Gravitational Waves After Galaxy Collisions

A new study explores whether the collision of very small galaxies could lead to the merging black holes detected in gravitational waves. Thanks to the Laser Interferometer Gravitational-wave Observatory (LIGO), we now know that black holes in our distant universe sometimes find each other in a dramatic inspiral and collision, releasing a burst of gravitational-wave emission that we can detect here on Earth.

But what happened earlier in these black holes’ lives to bring them to this point? A new study explores the possibility that LIGO’s black holes once lay at the centers of very small galaxies — until those galaxies collided. Since we discovered the first wiggles in spacetime signifying the distant merger of two black holes, LIGO has announced around ten confident detections of gravitational waves from black hole–black hole collisions — with the prospect of many more discoveries in the future.

But how did these black holes find each other? A team of scientists led by Christopher Conselice (University of Nottingham, UK) has proposed a picture that hinges on the central black holes we believe lie at the heart of most, if not all, galaxies. The team proposes that very low-mass dwarf galaxies contain central black holes of less than 100 solar masses. The mergers of pairs of these tiny galaxies ultimately lead to the inspirals and mergers of their central black holes — possibly accounting for the majority of LIGO’s detections of black hole–black hole collisions.

Conselice and collaborators have observed a relationship between galaxy mass and central black hole mass. By extrapolating this relationship to low masses, they find that ultradwarf galaxies can have central black holes of less than 100 solar masses — consistent with the LIGO-observed black holes of 10–70 solar masses.

Mergers of galaxies occurred more frequently in the early universe than they do today. Cosmological models indicate that galaxies don’t merge frequently enough today to reproduce LIGO’s observations — but at a redshift of z ~ 1.5 or higher, ultradwarf galaxies could merge often enough to match LIGO-measured gravitational-wave event rates.

Source: “Gravitational Waves After Galaxy Collisions” AAS NOVA, 21 February 2020.<https://aasnova.org/2020/02/21/gravitational-waves-after-galaxy-collisions/>

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