Most people are familiar with the concept of "love at first sight," but Northwestern University astrophysicists have discovered a phenomenon they describe as "love at first collision." Their study reveals that binary black holes, when formed in dense star clusters, tend to align their spins before merging.
Previous research suggested that the spins of these black holes should be chaotic and random. However, the new findings indicate that binary black holes often spin in alignment with their orbital motion. This behavior is attributed to collisions with massive stars within stellar clusters.
The study, which has been accepted for publication by The Astrophysical Journal Letters, describes how black holes consume debris from torn-apart stars. This process increases their mass and gravitational pull, causing their spins to realign in harmony with their orbit. As the black holes move closer together, this alignment becomes more pronounced.
Fulya Kıroğlu, a graduate student at Northwestern's Weinberg College of Arts and Sciences, led the study. She explained that when a massive star is disrupted by a binary black hole, it creates two streams of debris spiraling around each black hole. Initially random, these debris clouds gradually realign due to tidal forces as the black holes approach each other. Over time, this leads to mergers with aligned spins—a phenomenon observed in gravitational wave signals from merging binary black holes detected by LIGO/Virgo.
Kıroğlu's advisor is Professor Fred Rasio at Northwestern University. Both are part of the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA). The research also involved collaborators from Allegheny College and the University of California, San Diego.