Giving Justice to Intermediate Mass Black Hole Mergers!

Title: Intermediate Mass Ratio Inspirals in Milky Way Galaxies

Authors: Jillian Bellovary, Yuantong Luo, Thomas Quinn, Ferah Munshi, Michael Tremmel, James Wadsley

First Author’s Institution: Department of Physics, Queensborough Community College, 222-05 56th Ave, Bayside, NY 11364, USA

Status: Submitted to the Astrophysical Journal [open access]

Galaxies like our Milky Way are not just serene places hosting stars, gas, and dark matter. They also undergo a host of violent activities. Evidence strongly suggests that our Milky Way interacts with nearby dwarf galaxies, pulling them in entirely or tearing them apart through tidal forces. Dwarf galaxies, though small, often host black holes with masses between 10³ – 10⁵ M_☉ (Intermediate mass black holes (IMBH)). When these dwarfs fall into the Milky Way’s gravitational pull, tidal forces strip them of stars and gas, leaving their black holes to roam the galaxy halo. Some of these “wandering” black holes spiral toward the central supermassive black hole (SMBH). This leads to an event called inspiral, where black holes of differing masses slowly spiral together before merging.

Mergers between an IMBH and an SMBH are called Intermediate Mass Ratio Inspirals or IMRIs. They represent an intermediate case between major mergers involving equal-mass black holes and extreme mass ratio inspirals (EMRIs), where a stellar-mass black hole merges with a supermassive black hole. IMRIs are expected to generate gravitational waves that are detectable by the next-generation space-based gravitational wave detector, Laser Interferometer Space Antenna (LISA). However, their waveforms remain challenging to model as the origins and development of these IMRIs are not fully understood.

Tracking IMRIs with Simulations

In today’s paper, the authors use high-resolution simulations to study how IMRIs form and evolve, offering critical insights for the upcoming LISA mission. They used the DC Justice League simulation suite (The individual simulations are all named after women who have served on the U.S. Supreme Court: Sandra, Ruth, Sonia, and Elena!) to model four Milky Way-like galaxies at high resolution to trace the origins of IMRIs. These simulations track the formation and evolution of massive black holes in dwarf galaxies that eventually merge into larger galactic systems.

What Did the Simulations Reveal?

The study uncovered key features of IMRIs in Milky Way-like galaxies:  

• Prevalence: About half of all massive black hole mergers were IMRIs, emphasizing their importance in galaxy evolution.  
• Timing of Mergers: Most IMRI events occur early in the universe, approximately 3 billion years after the Big Bang, when galaxy mergers and interactions were more frequent (Figure 1).

• Inspiral Timescales: The duration of the inspiral process depends heavily on the compactness of the dwarf galaxy. Dense, compact dwarfs have faster inspiral time, whereas diffuse dwarfs slow down the process. A more compact galaxy can plunge deeper into the galaxy before being disrupted, resulting in a massive black hole that is closer to the center and will inspiral more quickly (Figure 2).

• Orbital Evolution: While some IMRIs become more circular over time, others maintain eccentric orbits until the merger.  

Why Does This Matter?

IMRIs offer a unique opportunity to study black hole demographics and galaxy assembly. The mass ratios and orbital eccentricities of these events are sensitive to the early conditions of black hole formation and the dynamics of galaxy mergers. However, IMRIs present challenges for gravitational wave detection. Unlike major mergers or EMRIs, their waveforms cannot be easily modeled using existing methods. The authors stress the need for a hybrid approach that combines post-Newtonian (commonly used for major mergers) and perturbative techniques (widely used for EMRIs) to simulate IMRI signals effectively. 

The study also highlights the importance of preparing for LISA, slated to launch in the 2030s. As a space-based gravitational wave observatory, LISA will be sensitive to low-frequency waves produced by IMRIs at unprecedented distances. Accurate waveform models are essential for detecting these signals and extracting their astrophysical information. LISA’s detections of IMRIs could constrain the masses of black holes in dwarf galaxies, shed light on SMBH seed formation mechanisms, and enhance our understanding of galaxy evolution across cosmic time.  

Looking ahead!

While the authors of today’s paper provide a detailed glimpse into the dynamics of IMRIs, they also highlight limitations. The small sample size and simplified black hole merger models call for broader studies using more comprehensive simulations. Future work must refine the physics of inspirals and develop robust waveform libraries to maximize LISA’s scientific return.

IMRIs are not just a niche class of black hole mergers; they are a treasure trove of information about the cosmic past. With LISA on the horizon, our ability to unlock these secrets is closer than ever! 

Astrobite edited by Tori Bonidie 

Featured image credit:  LIGO/A. Simonnet