Astronomers from The Netherlands, UK and Germany have developed a new method to find distant quasars (quasi-stellar objects) which better distinguishes them from other objects that look like them. Using machine learning techniques and spectroscopic data taken with the Isaac Newton Telescope (INT), the researchers discovered possibly the highest-redshift quasar ever observed with the INT.

A quasar is an extremely bright active centre of a galaxy, powered by a supermassive black hole that can be up to a billion times heavier than the Sun. Some supermassive black holes in the centres of galaxies are inactive, like the black hole at the centre of the Milky Way, but many are active, surrounded by a swirling disc of superheated gas. The black holes launch jets that reach hundreds of thousands of light-years into intergalactic space. They accelerate charged particles at speeds close to the speed of light, making them among the most powerful particle accelerators in the universe. Gas ejection by jets is essential for regulating mass and star formation in some of the most massive galaxies. Consequently, supermassive black holes play an important role in the formation and evolution of galaxies.

Quasars are therefore ideal objects for studying the evolution of the universe, especially in its earliest stages. One of the biggest challenges facing astronomers is finding these objects. Because they are so far away, quasars are seen as faint red dots in the sky.

Ironically, from Earth these powerhouses at the 'edge' of the universe look very similar to objects like red dwarfs. Red dwarf stars are much smaller than our Sun, and astronomers can only observe them within a few hundred light years. Because there are so many more red dwarf stars than quasars, most samples of promising quasar candidates have traditionally been heavily contaminated with such dwarf stars.

The astronomers implemented the new method on a catalogue of sources from the wide-field sky survey of Pan-STARRS, a set of optical imaging survey telescopes in Hawaii. This was supported by a catalogue of radio sources from LOFAR, ASTRON's low-frequency radio telescope in the Netherlands. Using the combined data, they identified sources that are most likely quasars. To properly identify these objects, they measured the spectra of a small number of candidates with the INT.

This study confirmed that one of the candidates is indeed a very bright quasar, from the time the Universe was less than a billion years old. The discovery of this never-before-seen quasar shows that this technique opens up new ways to discover more quasars in the early Universe, both in existing and future surveys. The researchers expect that hundreds of other quasars could be hidden, as the newly discovered quasar was found in a search of a relatively small area of the sky.