Massive Black Holes Maybe Merging On Edges Of Milky Way-Type Spiral Galaxies

As scientists improve their understanding of the fundamental aspects of the universe, the detection of gravitational waves, which are ripples in the fabric of space-time, has been a big jump in the field of astronomy. And while only a handful of them have been detected so far, a new paper suggests a lot more could be found in a previously unsuspected location.

The fringes of spiral galaxies like our very own Milky Way could be teeming with massive colliding black holes, the upcoming paper says. The merger of black holes is what creates gravitational waves, and these orbiting pairs of black holes (binary black hole systems) are thought to exist in regions of space that are sparsely populated. That means a relatively small number of stars, the area fee of heavy elements like iron, gold, and platinum which are produced in supernova explosions, and low-intensity winds that don’t affect the gigantic stars.

These conditions typically exist in dwarf or satellite galaxies, and the paper — accepted for publication in the Astrophysical Journal Letters — posits they exist on the outskirts of spiral galaxies too. The one big difference is that the much bigger and more massive spiral galaxies are significantly easier to find.

Sukanya Chakrabarti, assistant professor of physics at Rochester Institute of Technology and lead author of the paper said in a statement Monday: “The metal content in the outer disks of spiral galaxies is also quite low and should be rife with black holes in this large area.”

Richard O’Shaughnessy, assistant professor of mathematical sciences at RIT and a co-author of the paper, explained further: “This study shows that, when predicting or interpreting observations of black holes, we need to account not only for differences between different types of galaxies but also the range of environments that occur inside of them.”

O’Shaughnessy is also a member of the LIGO Scientific Collaboration which has been responsible for detecting all the five gravitational wave events observed so far, including a recent one which was caused by the merger of binary neutron stars.

If scientists could observe the host galaxies of binary black holes, it could help them understand how these pairs of black holes begin to orbit each other, before their eventual merger. Existing research has already shown that black holes — so-called because their strong gravity doesn’t let even light escape — do, in fact, have an optical component of radiation, due to the remnants of matter from the collapse of the stars that led to their formation.

“If you can see the light from a black-hole merger, you can pinpoint where it is in the sky. Then you can infer the parameters that drive the life cycle of the universe as a whole and that’s the holy grail for cosmology. The reason this is important is that gravitational waves give you a completely independent way of doing it so it doesn’t rely on astrophysical approximations,” Chakrabarti said.

The paper, titled “The Contribution Of Outer HI Disks To The Merging Binary Black Hole Population,” is available on the preprint server arXiv.