Wandering black holes in Milky-Way-type galaxies

Wandering black holes, a class of black holes that are not anchored to the centres of galaxies, are a fascinating yet elusive aspect of our universe. These black holes are not formed by the typical processes that create stellar black holes, which occur when massive stars run out of fuel and collapse under their own gravity. Instead, wandering black holes are believed to have formed through more complex cosmic events, such as galaxy mergers, gravitational interactions, or even ejections after violent collisions with other black holes. These objects, although smaller than the supermassive black holes found at the centres of galaxies, still have a significant mass and can be as much as 2,000 times the mass of the Sun.

For a long time, the existence of wandering black holes has been predicted, but they remain hard to detect directly because of their small size, and because they are not located at the galactic centres where most black holes are found. However, recent advancements in computational modelling have allowed scientists to predict their behaviour and distribution within galaxies like our own, the Milky Way. By using detailed computer simulations, astronomers are beginning to understand where these wandering black holes are likely to be found and how they are distributed within galaxies. These insights are important because they provide a glimpse into the early formation processes of galaxies, as well as the complex interactions that govern the motion and growth of black holes.

In a new study, researchers ¹ used a sophisticated tool called the L-Galaxies semi-analytical model. This model simulates the evolution of galaxies over time, helping scientists to trace the formation and growth of black holes, including the wandering variety, within a galaxy’s merger history. The model was applied on top of high-resolution simulations of the large-scale structure of the universe, particularly the Millennium-II merger trees. These trees trace the merging and interactions of dark matter halos, which are the invisible scaffolding around galaxies. By focusing on the Milky Way galaxy, the researchers were able to predict that a typical galaxy like ours could host around 10 wandering black holes.

These wandering black holes would represent about 2% of the total mass of all black holes in the Milky Way, a relatively small fraction but still a significant contribution. One of the most intriguing findings of this study is that the locations of these wandering black holes are closely tied to the ways in which they formed. There are different scenarios for how wandering black holes come into existence, and these scenarios affect where the black holes end up in the galaxy.

For example, some wandering black holes are thought to have formed when smaller galaxies collided and merged with a larger one. This process, called galactic merging, can result in black holes being displaced from their original positions and sent on a “wander” through the galaxy. These black holes are typically found in the disk of the galaxy, close to the galactic centre—about 1 kiloparsec (about 3,200 light-years) away from the nucleus. In contrast, black holes that are ejected from galaxies through high-speed interactions with other black holes, known as gravitational recoils, are found much farther away—about 100 kiloparsecs (about 300,000 light-years) from the centre.

This difference in location is a key observation that helps explain why wandering black holes are so hard to detect. The black holes that are near the galactic centre might be hidden in the dense, crowded environment of the galactic disk, while those that are farther out in the galaxy are too distant to be easily observed. The extreme distances involved make it challenging to spot these wandering black holes with current observational techniques, even though they are there, silently moving through the galaxy.

Remnants of early black holes

Another important insight from this study is the composition of the wandering black hole population. The researchers found that about 67% of these wandering black holes are the remnants of what are known as “seeds”—early black holes that formed in the early universe but did not grow much afterward. These seed black holes are thought to have formed in a variety of ways, such as through the collapse of massive stars in early, metal-poor galaxies, or through the merging of dense gas clouds. In many cases, these seed black holes remained relatively small, failing to accrete enough matter to grow into supermassive black holes over time.

These leftover seed black holes are an important part of the universe’s history. They offer clues about the early stages of black hole formation, particularly during the era when galaxies were first coming together. The study suggests that these leftover seeds are more common in metal-poor galaxies, which are older and had fewer heavy elements. This indicates that the formation of wandering black holes is strongly linked to the time and environment in which they were born. The seed black holes that became wandering black holes likely come from galaxies that were smaller and poorer in metals, making them more susceptible to remaining small and not growing as much as others.

The research also examined how different factors influence the number of wandering black holes in a typical galaxy. For example, the efficiency of black hole seeding—the process by which the first black holes are created—can significantly affect the number of wandering black holes in a galaxy. If the seeding process is more efficient, more black holes will be created, increasing the likelihood that some of them will become wandering black holes. This finding highlights the complexity of black hole formation and the need to understand the various processes that govern the birth and evolution of black holes across different cosmic epochs.

Interestingly, the study also found that the exact number of wandering black holes in the Milky Way predicted by their model is consistent with earlier predictions made using a different simulation method. However, the model used in this study predicts a wider range of locations for these black holes, likely due to differences in the assumptions made about how black holes grow and interact with their environment.

Ultimately, this work underscores the importance of understanding the formation and distribution of wandering black holes in the context of galaxy evolution. The wandering black hole population is not just a curiosity, but a piece of the puzzle that helps scientists understand how galaxies and their central black holes evolve over time. Despite their elusive nature, wandering black holes are an essential aspect of the broader picture of black hole formation, offering insights into the formation of the universe’s earliest galaxies and the ongoing processes that shape the large-scale structure of our universe.

Future research will likely delve deeper into the study of wandering black holes, especially in terms of their interaction with star clusters and their role in the wider galactic environment. By refining our understanding of these mysterious objects, we can gain new insights into the fundamental processes that govern the formation of galaxies, stars, and black holes across the cosmos.

Author: César Tomé López is a science writer and the editor of Mapping Ignorance

Disclaimer: Parts of this article may have been copied verbatim or almost verbatim from the referenced research paper/s.