A Wave of Ancient Quasars from the Dawn of the Universe…..Keck Observatory and ESA’s Euclid mission double the known population, including the two most distant quasars ever found

Maunakea, Hawaiʻi – Astronomers using the W. M. Keck Observatory on Maunakea, Hawaiʻi Island, have confirmed two-thirds (21 of 31) of the most ancient quasars from the Euclid mission, including two of the most distant ever observed.

Quasars represent a brief phase in a galaxy’s life during which large amounts of material spiral into the central supermassive black hole, releasing enormous amounts of energy. In this phase, the galaxy’s nucleus can become one of the brightest persistent sources in the Universe.

Astronomers have been hunting for the Universe’s earliest quasars for decades. These celestial archives reveal conditions in the earliest days of the cosmos, including how the first supermassive black holes and galaxies took shape. But accessing them isn’t easy. Quasars from this era are difficult to find: they are elusive, as few galaxies had time to grow large enough, and their light has traveled for more than 13 billion years to reach us, arriving faint and stretched into infrared wavelengths, where it can resemble nearby cool stars.

“We see these quasars as they were during the Universe’s infancy,” says Daming Yang, lead author of the study and graduate student at Leiden University in the Netherlands. “By finding and studying them, we can better understand how these enormous systems formed and grew so quickly – one of the greatest mysteries in astrophysics.”

The study, led by an international team in the Euclid consortium, is published in today’s issue of Astronomy & Astrophysics.

Confirming Ancient Quasars

The 31 quasars reported here were selected from ESA’s Euclid Wide Survey, which will cover more than one-third of the total sky once complete. While the Euclid mission identified promising candidates, confirming them required follow-up spectroscopy from ground-based observatories.

To confirm these quasars spanning redshifts from 6.5 and beyond, the team used three instruments at Keck Observatory, covering a broad wavelength range: the Multi-Object Spectrograph for Infrared Exploration (MOSFIRE), the Low Resolution Imaging Spectrometer (LRIS), and the Keck Cosmic Web Imager (KCWI).  With these instruments, the team analyzed the light from more than 100 candidates in the northern sky, searching each one for a telltale signature: a sharp drop in brightness known as the Lyman-alpha break. This cutoff is imprinted by hydrogen gas that pervaded the early universe, which absorbed the quasar’s light at specific wavelengths before it could reach us. The precise location of that drop confirms the object as a genuine quasar and pins down its distance.

The challenge is that these quasars appear incredibly faint due to their extreme distances, so detecting this signal requires the combination of highly sensitive instruments and the light-gathering capability of the world’s largest ground-based telescopes, such as Keck Observatory.

An elusive and distant population

Before Euclid, astronomers had identified only a handful of quasars at redshift 7 or higher after decades of searching — a measure of distance tied to how light stretches as the universe expands — corresponding to the first 770 million years after the Big Bang. Finding more and pushing further into the early universe is extraordinarily challenging for several reasons: so early in cosmic history, very few galaxies had grown large enough to power them; those that existed are so far away that their light has been travelling for over 13 billion years, arriving incredibly dim; and over that vast journey, the light has been stretched into infrared wavelengths, invisible to traditional optical telescopes.

The mission has already uncovered 12 new quasars above redshift 7, more than doubling the known population. For the first time, astronomers have a large enough sample to study these ancient quasars not as individual curiosities but as a population, building the first statistical picture of how supermassive black holes formed and grew in the Universe’s earliest epochs.

The two most ancient of the batch, EUCL J172902.75+641018.1 and EUCL J125308.55+705432.3, have redshifts of 7.77 and 7.69, respectively, setting a new record for the most ancient quasars ever found. Both lie more than 13 billion light-years away and emerged during the Universe’s first 670 million years.

Building a quasar chronicle

A multi-wavelength follow-up study of all high redshift quasars is currently underway. Using a collection of ground and space-based telescopes the team is constructing a quasar chronicle that will trace how supermassive black holes and their host galaxies evolved over the first billion years of cosmic history, and how reionization unfolded.

“We have a real sample at such high redshift for the first time, and we can finally start answering the questions that were unanswerable before,” said Yang.

“Every one of these 31 quasars is a new sightline into the early Universe, and the follow up science is just beginning” added Joseph Hennawi, co-author and professor at the University of California, Santa Barbara.

As the ESA Euclid space telescope continues to scan new sky, the probability of finding the first quasars beyond redshift 8 increases. A redshift 8 quasar would be shining when the Universe was less than about 630 million years old and would also place the strongest direct constraints yet on how early supermassive black holes could have formed and grown.