Record stellar black hole: 33 solar masses

A black hole of stellar origin, dormant and massive. Three characteristics that, together, had never been seen before in our galaxy. Three characteristics that, before the arrival of the Gaia satellite and its revolutionary astrometric precision, it was unthinkable to observe together. In an article published today on Astronomy and Astrophysics Lettersinstead, the news: a black hole of 33 solar masses, dormant, just 1926 light years away from us. Similar black holes had previously been detected only through the emission of gravitational waves during the merger between pairs of black holes, and always in other galaxies.

Third in order of discovery, for the Gaia satellite, but first in order of “weight”: member of a binary system together with an old star of low metallicity belonging to the halo of our galaxy, Bh3 – or black hole 3, this is the acronym used to indicate it – it is in fact the most massive black hole of stellar origin (i.e. which was formed at the end of the life of a star) that has been discovered so far in the Milky Way. The one with 70 solar masses that we reported in 2019, announced in an article on Naturehas in fact been downsized by subsequent studies, so the 33 solar masses of Bh3 currently place it at the top of the podium.

“I never expected to find such a massive black hole, so close to our home,” he says Average Inaf Pasquale Panuzzo52 years old, originally from Reggio Calabria, Cnrs research engineer at the Observatoire de Paris and main author of the study published today on A&AL, in which numerous INAF researchers also took part. «Only Gaia Bh1 is closer. And who knows how many other “monsters” like this wander around without being detectable.”

So let’s start by describing the characteristics of this unique object and the system it inhabits. First of all, astronomers define it as “dormant”, that is, a black hole that does not provide information about itself with scenic emissions of radiation (as active black holes at the center of galaxies do, for example). In other words, a black hole that is not gaining mass from other nearby celestial bodies. Because, when this happens, an accretion disk is normally formed, where the matter heats up due to friction and in which the temperatures reached in the innermost parts of the disk (of the order of millions of degrees) and in the corona cause this become bright in far ultraviolet and X-rays.

Almost all black holes of stellar origin discovered so far are of this type, they are “active”: they are found in a binary system in which the companion star orbits close enough to the black hole to give it mass, or produces a strong stellar wind that reaches to the dark object. They are then discovered through observations with X-ray-seeing telescopes, such as the Xmm-Newton and Chandra satellites.

Above, I compare three stellar black holes in our galaxy: Gaia Bh1, Cygnus Below, in the left panel, the orbital movement on the sky of the star (blue line) and the black hole (red line) Bh3, compared with the astrometric measurements of Gaia (black dots). The dotted line indicates the position of the periastron (i.e. the point in the orbit where the black hole and the star are closest). The figures indicate the position of the star every 2 years. In the right panel, the evolution of the star’s radial velocity (blue line) compared with the measurements obtained with Gaia’s RVS instrument (black dots) and with three spectrographs on ground-based telescopes (Uves at ESO’s VLT, Hermes at Mercator telescope (Las Palmas) and Sophie at the 1.95 meter telescope at the Observatoire de Haute Provence (click to enlarge). Credits: Gaia collaboration, Panuzzo et al. (2024), A&A Letters

Dormant black holes, however, can be discovered through the phenomenon of microlensing, when the black hole passes between us and a more distant star, and as a consequence we see the brightness of the distant star increase due to the gravitational lens generated by the black hole; or, if the black hole has a companion star, they can be found by measuring the orbit of the companion around the black hole with the radial velocity technique, or by measuring its astrometry as in the case of the three black holes discovered by Gaia in our galaxy .

In the case of Bh3, therefore, the orbit of the companion star around the common center of mass is about 11.6 years. It means that, considering the 5.5 years of data already processed by the satellite, Gaia was able to map half of its orbit. Enough time to distinguish the oscillation in the position and motion of the companion star.

“The star’s orbit around Bh3 is very large, 27 milliarcseconds, compared to the precision of Gaia’s astrometric measurements (a few tenths of a milliarcsecond),” explains Panuzzo. «The fact that it was found by Gaia and not by others is due first of all to its very long orbital period, and secondly to the fact that it is a rare object, and therefore the whole sky must be observed to have the concrete possibility of find it.”

After observing it with Gaia, to confirm its nature the system was also observed with several ground-based telescopes. First of all, the spectrum was searched in the ESO archive, and observations were then carried out follow up with the Hermes spectrograph at the Mercator telescope in La Palma (Canary Islands), and with the Sophie spectrograph at the Observatoire Haute Provence in France. The radial velocities obtained with these ground-based observatories confirmed the orbital characteristics of the system.

Not only that, photometric observations have allowed us to estimate that the age of the companion star is around 11 billion years, and spectroscopic ones (coming from the Uves spectrograph of the Vlt) to state that it has a low metallicity. In other words, Bh3’s companion star is very old and formed in a metal-poor, and therefore almost pristine, environment. It would be part of the so-called Population II of stars, among the first to have formed in a universe in which many places were still “virgin”.

This feature supports one of the most accredited theories about the formation of such massive stellar black holes.

“Black holes of this mass have been observed with gravitational waves in external galaxies, but stellar evolution models cannot explain them, except by assuming that they are formed by massive stars with low metallicity,” says Panuzzo. «Our black hole is therefore the first discovered in our galaxy equivalent to the high-mass black holes observed with gravitational waves. Furthermore, the fact that it has a low metallicity star as a companion tells us that it was also formed by a low metallicity star. This discovery is therefore the first confirmation of those models that explain the high-mass black holes seen with gravitational waves as being due to stars of low metallicity.”

Pasquale Panuzzo, 52 years old, originally from Reggio Calabria, main author of the article describing the discovery of Bh3.

Finally, a peculiarity: this system seems not to be our own. Of that of the Milky Way, of course.

«An important point that we did not cover in the article, and which we will delve into as soon as possible, is the origin of this system», explains Panuzzo. «We know that it has a retrograde orbit in our galaxy (that is, it rotates in the galaxy in the opposite direction of the stars in the galactic disk), and probably belongs to an ancient globular cluster now destroyed. If this were true, it would justify some models that say that wide-orbiting black hole-star binary systems (like those found by Gaia, and like this one), are produced in clusters via a dynamic exchange process, in which the hole black “steals” a star from another binary system by passing close to it.”

A unique discovery, that of Bh3, which however leaves many question marks. So much so that these observations only define the beginning of the study of this system.

«Obviously we have something in mind follow up“, concludes Panuzzo, “but I would prefer it to be underlined that the discovery was announced to allow the entire community to do your own follow-ups. The Dpac consortium (the one that produces the catalogs of Gaia observations, ed) was made to provide Gaia data to the community, and is therefore a service mission to the community, which will use the data to do research. I am sure that on the same day of publication there will be those who will propose observations, for example, with Chandra and Xmm-Newton in X-rays, to see if the Bh3 companion does not produce a bit of stellar wind “swallowed” by the black hole”.

To know more:

Watch the video service on MediaInaf TV: