Astronomers from the University of California, Berkeley have discovered a unique phenomenon in the universe – not one, but two massive black holes simultaneously devouring stars within the same galaxy. This extraordinary discovery sheds new light on the evolution of galaxies and the interaction of their central supermassive black holes, which may play a key role in understanding future gravitational waves.

An unusual discovery: a black hole outside the galactic center

Until now, astronomers have primarily found massive black holes destroying stars within the dense centers of large galaxies. However, a new study from Berkeley presents an exception – a black hole with a mass of about one million suns, located outside the center of the galaxy, in its central bulge, about 2,600 light-years from the center. This black hole was discovered thanks to an extremely rare event known as a tidal disruption event (TDE), during which a black hole tears apart a star with powerful gravitational forces that stretch the star into a "spaghetti" shape, creating intense bursts of light.

Usually, massive black holes are found precisely at the center of galaxies, like our Milky Way and its central object Sagittarius A*, which weighs about 4 million solar masses. This new black hole, however, is a “wanderer” in the central part of the galaxy, and thus represents the first optically detected case of a TDE outside the galactic nucleus.

Two massive devourers – a possibility of a future merger

The galaxy in which this remarkable black hole was discovered also contains its own central supermassive black hole with a mass of about 100 million solar masses, which is also currently consuming surrounding gas. Such situations are not unknown to scientists, as large galaxies often merge during cosmic collisions, which can lead to the simultaneous existence of two or more massive black holes within a single galaxy. Over time, these black holes can approach each other and merge into an even more massive object, and such events create gravitational waves that can be detected by special instruments.

How did they discover this rarity?

The new black hole and its TDE event, designated AT2024tvd, were observed using the Zwicky Transient Facility (ZTF) – an optical telescope at the Palomar Observatory in California. Confirmation came from additional studies using X-rays and radio waves, as well as observations with the Hubble Space Telescope. These methods allowed for the precise location of the light flash's source and confirmed that the event did not occur in the very center of the galaxy, but on its outskirts.

Such “wandering” objects and their TDE events are very rare because astronomers estimate that, on average, a massive black hole destroys a star about once every 30,000 years. This is precisely why this discovery is so significant – it opens up the possibility of finding many other similar “wandering” black holes that were formed during the history of galaxy mergers.

What does this mean for the future of astronomy?

The discovery of such black holes that are not located in the center of a galaxy has major implications for the development of theories about the evolution of galaxies and the behavior of their central objects. There are two main theories about the origin of this “wandering” black hole. The first suggests that it is the remnant of a small galaxy that merged with a larger one long ago, and the black hole is still orbiting within it. The second, more intriguing theory, proposes that this black hole was once part of a triple black hole system at the galaxy's center, but was ejected onto this distant path due to gravitational interactions.

Erica Hammerstein, one of the researchers, studied the Hubble images in detail and found no clear traces of a past galaxy merger, which further fuels discussions about the real cause of the black hole's position.

The search for TDE events and their significance

The ZTF system is of key importance in the search for such rare light explosions. This telescope, although primarily intended for discovering supernovae, has proven to be extremely effective in detecting TDEs as well. To date, ZTF has identified nearly one hundred such events, and almost all of them are located in the center of galaxies. The exception, AT2024tvd, significantly changes the paradigm, as it opens the door for researching other extraordinary cases.

TDE events are invaluable to astronomical science because they allow us to understand the physics of material accretion around black holes and reveal the conditions under which black holes can create powerful jets and winds.

Laser Interferometer Space Antenna (LISA) – a look into the future

The discovery of such black holes and their interactions will be of particular importance for the upcoming LISA space mission, which is expected to begin hunting for gravitational waves from the mergers of supermassive black holes in the next decade. While existing detectors like LIGO and Virgo track smaller black holes and neutron stars, LISA will focus on objects with masses of millions of suns, like those in the AT2024tvd galaxy.

As LISA becomes operational, astronomers will be able to track the mergers of such massive objects in more detail and better understand the dynamics of galactic nuclei, as well as the evolution of the largest structures in the universe.

The importance of systematic searches for unusual black holes

Yuhan Yao, the lead researcher, points out that this is the first time such a TDE has been discovered outside the center of a galaxy and that the discovery encourages scientists to consider how common such events are and how many “wandering” black holes actually exist within galaxies.

Previous searches have typically concentrated on galactic centers, but new findings suggest that future surveys should broaden their focus to find more of these rare phenomena. Also, developing algorithms that distinguish the light flashes of supernovae from TDE events allows for more precise and faster discoveries.

Global collaboration and future challenges

The research involved 34 co-authors from various international institutions, highlighting how interdisciplinary and internationally connected this field of astronomy is. The ZTF partnership and the support of the U.S. National Science Foundation made this pioneering analysis possible and revealed a new dimension in the study of cosmic mysteries.

As science advances, the search for “wandering” black holes and the monitoring of their TDEs will become one of the key tasks of modern astrophysics, and understanding them will help us uncover the hidden processes that shape our cosmic environment.

Source: UC Berkeley