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Supermassive black holes reside at the center of most large galaxies, gravitational giants with masses millions or even billions of times that of our sun. While many remain relatively dormant, occasionally consuming nearby matter, a select few are remarkably active.
These actively feeding black holes voraciously ingest material, launching powerful jets of energy and radiation into space. They power active galactic nuclei (AGN), which appear as incredibly bright beacons across the universe. For decades, scientists have debated what triggers the transformation of these quiescent black holes into intensely active ones.
Galactic Collisions as a Cosmic Wake-Up Call
New data from the European Space Agency’s Euclid space telescope may have provided an answer. Observations suggest that galactic collisions and mergers could be the key to activating these cosmic engines. This discovery offers new insight into the evolution of galaxies and the role supermassive black holes play in shaping them.
When galaxies collide, it’s not a direct impact like cars. Instead, they pass through each other due to the vast distances between stars. However, the gravitational disruption caused by the encounter sends gas, dust, and stars hurtling through space, often over immense distances. Some of this material is driven towards the central supermassive black hole, forming a massive accretion disk. As matter spirals into the black hole, the galactic core flares with intense energy.
Scientists have long theorized about this connection, but previous studies were limited by small sample sizes and insufficient image quality to reliably identify both ongoing collisions and faint active galactic nuclei. The Euclid telescope has changed that.
Launched two years ago, Euclid is proving to be a remarkably powerful observatory. It’s equipped with a 600-megapixel sensor for capturing visible light, complemented by a spectrometer and photometer for near-infrared observations. In just one week, Euclid has captured high-quality images of a larger area of the universe than the Hubble Space Telescope did in over thirty years of observation.
To effectively analyze this massive dataset, researchers at the Netherlands Institute for Space Research (SRON) developed a new artificial intelligence tool. This tool can deconstruct images of galaxies into their individual components and identify AGN that would otherwise remain hidden. It also allows for precise measurements of their energy output. When applied to a sample of one million galaxies – far larger than any previous study – the results were conclusive.
A Pivotal Moment in Galactic Life
The data clearly demonstrates that merging galaxies contain significantly more active black holes than isolated galaxies. The proportion varies depending on the stage of the collision: in the early, dynamic, and dust-rich phases, where activity is primarily visible in infrared light, up to six times more AGN are observed.
In later stages, as the galaxies approach complete merger and dust no longer obscures X-ray emissions, approximately twice as many active nuclei are found compared to isolated galaxies. Some seemingly “lonely” galaxies may actually be remnants of ancient collisions, leaving only subtle traces behind.
The brightest and most energetic AGN are almost exclusively found in merging galaxies. This suggests that while milder activity can be triggered by other processes, cosmic collisions are crucial – and perhaps even necessary – for the formation of the most powerful black holes in the universe.
A recently published study, currently awaiting peer review on the scientific platform arXiv, also reveals that as galaxies merge throughout cosmic history, their central black holes not only grow but also briefly flare with tremendous intensity. This energetic outburst can dramatically impact their surroundings – powerful radiation and outflows of matter can heat or expel gas needed for star formation, effectively halting star birth in the newly formed galaxy.
The new Euclid telescope provides the clearest picture yet of how the fates of galaxies and their black holes are intertwined – and how violent collisions shape the universe as we observe it today.
