Merging Black Holes vs. Gas and Stars

When galaxies merge, what happens to the massive black holes at their centers? Today’s article explores the math behind the merger.

Hubble image of two spiral galaxies in the process of merging

When galaxies merge, it shakes up star formation and sets the stage for a massive black hole merger. [NASA, ESA, the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration, and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University)]

An Emerging Question 

Two galaxies, adrift in the universe, pass near one another. If they become gravitationally entangled, the billion-year process of merging begins as they gradually coalesce into a single galaxy. As part of this process, the massive black holes at the centers of the colliding galaxies undergo a merger of their own.

As these massive black holes begin their death spiral, they encounter other galactic material like stars and gas. While simulations have shown that interacting with nearby stars causes the black-hole binary to spiral inward more quickly, the results aren’t as clear when it comes to gaseous material. Some studies have found that the presence of gas hastens the merger, while others suggest that it delays the merger instead.

The rate at which massive black holes merge has implications for upcoming gravitational-wave observatories, like the Laser Interferometer Space Antenna (LISA). Massive black-hole mergers at the centers of colliding galaxies are expected to be the loudest source of low-frequency gravitational waves in upcoming surveys — but if some process prevents these mergers, there may be nothing to listen to.

binary black hole merger

Artist’s illustration of the merger of two black holes in space. [LIGO/T Pyle]

Black Holes on Paper

Elisa Bortolas (University of Milano-Bicocca, Italy) and collaborators used a mathematical model of a black-hole merger to understand how interactions with stars and the presence of gas affect the inspiraling of the binary. Unlike most previous work, the set of differential equations developed by Bortolas and coauthors allowed for the effects of stars and gas to be considered simultaneously rather than separately.

The authors find that stars and gas tend to compete with one another as the black holes merge. If the black-hole pair accretes only a little mass from the surrounding material, gravitational interactions with nearby stars cause the black-hole pair to tighten inward. If the accretion rate is higher, the presence of a gaseous disk works to expand the binary pair, delaying the merger. Eventually, though, the stars win out, and the binary pair draws close enough to shed massive amounts of energy in the form of gravitational waves, sending the black holes on a collision course.

Conceptual artwork of a flat-topped circular spacecraft with two red laser-beam arms against a starry background

The LISA mission, planned for the 2030s, aims to to detect gravitational waves from space using its extremely long (2.5-million-km) “arms.” [LISA Consortium]

Looking Ahead to Future Detections

The results from Bortolas and coauthors showed that while the presence of gas can delay a merger, it won’t prevent it altogether. Under the conditions the authors explored, the presence of gas increased the time to the merger by a factor of a few, but all mergers occurred within a few hundred million years.

This is good news for LISA and other gravitational-wave detectors, and there are implications for the non-gravitational-wave detections of these events as well; the presence of gas in the black holes’ surroundings seems to make them pause with just a few light-years between them, increasing the chance that a survey might detect them in this phase.


“The Competing Effect of Gas and Stars in the Evolution of Massive Black Hole Binaries,” Elisa Bortolas et al 2021 ApJL 918 L15. doi:10.3847/2041-8213/ac1c0c