Galactic Gravity’s Secret Role in Black Hole Collisions

Galactic gravity can dramatically impact wide binary stars, pushing them towards unexpected mergers or collisions.

The detection of gravitational waves from merging black holes has sparked a fascinating question: how do these black holes get close enough to collide? Researchers at the Max Planck Institute for Astrophysics (MPA) suggest that some of these black holes may have started their journey as massive stars orbiting each other at immense distances — 1,000 to 10,000 times the Earth-Sun distance. After these stars complete their life cycles and collapse into black holes, the gravitational forces of their host galaxy can gradually distort their orbits. Over time, this process can bring the black holes closer together, ultimately leading to their merger.

Binary Star Systems

Many stars in the universe don’t exist in isolation. Unlike our Sun, many have a stellar companion, forming what is known as a binary system. The distance between these binary stars plays a crucial role in their evolution. Stars in very close orbits often exchange mass, leading to complex and dynamic interactions.

For massive stars, these interactions can result in the formation of binary black holes that may eventually merge due to energy loss through gravitational waves. In contrast, binary stars with wider separations were traditionally thought to evolve quietly, behaving like single stars and producing binary black holes too far apart to ever merge.

Rethinking Wide Binary Star Evolution

However, a recent study published in The Astrophysical Journal Letters challenges this conventional understanding. Led by Jakob Stegmann, a research fellow at the Max Planck Institute for Astrophysics (MPA), the study highlights how this view changes when the binary stars are placed in the context of a galactic environment.

Wide binaries — those separated by more than 1,000 times the Earth-Sun distance — are susceptible to disturbances from the gravitational forces of their host galaxy and close encounters with passing stars. When these galactic influences are factored in, the study reveals that wide binaries can experience unexpected and dramatic interactions, impacting both stars and their compact remnants.

Galactic Influence on Binary Black Holes

These interactions are a consequence of the extremely low binding energy that holds very wide binary black holes together. Thus, the gravitational pull of the entire host galaxy can slowly deform the shape of the orbit on which the two black holes move around each other and make it more and more elongated. On these highly elliptical orbits the two black holes remain widely separated for most of the time, but pass close to each other once per orbit (see animation).

This leads to a counterintuitive result: In order to bring two black holes closer than a few kilometers so that they can merge, we could nevertheless start with a wide separation of more than 1,000 times the distance between Earth and Sun. The clue lies in the ellipticity of their orbit which slowly grows due to the disturbing effect of the galaxy’s gravity.

Implications for Low-Mass Star Collisions

This mechanism of driving two black holes closer together could also be relevant for the evolution of wide low-mass binary stars. Recently, researchers at MPIA in Heidelberg have searched for wide binaries in the data from the ESA-led mission Gaia. Surprisingly, they found that about ten percent of all low-mass stars possess a distant stellar companion.

While systems like those are not massive enough to develop black holes, in this case, the MPA study shows that the gravity of the galaxy could drive the stars to a head-on collision. These collisions would not lead to detectable emission of gravitational waves, but could be visible as energetic flares, so-called Luminous Red Novae.

Advancements in Binary Star Research

The results of this study represent progress in investigating the plethora of evolutionary pathways of binary stars and their compact remnants. While previous work on wide binaries has mostly focused on ruling out the existence of a distant companion to our Sun (referred to as the “Nemesis hypothesis”), on the one hand, and understanding the upper limit of their separation to remain bound, on the other hand, little attention has been paid to studying the interactions between wide binary stars. With future data releases of Gaia expanding the catalog of wide binary stars at an unprecedented rate, the MPA study makes an important step toward understanding their co-evolution with the Milky Way.

Investigating their dynamics in detail allows us to understand how systems previously thought uneventful could in fact lead to some of the most energetic transients in the Universe.

Reference: “Close Encounters of Wide Binaries Induced by the Galactic Tide: Implications for Stellar Mergers and Gravitational-wave Sources” by Jakob Stegmann, Alejandro Vigna-Gómez, Antti Rantala, Tom Wagg, Lorenz Zwick, Mathieu Renzo, Lieke A. C. van Son, Selma E. de Mink and Simon D. M. White, 2 September 2024, The Astrophysical Journal Letters.
DOI: 10.3847/2041-8213/ad70bb

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