Euclid discovers the most ancient quasars: how redshift lets us see the early Universe

What happened?

The European Space Agency announced that its Euclid space telescope has found 31 very early quasars. The record-holder, EUCL J172902.75+641018.1, has a measured redshift of 7.77; that means the light we receive now left it when the Universe was about 670 million years old.

A quasar is the intensely bright centre of a galaxy where material is falling towards a supermassive black hole. Gas in the accretion disc becomes extremely hot as it loses energy and angular momentum, so the central region can outshine the rest of its galaxy by a huge amount.

These objects are difficult to find. Their ancient light is faint and can be confused with much closer objects, including cool stars in our own Galaxy. Euclid surveyed a very large area of sky, then astronomers selected likely candidates and confirmed their distances using observations of their light.

Euclid is especially useful because it can observe near-infrared light. For a very distant quasar, light that began in the ultraviolet or visible region has had its wavelength stretched during its journey. By the time it reaches us, important spectral features can lie in the infrared.

The simple version

Think of an atom's spectrum as a barcode. Atoms absorb or emit particular wavelengths, so a spectrum contains recognisable lines. Astronomers compare the wavelength of a known line in a laboratory with the wavelength of the same line from a distant object.

If every line has moved to a longer wavelength, the light is redshifted. At small redshifts this is similar to the Doppler effect you meet with sound. Across most of the Universe, however, the more useful explanation is that space expanded while the light was travelling, stretching the wave with it.

A redshift of 7.77 does not mean the quasar is coloured 7.77 times redder. It means the observed wavelength is 8.77 times the wavelength at which that light was emitted. That is why an ultraviolet feature can arrive at an infrared detector.

Looking farther into space also means looking further back in time, because light has a finite speed. The Euclid result is not a live picture of these quasars today. It is a view of their galaxies when the Universe was still in its first billion years.

Worked equations

Why it matters

The result is more useful than one new distance record. Euclid has found enough early quasars for astronomers to begin studying a population rather than relying only on a few unusually bright examples. That makes comparisons with models of early galaxy and black-hole growth more meaningful.

The quasars come from the epoch of reionisation, when energetic light from the first stars and galaxies changed much of the hydrogen between galaxies from neutral atoms into ionised plasma. Quasars act as bright background sources that help researchers test what that early material was like.

It is also a measurement story. A rare discovery required a telescope, infrared detectors, an enormous survey, careful data processing and follow-up observations. That is a useful reminder that modern physics is often about designing instruments that can distinguish a faint real signal from many convincing lookalikes.

Physics you already know

Photons carry electromagnetic radiation across space. Their wavelength tells us where they lie on the electromagnetic spectrum, and their energy changes as wavelength changes. Euclid uses that relationship when it detects very stretched light in the near-infrared.

Spectra are evidence, not decoration. A laboratory wavelength gives astronomers a reference value. Comparing it with an observed wavelength lets them calculate redshift, just as a shift in a familiar sound frequency can give information about motion.

The worked example is a useful standard form and unit-conversion exercise: nanometres, micrometres and metres are all the same physical wavelength expressed on different scales. Keeping powers of ten clear is essential before comparing detector ranges.

The story connects the life cycles of stars and galaxies with cosmology. Early stars made energetic radiation, while the black holes at the centres of some galaxies were already growing quickly enough to power quasars. Observations of their redshift support the picture of an expanding Universe.

Science ideas to understand

A Level stretch

For cosmological redshift, physicists often write 1 + z = a_today / a_emit, where a is the scale factor of the Universe. At z = 7.77, the scale factor at emission was about 1 / 8.77 of its value today. This describes the expansion of large-scale space, not the stretching of atoms, people or galaxies held together by forces.

Do not use the small-redshift approximation v = zc for this quasar. At z = 7.77 it would wrongly imply a speed greater than c. Very large cosmological redshifts need general-relativistic cosmology, where the recession rate comes from the changing scale factor rather than a simple object flying through static space.

A high-redshift candidate can be selected from its brightness in several filters because intervening hydrogen absorbs parts of its ultraviolet spectrum. Follow-up spectroscopy then checks whether the pattern of shifted features is consistent with a quasar at the claimed redshift instead of a much closer star or galaxy.

There is a distinction between lookback time, comoving distance and light-travel distance. Saying that a quasar is seen as it was 670 million years after the Big Bang is a time statement. Its present-day distance is not found by simply multiplying 13 billion years by c, because the Universe expanded while the light travelled.

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