Black holes are massive, strange and incredibly powerful astronomical objects. Scientists know that supermassive black holes reside in the centers of most galaxies.

And they understand how certain stars form the comparatively smaller stellar mass black holes once they reach the end of their life. Understanding how the smaller stellar mass black holes could form the supermassive black holes helps astronomers learn about how the universe grows and evolves.

But there’s an open question in black hole research: What about black holes with masses in between? These are much harder to find than their stellar and supermassive peers, in size range of a few hundred to a few hundred thousand times the mass of the Sun.

We’re a team of astronomers who are searching for these in-between black holes, called intermediate black holes. In a new paper, two of us (Krystal and Karan) teamed up with a group of researchers, including postdoctoral researcher Anjali Yelikar, to look at ripples in space-time to spot a few of these elusive black holes merging.

Take me out to the (gravitational wave) ball game

To gain an intuitive idea of how scientists detect stellar mass black holes, imagine you are at a baseball game where you’re sitting directly behind a big concrete column and can’t see the diamond. Even worse, the crowd is deafeningly loud, so it is also nearly impossible to see or hear the game.

But you’re a scientist, so you take out a high-quality microphone and your computer and write a computer algorithm that can take audio data and separate the crowd’s noise from the “thunk” of a bat hitting a ball.

You start recording, and, with enough practice and updates to your hardware and software, you can begin following the game, getting a sense of when a ball is hit, what direction it goes, when it hits a glove, where runners’ feet pound into the dirt and more.

Admittedly, this is a challenging way to watch a baseball game. But unlike baseball, when observing the universe, sometimes the challenging way is all we have.

This principle of recording sound and using computer algorithms to isolate certain sound waves to determine what they are and where they are coming from is similar to how astronomers like us study gravitational waves. Gravitational waves are ripples in space-time that allow us to observe objects such as black holes.

Now imagine implementing a different sound algorithm, testing it over several innings of the game and finding a particular hit that no legal combination of bats and balls could have produced. Imagine the data was suggesting that the ball was bigger and heavier than a legal baseball could be. If our paper was about a baseball game instead of gravitational waves, that’s what we would have found.

Listening for gravitational waves

While the baseball recording setup is designed specifically to hear the sounds of a baseball game, scientists use a specialized observatory called the Laser Interferometer Gravitational-Wave Observatory, or LIGO, to observe the “sound” of two black holes merging out in the universe.

Scientists look for the gravitational waves that we can measure using LIGO, which has one of the most mind-bogglingly advanced laser and optics systems ever created.

In each event, two “parent” black holes merge into a single, more massive black hole. Using LIGO data, scientists can figure out where and how far away the merger happened, how massive the parents and resultant black holes are, which direction in the sky the merger happened and other key details.

Most of the parent black holes in merger events originally form from stars that have reached the end of their lives – these are stellar mass black holes.

The black hole mass gap

Not every dying star can create a stellar mass black hole. The ones that do are usually between about 20 to 100 times the mass of the Sun. But due to complicated nuclear physics, really massive stars explode differently and don’t leave behind any remnant, black hole or otherwise.

These physics create what we refer to as the “mass gap” in black holes. A smaller black hole likely formed from a dying star. But we know that a black hole more massive than about 60 times the size of the Sun, while not a supermassive black hole, is still too big to have formed directly from a dying star.

The exact cutoff for the mass gap is still somewhat uncertain, and many astrophysicists are working on more precise measurements. However, we are confident that the mass gaps exist and that we are in the ballpark of the boundary.

We call black holes in this gap lite intermediate mass black holes or lite IMBHs, because they are the least massive black holes that we expect to exist from sources other than stars. They are no longer considered stellar mass black holes.

Calling them “intermediate” also doesn’t quite capture why they are special. They are special because they are much harder to find, astronomers still aren’t sure what astronomical events might create them, and they fill a gap in astronomers’ knowledge of how the universe grows and evolves.

Evidence for IMBHs

In our research, we analyzed 11 black hole merger candidates from LIGO’s third observing run. These candidates were possibly gravitational wave signals that looked promising but still needed more analysis to conclusively confirm.

The data suggested that for those 11 we analyzed, their final post-merger black hole may have been in the lite IMBH range. We found five post-merger black holes that our analysis was 90% confident were lite IMBHs.

Even more critically, we found that one of the events had a parent black hole that was in the mass gap range, and two had parent black holes above the mass gap range. Since we know these black holes can’t come from stars directly, this finding suggests that the universe has some other way of creating black holes this massive.

A parent black hole this massive may already be the product of two other black holes that merged in the past, so observing more IMBHs can help us understand how often black holes are able to “find” each other and merge out in the universe.

LIGO is in the end stages of its fourth observing run. Since this work used data from the third observing run, we are excited to apply our analysis to this new dataset. We expect to continue to search for lite IMBHs, and with this new data we will improve our understanding of how to more confidently “hear” these signals from more massive black holes above all the noise.

We hope this work not only strengthens the case for lite IMBHs in general but helps shed more light on how they are formed.

Bill Smith receives funding from an NSF Research Trainee Grant called EMIT.

Karan Jani is a member of the LIGO Scientific Collaboration.

Krystal Ruiz-Rocha receives funding from an NSF research grant called EMIT.