Using the European space telescope Euclid, astronomers have tracked down the earliest known quasars in the universe. These brilliant objects formed less than 670 million years after the Big Bang and shone as brightly as trillions of suns – a glimpse into an era from which almost nothing had been visible before.
Quasars are powered by supermassive black holes that devour enormous amounts of matter at the centres of distant galaxies, releasing tremendous radiation in the process. Because that glow remains visible across billions of light-years, quasars act as cosmic beacons – and pose a puzzle at the same time: many of them already existed in the early cosmos. How their black holes could grow to millions or even billions of solar masses in so short a time is one of the great open questions in astrophysics, because the so-called Eddington limit caps how fast a black hole can grow at all.
To find answers, researchers hunt for quasars as early as possible – but they are hard to spot. Their light is heavily stretched by the expansion of the universe and reaches us only as a faint infrared point that is difficult to tell apart from nearby stars. "For every early quasar there are thousands of stars that look almost identical in the images," explains lead author Daming Yang of Leiden University.
This is where Euclid plays to its strengths. Launched in 2023, the telescope captures large patches of sky in sharp detail in a single pass and can gather the stretched light with its near-infrared spectrometer, NISP. To sift the rare candidates out of the flood of data, the team turned to an artificial intelligence trained to recognise the tell-tale signatures of early quasars.
The objects found show a redshift of around seven – a measure from which distance and age can be derived, placing the discoveries deep in the youth of the universe. For science, such quasars are valuable witnesses: they reveal how the first supermassive black holes and galaxies took shape. Euclid, which is designed primarily to map dark matter and dark energy, thus proves to be a finder of the oldest lighthouses in the cosmos as well.