Mysterious black holes that defy conventional understanding may contain enigmatic clues about the origins and dynamics of the early universe
At the heart of our galaxy lies an insatiable devourer—a colossal black hole that engulfs everything in its vicinity, much like the mythical Kammapa of the Sotho people in southern Africa. This supermassive black hole in the Milky Way steadily grows more massive as it consumes anything within its reach. Astonishingly, this phenomenon is not unique to our galaxy. Almost all known massive galaxies harbor black holes of immense proportions, weighing thousands, millions, or even billions of times the mass of our Sun.
For a long time, scientists believed that only massive galaxies possessed the necessary resources to satiate these monstrous entities. However, around two decades ago, computer simulations of early black holes yielded peculiar findings—unexpectedly large black holes in unconventional locations. Initially dismissed as mere anomalies, some scientists pondered whether these oddities held untapped insights into the universe's infancy and adolescence, should they be found in the nearby cosmos.
The notion remained theoretical until recently when the existence of these atypical black holes became increasingly challenging to ignore. Astronomers have uncovered evidence of remarkably massive black holes in the smallest galaxies, and intriguingly, some of these black holes do not reside at the centers of their galaxies. Furthermore, rare instances of black holes wandering at the edges of their galaxies or being ejected into intergalactic space have been observed.
These unconventional black holes may not be cosmic outliers but rather integral to unraveling the story of our universe. If that is the case, they offer a means to explore one of astrophysics' greatest enigmas—how the supermassive black holes we observe today came into existence.
"To comprehend the evolution of galaxies, one must first understand what black holes are up to," explains Xiaohui Fan, a cosmologist at the University of Arizona in Tucson. Without this understanding, it remains impossible to explain the cosmic landscape.
According to our current understanding, as galaxies grow, collide, and merge throughout cosmic history, they accumulate vast amounts of stars, gas, and dust. Black holes at their centers expand in tandem, growing as they merge with one another and feed on the newly acquired material. Rough estimates suggest that a supermassive black hole weighs around one-thousandth the mass of its host galaxy.
In this framework, dwarf galaxies—the tiniest galaxies in the universe—likely underwent minimal mergers in the past. Weighing just a fraction of the Milky Way's mass, they were expected to harbor relatively small black holes or none at all.
However, in the late 2000s, astrophysicist Marta Volonteri of Institut d'Astrophysique de Paris at Sorbonne University conducted computer simulations that traced the evolution of massive black holes from their birth to the present day. Surprisingly, even the smallest galaxies could harbor significantly large black holes from their inception. As time progressed, some of these galaxies remained unchanged, devoid of growth or mergers, preserving their unaltered state over billions of years of cosmic evolution.