Hubble may have discovered the closest known intermediate-mass black hole

We remember that in 2020 the Swedish Academy awarded the Nobel Prize in Physics to Andrea Ghez and Reinhard Genzel ” for the discovery of a supermassive compact object at the center of our Galaxy “. The exact formulation is important, because if the majority of astrophysicists think that this object is indeed a supermassive black hole, which has not been denied, quite the contrary, by the observations of the collaboration Event Horizon Telescope, we do not yet have a certain proof, but very close. More generally, it is reasonable to think that this proof will come from the study of gravitational waves.

The two researchers and the teams of their collaborators arrived at the discovery of this compact object by studying for more than a decade the movements of stars near the center of the Milky Way, orbiting around a mysterious source of radio waves. named Sgr A*. This source therefore happens to be associated with a distribution of compact mass in a volume which, according to the equations of general relativity, makes it a black hole of just over 4 million solar masses. Indeed, this object does not radiate by itself and the electromagnetic waves that we detect come from the matter surrounding this compact star.

This simulation shows the orbits of a small group of stars located near the supermassive black hole at the center of the Milky Way. During 2018, one of these stars, called S2, passed very close to the black hole and was the subject of an intense campaign of observations using ESO telescopes. Its behavior was consistent with the predictions of Einstein’s theory of general relativity – however incompatible with Newton’s theory of gravitation. © ESO, L. Calçada,

A universal law for the size of black holes?

In fact, it has been believed since the late 1960s, and observations have only reinforced this thesis, that there exist at the heart of all large galaxies supermassive black holes containing at least a million solar masses. We also note that the mass of the detected black holes is very often proportional to the mass of the host galaxy.

We also know that in the Milky Way there are many black holes of about ten solar masses each, most often. Some researchers have therefore wondered whether the law of proportionality observed in large galaxies could not also be valid for dwarf galaxies and also globular clusters.

If so, there should be so-called intermediate-mass black holes containing roughly between a hundred solar masses and a million solar masses. We are tracking these objects and we know a few candidates outside our Galaxy like 3XMM J215022.4−055108, which the Hubble telescope discovered in 2020, and HLX-1, identified in 2009. They reside in dense star clusters at the periphery of other galaxies. Each of these possible black holes has the mass of tens of thousands of suns and some of the clusters, where they are observed, were possibly the nuclei of now extinct dwarf galaxies, dislocated by the tidal forces of galaxies from which they are passed too close.

Today, as an article published in Monthly Notices of the Royal Astronomical Society by a team of researchers led by astronomer Eduardo Vitral from Space Telescope Science Institute from Baltimore, Maryland in the United States, it is still the Hubble telescope which delivers observations suggesting that intermediate black holes do indeed exist. Above all, for the first time, these observations concern the Milky Way since they relate to the movements of the stars in the globular cluster Messier 4 (M4) which is only about 7,000 light-years from the Solar System.

This video begins with a wide view of the spectacular central parts of the Milky Way. We are approaching the constellation of Scorpius. Near its nearest star, Antares, is the globular cluster Messier 4, one of those star systems closest to Earth. The detailed images of the cluster at the end of this video are from the WFI camera on the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory in Chile, with a close-up image of the area to finish power plant produced by the NASA/ESA Hubble Space Telescope. © ESO-ESA-Nasa, Digitized Sky Survey 2, Nick Risinger (

Closest globular cluster to the Sun

Still called NGC 6121 and located in the constellation Scorpio, M4 is in fact the closest globular cluster to the Sun. It was discovered by the Swiss astronomer Jean Philippe Loys de Cheseaux in 1746. Eduardo Vitral and his collaborators now suggest that it contains a compact body containing around 800 solar masses. Again, given the movement of the stars around it, it must be a black hole and in this case an intermediate-mass black hole.

After more than 12 years of observations, astrophysicists have reached this conclusion by eliminating other alternative hypotheses for a compact star, such as a concentration of neutron stars or stellar black holes produced by the collapse of stars. The high resolution for the images taken by Hubble was necessary to accomplish this feat by making it possible to observe point stars in orbit and to make precise measurements of their positions and velocities while the cluster contains a large number of stars in a volume with a radius of only 35 light years.

In a statement from NASA, Eduardo Vitral specifies on this subject: “ We are convinced that we have a very small region with a lot of concentrated mass. It is about three times smaller than the densest black mass we have found before in other globular clusters. The region is more compact than we can reproduce with numerical simulations when we consider a collection of segregated black holes, neutron stars and white dwarfs at the center of the cluster. They are not able to form such a compact concentration of mass. »

The same press release also states: A grouping of closely related objects would be dynamically unstable. If the object is not a single intermediate-mass black hole, it would take about 40 smaller black holes crammed into just a tenth of a light-year space to produce the observed stellar motions. The consequences are that they would merge and/or be ejected in an interstellar pinball game. »

A presentation of the discovery made with Hubble concerning M4. To obtain a fairly accurate French translation, click on the white rectangle at the bottom right. The English subtitles should then appear. Then click on the nut to the right of the rectangle, then on “Subtitles” and finally on “Translate automatically”. Choose “French”. © Nasa’s Goddard Space Flight Center; Lead Producer: Paul Morris; Computer Representation of the Stellar Motions in the Core of M4: Mattia Libralato (AURA/STScI for ESA)

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