 231, sits about 581 million light-years away toward the Plough. EUCL J1729 and EUCL J1253 lie more than 13 billion light-years off, a scale we unpack in our guide to measuring cosmic distances in billions of light-years — so far that their light has been travelling for almost the entire history of the cosmos.

How did Euclid find quasars that eluded astronomers for years?

Euclid found these quasars by pairing machine-learning and probabilistic methods with its sharp visible and near-infrared imaging, then confirming each candidate with ground-based spectroscopy. Two cameras work side by side aboard the 1.2-metre telescope: the visible camera (VIS) and the near-infrared spectrometer and photometer (NISP). Together they reach depths a wide-field survey has never managed before, across both optical and infrared light.

The trick for spotting a z ≈ 7 quasar is a colour signature called a dropout. Neutral hydrogen along the line of sight swallows the object’s ultraviolet light, so it vanishes in Euclid’s optical band while staying visible in the infrared. The team hunted for exactly those point-like sources with three independent selection algorithms, cross-checked their guesses, and trimmed away look-alikes.

Then came the hard part. Cool stars and faint red galaxies can mimic a distant quasar, so spectra are the only way to be sure. Astronomers pointed the Keck, Magellan and Large Binocular telescopes at 123 candidates over roughly 20 nights. Just 31 turned out to be genuine quasars; the rest were mostly brown dwarfs and other contaminants.

Why does finding faint, distant quasars matter?

These quasars matter because they let us test how supermassive black holes grew so large so early, and because their light probes the gas that filled the young Universe. Under the usual growth limit, a black hole doubles its mass roughly every 45 million years. Squeeze in a quasar at 662 million years and the arithmetic gets tight: the seed had very little time to reach millions of solar masses. Each new record narrows the puzzle of how the first supermassive black holes grew so fast in the early Universe.

What sets this sample apart is its faintness. Most earlier z ≳ 7 quasars were rare, luminous giants — the tip of the iceberg. Euclid reaches one to two magnitudes fainter, into a population that had barely been sampled at these redshifts. Faint quasars sit closer to ordinary galaxies in brightness, so they offer a more representative look at black-hole growth in the first billion years.

Their light also carries a record of the surrounding gas. During the epoch of reionisation, ultraviolet radiation from the first stars to light up the cosmos and the earliest black holes flipped the intergalactic medium from neutral to ionised. Beams from distant quasars pass through that gas on their way to us, letting astronomers read the state of the cosmos at different moments. Two of the new quasars even show up in radio surveys with the LOFAR array, hinting at powerful jets in the early Universe.

What comes next in the search for the first quasars?

The hunt has only just begun: as Euclid’s map grows, the team expects the first quasars beyond redshift 8 to surface soon. A 2019 forecast for the full survey predicted more than 100 quasars between z = 7.0 and 7.5, around 25 beyond z = 7.5, and a handful past z = 8. This first slice already lines up with that outlook, and the survey will run until 2030.

Ground telescopes will struggle to keep pace at the faint end, where confirming a single quasar can eat one to two hours on a 10-metre giant. For the dimmest targets, space observatories such as JWST become the only realistic option, while NOEMA and ALMA can weigh the gas and dust in the host galaxies. The frontier is moving, and Euclid is the machine pushing it.

What have we learned from Euclid’s quasar haul?

In barely eighteen months, one telescope has more than doubled our tally of the earliest known quasars and reset the distance record. We now have a fresh population of faint, far-off black-hole beacons to study — objects that stretch our models of how monsters grow and let us read the gas of the reionisation era. The real prize is not a single record but a doorway: a chance to watch the first galaxies and their black holes taking shape.

Take a moment with that thought. The photons reaching Euclid tonight began their journey when the Universe was a toddler, and they crossed almost all of time to land on a mirror we built. What else is waiting in that first billion years, still unseen?

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