Signatures of Massive Black Hole Merger Host Galaxies from Cosmological Simulations: Unique Galaxy Morphologies in Imaging

Jaeden Bardati, John J. Ruan, Daryl Haggard, and Michael Tremmel

The recent detection of ripples in space-time called gravitational waves from the merger of two small black holes was a breakthrough discovery that led to the award of the 2017 Nobel Prize in Physics. Over the next decade, gravitational wave experiments are expected to also detect the mergers of more massive black holes, such as the massive black holes at the centres of galaxies. As galaxies such as the Milky Way grow over cosmic time through mergers with other galaxies, we also expect their central massive black holes to merge due to gravity, generating gravitational waves. When we detect the gravitational waves from a massive black hole merger, identifying the host galaxy in the sky with telescopes will enable a first look at the environment around the merging black holes. However, gravitational wave detectors have poor spatial resolution, and can only localize the massive black hole merger to a large region of the sky in which there lies many galaxies: how do we identify the exact galaxy hosting the massive black hole merger detected by gravitational waves?

A recent manuscript published in The Astrophysical Journal by Jaeden Bardati (BU ‘23, now PhD student in physics at Caltech) under the supervision of Prof. John Ruan in the Department of Physics & Astronomy at Bishop’s aims to answer this question. They propose that the host galaxies of merging massive black holes may have unique shapes, which can be used to identify the exact host galaxy. To understand these unique shapes, they analyzed a cosmological simulation, in which they used supercomputers to simulate the formation and evolution of galaxies in a large volume of the Universe, based on the laws of physics. They selected galaxies in the simulation that host massive black hole mergers and generated synthetic telescope images by computing the expected light from stars in each galaxy. The figure below shows examples of synthetic images of three galaxies from the simulation hosting massive black hole mergers (top row), as well as three similar galaxies from a control sample without massive black hole merger for comparison (bottom row). They measured and studied the shapes of these galaxies using machine-learning techniques. The key finding is that galaxies with massive black hole mergers have more prominent bulges, which are the massive spherical clumps of stars at the centres of galaxies. This bulge component of galaxies is thought to be produced by galaxy mergers. Since these galaxy mergers also lead to massive black hole mergers, the prominent bulges in galaxies naturally act as signposts for massive black hole mergers detected in gravitational waves.

In the coming years, gravitational wave experiments are expected to make the first detection of a massive black hole merger at the centre of a galaxy. When this landmark discovery occurs, teams of astronomers will use telescopes to identify the host galaxy (in part through their prominent bulges based on this study) and investigate the properties of the merger. These frontier multi-messenger observations that combine the ‘cosmic messengers’ of gravitational waves and light promise to unveil the mysterious origin of massive black holes in the early Universe, and their growth over cosmic time.