Searching for the relics of monstrous black holes: Astronomers have been looking with the wrong tools

In the heart of many galaxies lurk monstrous black holes with masses from a million to over 1 billion times the mass of the Sun. While the origins of black holes with masses up to a few tens of the mass of the sun are understood to be formed through the death of a massive star, how these monstrous black holes acquire these tremendous sizes has baffled astronomers for the past several decades. Do they start out tiny and feed voraciously on surrounding matter to grow to such tremendous sizes? Or do they start out big, and graze gradually on the host galaxies through cosmic time?

The clue to unraveling this puzzle lies in finding black holes that bridge the gap between the “tiny” black holes formed through star deaths to the supermassive black holes that lie in the cores of giant galaxies. As result, astronomers are actively hunting for these so-called intermediate mass black holes. Unfortunately, this is no easy task. This is because smaller mass black holes don’t exert a significant enough gravitational pull on surrounding stars that would allow us to detect the stellar movements, and the black hole’s presence, in galaxies outside our own. In order to see them, we must rely on them feeding off the surrounding gas. As black holes feed, the matter spiraling in towards the center heats up, emitting tremendous amounts of light that causes the surrounding gas to glow with unique spectral signatures that can be observed by ground-based telescopes. For over fifty decades, astronomers have been using a set of unique spectral signatures that are the fingerprints of these feeding black holes. These unique fingerprints have uncovered thousands of feeding black holes throughout the universe with masses from several tens of thousands times the mass of the sun, to over a billion times the mass of the sun. But as yet, not one single black hole has been found with a mass between about 60 times the mass of the sun to about 50,000 times the mass of the sun. The existence of this “mass desert” has remained a puzzle. Do black holes even exist in this mass range? Why can’t we see them?

An artist’s impression of an accretion disk around a black hole. Image credit: XMM-Newton / ESA / NASA.
An artist’s impression of an accretion disk around a black hole. Image credit: XMM-Newton / ESA / NASA.

In a recent paper in the Astrophysical Journal by our group, we think we know why. The gas spiraling into a medium sized black hole is much closer in than the gas spiraling around a supermassive black holes. Our group modeled for the first time the impact that this change in mass would have on the spectral signatures of the surrounding galaxy and found that, remarkably, the spectral signatures used to identify supermassive black holes don’t actually work for lower mass black holes. Our work showed that people have been searching for intermediate mass black holes using the wrong set of tools to find them, suggesting that the dearth of intermediate mass black holes may really not be due to their rarity, but to the method by which astronomers are trying to hunt for them. In fact, there may be hundreds of accreting black holes right in front of us, but we simply couldn’t see them! In another recent paper by our group, we provide powerful new spectroscopic signatures that will be the ideal tool to use to search for intermediate mass black holes. These spectral signatures will be accessible by the James Webb Space Telescope, which will hopefully discover thousands of black holes in the black hole mass desert.

“The real voyage of discovery consists not in seeing new landscapes, but in looking with new eyes.”
-Marcel Proust