For the first time, this type of black hole has been discovered in the Milky Way?

An intermediate-mass black hole lurks at the center of the Omega Centauri globular cluster. Image: ESA/Hubble & NASA, M. Häberle (MPIA)

What is a black hole? In astrophysics, it is considered to be a dense celestial body whose gravity is so strong that even light cannot escape once it crosses its boundary. In the past few decades of observation, astronomers have observed two types of black holes: one is a stellar black hole with a mass between 5 and 150 times the mass of the sun. This type of black hole is very common, with about hundreds of millions in the Milky Way alone; the other is a supermassive black hole with a mass of hundreds of thousands to billions of times the mass of the sun, which is located at the center of the galaxy.

Astronomers have long suspected that there should be intermediate-mass black holes between these two types of black holes. However, finding such black holes has proven to be extremely difficult, and only a few intermediate-mass black hole candidates have been discovered in the past, but they have always been controversial. On July 10, 2024, in a study published in the journal Nature, astronomers announced that they had found conclusive evidence of the existence of an intermediate-mass black hole in the center of the Omega Centauri globular cluster.

So why is it so difficult to find intermediate-mass black holes?

First of all, we need to know that black holes themselves do not emit light, so we cannot see them directly. But astronomers have found several ways to indirectly observe their existence:

The first method is to detect the light emitted by matter around a black hole: There are many binary star systems consisting of black holes and stars in the universe. If they are close enough, the strong gravity of the black hole will attract the gas on the star. These gases will not fall directly into the black hole, but will form an accretion disk around the black hole. The gas in the accretion disk heats up as it rotates inward, so it will emit detectable X-rays.

The second method is to capture gravitational waves: when two black holes approach each other and merge, part of their mass will be converted into gravitational waves and radiate outward. By analyzing the received gravitational wave signals, the mass of the black holes before the merger can be inferred.

The third method is to track stars: Since the gravity of a black hole is very strong, it will seriously affect the trajectory of nearby stars. If we can observe some stars with strange behavior orbiting an invisible point, it is very likely a black hole. Astronomers used this method to determine that there is a supermassive black hole with a mass of more than 4 million times the mass of the sun at the center of the Milky Way.

The fourth method is photography: Scientists can image the supermassive black hole at the center of the galaxy by connecting radio telescopes scattered around the world. We will see a bright ring structure surrounding a dark central area as predicted.

In theory, these methods are all suitable for finding medium-mass black holes. However, problems arise in actual observations. For example, the most likely place to hide such black holes is in the center of globular clusters, but the matter density of most star clusters is not actually high, so the black holes have no gas to interact with, and therefore will not emit easily detectable X-ray signals. For another example, the merger of two medium-mass black holes will emit low-frequency gravitational wave signals, but current gravitational wave detectors are not sensitive to low-frequency gravitational waves.

In this study, the method used by astronomers to track stars was to focus on the Omega Centauri globular cluster, which is 17,000 light-years away from us.

Omega Centauri contains about 10 million stars and is the largest known globular cluster in the Milky Way. Astronomers generally believe that it is the core of an ancient dwarf galaxy that was swallowed by the Milky Way. During the annexation, the dwarf galaxy lost all the stars except those in the core. The black hole at the center will also be "frozen in time", which means that it will not be able to continue to grow and will remain the same size as when Omega Centauri was swallowed by the Milky Way. In order to verify this hypothesis, it is necessary to detect the black hole at the center of Omega Centauri.

In the latest study, the researchers realized that if they could identify the expected fast-moving stars around the central black hole of the Omega Centauri globular cluster, it would be a smoking gun. However, creating a huge catalog for the movement of stars is not an easy task. The researchers measured the speeds of 1.4 million stars located in the cluster by studying more than 500 images of the Omega Centauri globular cluster taken by the Hubble Space Telescope. In the end, they were lucky enough to find seven stars in a small area in the center of the Omega Centauri globular cluster that were moving at escape velocities exceeding that of Omega Centauri. If there were no central black hole exerting additional gravity, these stars should have escaped from the center of the cluster.

Using the most reliable measurements of the five fastest-moving stars, the researchers calculated that there should be a black hole with a mass of at least 8,200 times that of the sun, which is an intermediate-mass black hole.

An intermediate-mass black hole lurks at the center of the Omega Centauri globular cluster. Image: ESA/Hubble & NASA, M. Häberle (MPIA)

Of course, some people have questioned this, believing that there should be a group of much smaller black holes there, rather than an intermediate-mass black hole. This question highlights the complexity and challenge of studying dense stellar environments. Therefore, more observations are needed in the future to prove whether there is really an intermediate-mass black hole at the center of the Centauri Omega globular cluster.

So why do we look for this type of black hole? The stellar black holes we just mentioned are ubiquitous in the universe. These black holes are usually formed when massive stars exhaust all their fuel and their cores eventually collapse under the influence of gravity. Of course, not all stellar black holes are produced in this way. Some very massive stars will collapse directly and convert all their mass into black holes; some black holes are the final product of the merger of two neutron stars. However, what astronomers have not been able to confirm is how the supermassive black holes at the center of galaxies are formed.

According to the current picture of galaxy evolution, the earliest galaxies should have medium-sized central black holes, and as these galaxies evolve, the black holes will grow over time, swallowing up smaller galaxies around them (like the Milky Way did), or merging with larger galaxies. In other words, intermediate-mass black holes may provide the first seeds for the growth of supermassive black holes, so finding intermediate-mass black holes is expected to help solve the mystery of the formation of supermassive black holes.

This article is a work supported by the Science Popularization China Creation Cultivation Program. Author: Principle

Reviewer: Han Wenbiao, researcher at Shanghai Astronomical Observatory, Chinese Academy of Sciences

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.