Astrophysicists have announced the most significant discovery of black holes in history.

DESI (Dark Energy Spectroscopic Instrument) is a modern astronomical instrument capable of simultaneously capturing light from five thousand galaxies and breaking it down into spectra. This spectroscopic tool has been operational since 2020 and is installed on the four-meter Mayall telescope at the Kitt Peak National Observatory in Arizona, USA.

The DESI instrument is designed to study dark energy, specifically examining its influence on the rate of the universe's expansion. It captures optical spectra from tens of millions of galaxies and quasars, constructing a three-dimensional map that covers the universe up to a distance of 11 billion light-years. By 2026, DESI aims to gather data on 40 million such objects.

An international team of astrophysicists led by Ragadeepika Pucha from the University of Utah (USA) analyzed a vast dataset from DESI, encompassing 20 percent of the observations from the instrument's first year of operation. The sample included spectra from 410,000 galaxies, among which were 115,000 dwarf galaxies—dim systems with a small amount of gas.

Dwarf galaxies are small cosmic objects that contain significantly fewer stars (ranging from a few thousand to several billion) compared to large galaxies like our Milky Way. Due to their modest size and faint glow, scientists have struggled to study the processes occurring in the centers of dwarf galaxies. Modern telescopes have been unable to resolve fine details.

The situation changed when researchers focused on finding active galactic nuclei (AGN). This term refers to black holes that actively consume surrounding matter—gas, dust, and stars. During this "feasting," the matter is accelerated to near-light speeds, heated up, and begins to glow brightly, emitting vast amounts of energy across various electromagnetic radiation ranges. This transforms the black hole into a powerful cosmic "spotlight" that can be detected from great distances, even within a faint dwarf galaxy.

It is precisely these signals that DESI began to search for. Its highly accurate sensors can detect the characteristic "signatures" of active black holes in light spectra—such as the lines of ionized gas swirling around the black hole at tremendous speeds. As a result, the instrument has uncovered thousands of hidden objects that previously went unnoticed due to technical limitations.

Among the spectroscopic data, researchers identified 2,500 candidates for dwarf galaxies with active nuclei. This figure is four times greater than what previous similar instruments have found. Further analysis revealed that only two percent of dwarf systems exhibit signs of AGN. This suggests that most black holes within them are either dormant or remain undetected.

A separate breakthrough is the discovery of 300 candidates for intermediate-mass black holes (ranging from hundreds to millions of solar masses). Until now, science was aware of only about 100-150 candidates for such objects.

The majority of known black holes fall into two categories: either stellar (from 10 to several tens of solar masses) or supermassive (over a million times the mass of the Sun). Intermediate-mass black holes are poorly understood, but they are thought to be the "seeds" of supermassive black holes that are now located at the centers of large galaxies. Additionally, there is a possibility that these objects are relics of the very first black holes formed after the Big Bang. If they have survived to the present day, they represent "living relics" of early cosmology, preserving information about the physical conditions and processes of a very young universe.

Paradoxically, only 70 out of the 300 candidates for intermediate-mass black holes were associated with dwarf galaxies. This contradicts the existing hypothesis that these objects are more frequently formed in dwarf systems.

“We may be encountering new mechanisms for black hole formation or features of their growth,” explained American astrophysicist Stephanie Juneau, a co-author of the study from the National Optical-Infrared Astronomy Research Laboratory.

The success of DESI is linked to its technology. Each of the 5,000 fiber-optic sensors in the instrument has a diameter of just 10 microns, allowing for a focus on the cores of galaxies while avoiding "light pollution" from their outskirts.

The findings from Pucha's team raise new questions. Why are so few intermediate-mass black holes associated with dwarf galaxies? How do they influence the evolution of their "hosts"? Is there a connection between the mechanisms of black hole formation and the types of galaxies in which they reside? The search for answers could change scientists' understanding of black hole formation. With DESI's continued operation, researchers anticipate even more surprises.

The scientists' conclusions are presented in a paper published in The Astrophysical Journal.