Probing the host galaxy of one of the most distant quasars

To the point:

• Super-effective distant quasar hunter: the ESA space telescope Euclid has found a treasure trove of early quasars, including two at record-breaking distances, and is poised to find many more.
• First look at ordinary early quasars: This is the first time astronomers can examine very early quasars that are ordinary, instead of seeing only the very brightest quasars.
• A massive, star-forming host galaxy: Follow-up on the galaxy hosting one of those ordinary quasars reveals a massive galaxy forming many stars – a new piece for the puzzle of galaxy formation.

ESA’s space telescope Euclid has opened up a new chapter in the study of early galaxies. New research by Silvia Belladitta (Max Planck Institute for Astronomy, MPIA) and colleagues has followed up on one of the Euclid discoveries to uncover key properties of the host galaxy of one of the earliest supermassive black holes known in the universe.

Bright objects with a dark center

Active galactic nuclei known as quasars are responsible for some of the brightest celestial objects we see in the sky. The “engine” behind that enormous luminosity is matter falling onto a central supermassive black hole – a black hole with masses of millions, billions or an even greater number of solar masses. Such supermassive black holes are found in all but the smallest galaxies. Energy from the 'central engine' influences star formation in a galaxy (in particular the most massive galaxies): by either heating or compressing the gas that is the raw material for new stars, limiting star formation in the first case, enhancing it in the second.

How the first galaxies and their central black holes emerged is a highly active area of research. Finding the earliest quasars and examining their properties and the properties of their host galaxies is an important piece of the puzzle. But targeting the earliest quasars is challenging. Objects that we see as they were in the early universe are necessarily very far away. When light reaching our telescopes today shows us a quasar as it was 13.4 billion years ago, that is because the light needed 13.4 billion years to travel from its source to our telescopes.

Searching for “ordinary” quasars

At such great distances, even intrinsically bright objects like quasars appear rather dim. Easiest to observe are particularly bright specimens  – but those, being exceptionally bright, are unlikely to be representative of their more normal siblings. When it comes to the population of quasars, we had so far seen only the tip of the iceberg. This is changing:  Euclid’s combination of sensitivity and the ability to scan large areas of the sky at once makes it an ideal search machine for quasars in the early universe. Follow-up observations with ground-based telescopes confirm Euclid’s remarkable quasar-finding power..

Eduardo Bañados, group leader at MPIA and co-lead of the Euclid Quasar Work Package from 2022 to 2025, says: “Seeing Euclid deliver on its potential is immensely satisfying. But more than that, it marks a genuine shift: For the first time, we can study the typical early-universe quasar, not just exceptional outliers. We now have a real window onto how the bulk of the first black holes grew — and how they shaped the galaxies around them."

After only 1.5 years of data-taking, Euclid has more than doubled the number of known early quasars, from nine to 21 (“redshift z>7 quasars”, seen as they were less than 800 million years after the Big Bang). In fact, within a few months, Euclid broke the quasar redshift record not once, but twice!

Probing an extremely distant host galaxy

One refreshingly ordinary early quasar is the one that Belladitta and her team examined more closely. The quasar has the designation EUCL J125308.55+705432.3 (in the usual astronomy fashion, less a name than a detailed sky position). Light we receive today from this quasar was emitted 13 billion years ago, a mere 800 million years after the Big Bang (“z=7.7”). Its UV light amounts to only about 15% the brightness of previous redshift record-holding quasars.

For their follow-up, the astronomers used the NOEMA (NOrthern Extended Millimeter Array) observatory on the Plateau de Bure in the French Alps. NOEMA’s twelve 15-m-antennas act in concert like a single, much larger telescope. The astronomers observed submillimeter light at two carefully chosen wavelengths, each of which traces a different property of the quasar’s host galaxy.

Star formation and dust content

The first type of light is what astronomers call the [CII] line. This kind of light is produced in clouds of molecular gas where new stars are being born. The brightness of this line therefore indicates a galaxy’s star formation rate. The light also allows for a mass estimate: If you have ever heard the way that an emergency vehicle’s siren sound changes as the vehicle passes by, you know how motion influences the wavelength of waves. Applying the same principle in reverse, the way that the [CII] line is shaped allows astronomers to reconstruct the motion of gas in the quasar’s host galaxy, which in turn yields an estimate of its total mass.

The second type of light is thermal radiation from the cold dust in a galaxy. The intensity of this light reveals how much dust is present. The amount of dust is typically associated with the amount of molecular hydrogen, the raw material for star formation – of which this quasar appears to have a lot!

Reconstructing the galaxy’s star-formation rate

Taken together, Belladitta and her colleagues were able to reconstruct key properties of the galaxy that is hosting the quasar. The galaxy is forming stars at a rate of more than 250 solar masses per year – an impressive amount compared to the one solar mass per year of our own Milky Way, but not unexpected given previous finds for less distant quasars. The galaxy’s mass is estimated at around ten billion solar masses, a factor ten less than our own Milky Way. This is consistent with early galaxies that still have a lot of growth ahead of them.

"We found a galaxy that has all the ingredients to build a giant system: it is as massive as the hosts of the brightest early quasars and contains a huge reservoir of molecular gas to fuel intense star formation,” says Silvia Belladitta, a postdoctoral researcher at MPIA. Belladitta, who is the lead author of the study and the new co-leader of the Euclid Quasar Work Package, adds: “This raises an intriguing possibility. UV-faint quasars like EUCL J125308.55+705432.3 may be in a different evolutionary phase than their brighter cousins. Either the black hole is growing more slowly than in the brightest quasars, or else much of its activity is hidden behind thick clouds of dust. Distinguishing between these possibilities will be an exciting challenge for future observations.”

Future plans

For the big picture of galaxy evolution, these are incremental results. But they are pioneering achievements nonetheless, and they point the way forward: with the full 6-year Euclid survey expected to uncover hundreds of additional early quasars of this kind, and with follow-up observations like those of Belladitta and her colleagues providing an ever-larger set of information about star formation rates and galaxy masses, astronomy is steadily building a picture of the earliest galaxies and supermassive black holes in the universe. This will bring the story of the origin of galaxies, and of ourselves, into ever sharper focus.

Background information

The results described here have been published as Belladitta et al. “Euclid: A UV-faint quasar in a highly luminous star-forming host galaxy at z≈7.7” in the journal Astronomy & Astrophysics, doi: 10.1051/0004-6361/202659319. The Euclid quasar search results have been published as D. Yang et al., “Euclid: Discovery of 31 new quasars at 6.6 < z < 7.8” in the journal Astronomy & Astrophysics, doi: 10.1051/0004-6361/202658883.    

The MPIA scientists involved are Silvia Belladitta, Eduardo Banados, Fabian Walter, Knud Jahnke, Sarah Bosman, Julien Wolf, and Mischa Schirmer, in collaboration with Roberto Decarli (INAF Observatory, Bologna), Daming Yang (Leiden Observatory), Francesco Guarneri (University of Hamburg) and the rest of the EuclidCollaboration.

Euclid is ESA's mission to characterize Dark Energy and Dark Matter across cosmic time. Launched in 2023, Euclid will survey a third of the sky, recording images for two billion galaxies and precise distances of 50 million galaxies. Euclid's first major data release DR1 will provide data to the world for almost 2000 square degrees in November 2026. The Max-Planck-Institute for Astronomy (MPIA) is a founding member of the Euclid Consortium, a group of now more than 150 institutions across Europe, Canada, Japan, and the USA. During its construction MPIA has contributed hardware for the near-infrared instrument onboard Euclid. Now MPIA scientists are involved in its operation in orbit and are leading Euclid's overall calibration work.