Inaf, a fossil galaxy discovered 3 billion light years away. A Neapolitan scientist leads the research

Thanks to extremely high-resolution observations with the Large Binocular Telescope in Arizona, a team led by the National Institute for Astrophysics (INAF) has confirmed the existence of a galaxy that has remained virtually unchanged for about seven billion years: a true cosmic fossil that allows us to study the formation of the first galaxies in the history of the universe.

Throughout the history of the cosmos, galaxies tend to grow and evolve through mergers with other galaxies. But there are rare examples that behave like a time capsule: these galaxies, called fossils or relics, formed very rapidly in the very early stages of the universe, producing almost all of their stars in less than three billion years after the Big Bang, and have remained virtually intact since then. Observations show them as dense and compact, populated by stars rich in heavy elements, and without any signs of ongoing star formation.

A new study has now observed the most distant relic galaxy ever discovered: a cosmic fossil, unchanged for about 7 billion years. It is called KiDS J0842+0059 and is the first massive fossil galaxy confirmed outside the local universe, through spectroscopic observations and high-resolution images.

The discovery, made by an international team of researchers led by the National Institute for Astrophysics (INAF), was made possible thanks to the Large Binocular Telescope (LBT), a telescope operated by Italy, Germany and the United States on the summit of Mount Graham, in Arizona. The results are published in the July issue of the journal Monthly Notices of the Royal Astronomical Society.

“We have discovered a galaxy that has been 'perfectly preserved' for billions of years, a true archaeological find that tells us how the first galaxies were born and helps us understand how the universe has evolved up to today,” explains Crescenzo Tortora, researcher at the INAF-Capodimonte Astronomical Observatory and first author of the work. “Fossil galaxies are like the dinosaurs of the universe: studying them allows us to understand the environmental conditions in which they formed and how the most massive galaxies we see today evolved.”

The galaxy, which we observe as it was about three billion years ago, was initially identified in 2018 within the KiDS (Kilo Degree Survey) project, a public survey of the European Southern Observatory (ESO) carried out by the Italian VST (VLT Survey Telescope) telescope located at the Paranal Observatory in Chile. The KiDS images provided an estimate of the mass and size of the galaxy, whose properties were further characterized through observations with the X-Shooter instrument on ESO's Very Large Telescope, also in Chile. All its characteristics seemed to indicate that it was a fossil galaxy: from its stellar mass, equal to about one hundred billion solar masses, to its lack of star formation for most of the galaxy's life, to its dimensions, more compact than those of galaxies with the same stellar mass.

However, some uncertainties remained about the size and structure of the galaxy. To confirm the compactness of the galaxy, new observations made with the Large Binocular Telescope (LBT) were crucial, capable of obtaining much sharper images thanks to the SOUL adaptive optics system, which compensates in real time for the effects of atmospheric turbulence. The observations of the galaxy KiDS J0842+0059 collected with the LBT have a degree of detail ten times greater than the data from the KiDS survey: they are the most detailed images of a relic galaxy at this distance and allow us to study its shape and size like never before.

“The data from the Large Binocular Telescope have allowed us to confirm that KiDS J0842+0059 is indeed compact and therefore a true relic galaxy, with a shape similar to NGC 1277 and the compact galaxies that we observe in the early Universe,” explains co-author Chiara Spiniello, a researcher at the University of Oxford, an INAF associate and principal investigator of the INSPIRE project, who contributed to the characterization of the properties of this galaxy. Until now, NGC 1277 was one of the few confirmed prototypes of this rare class of galaxies. “This is the first time that we have been able to do this with such high-resolution data for such a distant relic galaxy.”

The existence of massive relic galaxies such as KiDS J0842+0059 or NGC 1277 demonstrates that some galaxies can form rapidly, remain compact, and then remain dormant for billions of years, escaping the growth that most of their counterparts experienced through mergers with other galaxies.

“Studying these cosmic fossils helps us reconstruct the formation history of the nuclei of today's massive galaxies, which — unlike relic galaxies — have undergone merger processes, accreting matter precisely around those first (compact) galaxies from which they originated,” concludes Tortora. “With cutting-edge technologies such as adaptive optics and the support of telescopes such as the LBT, we can improve our understanding of this type of galaxies. In the near future, moreover, we will take a step forward, aiming to search for, confirm and study new relic galaxies through the data of unique quality and resolution of the Euclid space telescope.”