The Milky Way’s biggest stellar-mass black hole? In MY unreleased Gaia data? It’s more likely than you think!

Title: Discovery of a dormant 33 solar-mass black hole in pre-release Gaia astrometry

Authors: Gaia Collaboration (P. Panuzzo et al.)

First Author’s Institution: GEPI, Observatoire de Paris, Université PSL, CNRS, Meudon, France

Status: Accepted for publication in A&A Letters [open access]

Over the last decade, ESA’s Gaia satellite has revolutionized our understanding of the Milky Way by measuring the distances, positions, and proper motions of more than one billion stars. These measurements are based on parallaxes: tiny, periodic wobbles in the relative positions of stars caused by the Earth’s motion around the Sun. But the motion of the Earth is not the only possible source of these wobbles. 

Many of the stars that look like single points of light in the sky are actually binary systems. As the two stars orbit each other, the “single star” that Gaia sees wobbles more than a true single star would. Starting with the third data release (DR3, released in June 2022), the Gaia team has deployed the non-single star pipeline (NSSP) to derive the orbital parameters of these hidden binaries. DR3’s NSSP results increased the number of known Milky Way binaries by a whopping two orders of magnitude, and the next data release (DR4) is expected to add even more. 

Though DR4 won’t become public until the end of 2025, the Gaia team is already hard at work preparing the next generation of data. Today’s paper reports the discovery of a very unusual binary system in preliminary DR4 NSSP results: Gaia BH3, a red giant branch star with a black hole companion. Weighing in at almost 33 solar masses (M_☉), the black hole is the most massive stellar-origin black hole known in our Milky Way. Its existence is an important test for our theories of stellar evolution, which have struggled to explain how black holes can get so big.

An unexpected discovery

In a preliminary DR4 NSSP run, Gaia BH3’s wobbles clearly matched a binary model better than a single star model (see Figure 2). It also stood out as the binary with the largest mass function, which is used to constrain the mass of an unseen object from observations of its visible companion. 

Unseen companions aren’t always black holes – in many cases, they are simply smaller stars drowned out by their larger companion’s light. To determine whether a companion is truly “dark” (like a black hole), the NSSP includes both astrometric data (the wobbles mentioned earlier) and radial velocities in its model-fitting routines. If the companion is contributing light to the system, the orbit derived from astrometric data will be different than the orbit derived from the radial velocities. In the case of Gaia BH3, the two orbits were exactly the same.

Using Gaia photometry, Gaia spectra, and additional spectra from three other telescopes, the Gaia collaboration derived the physical parameters of the visible star in Gaia BH3. From the spectra, they learned that the star’s metallicity – a measure of how much of the star is made up of elements heavier than helium – is over 100 times lower than that of our Sun. By comparing the photometry to theoretical models, they also learned that the star is on the red giant branch, with a mass of around 0.76 M_☉. 

Combining this estimate with the mass function yielded a mass of 32.70 ± 0.82 M_☉ for the dark companion. Though a single black hole is the simplest explanation, the authors note that the companion could also consist of two black holes orbiting each other, or a black hole and a neutron star. Further observations will be needed to rule out these possibilities. For now, the authors assume the companion is a single black hole, which would make it the first known Milky Way black hole with a mass greater than 20 M_☉!

How do black holes get so big?

Stellar-mass black holes like the one in Gaia BH3 (not to be confused with supermassive black holes) are formed when massive stars run out of fuel and collapse under their own weight. Unless they’re actively accreting material – such as when they’re in an X-ray binary – they emit no light and are nearly impossible to observe directly. 

The few stellar-mass black holes that we have managed to observe (including Gaia BH1 and BH2, which were found in DR3!) typically have masses at or below 10 M_☉, which aligns with the predictions of stellar evolution models. While stars over 20 M_☉ exist, they should lose a significant amount of mass through stellar winds, resulting in black holes with masses below 20 M_☉. However, these models fail to explain the population of black holes detected outside of our Milky Way through gravitational wave signals, which have masses as high as 85 M_☉. How can such massive black holes form if not from the collapse of massive stars?

Astronomers have proposed two scenarios. The first involves star clusters, dense environments where smaller black holes would be more likely to merge with each other to form larger black holes. (To learn more, check out our interview with Dr. Carl Rodriguez, an expert on this topic!) The second is the collapse of massive stars with low metallicities. Metal-poor stars are expected to have weaker stellar winds and smaller radii than metal-rich stars. This means that metal-poor stars lose less mass and are less likely to merge with their binary companions, both of which are favorable conditions for producing systems like Gaia BH3. 

The mysterious origins of Gaia BH3

So how did Gaia BH3 form? The presence of a metal-poor giant in the system implies that the black hole progenitor was metal-poor as well, since stars that form together in binary systems (almost) always have the same metallicity. But the real story is a bit more complicated.

The orbital parameters derived by the NSSP show that at their closest point, the components of Gaia BH3 are about 1000 solar radii apart, which is comparable to the distance between the Sun and Jupiter. However, massive stars can easily grow larger than this during their transition from the main sequence to the red giant branch. During its red giant phase, the black hole progenitor in Gaia BH3 probably wouldn’t have fit within its current orbit – instead, it would’ve engulfed its companion, disrupting the binary before the black hole could form. 

Since we know that Gaia BH3 is a stable binary system, it’s unlikely the two components formed together. Instead, the red giant may have been captured by the black hole after it formed. This alternate method of forming binaries is common in globular clusters, where stars are so densely packed that interactions between them are frequent. The fact that Gaia BH3 seems to be located in the middle of a disrupted globular cluster lends support to this theory. However, the authors acknowledge that more work is needed before we can confidently explain Gaia BH3’s formation. 

There’s more where that came from!

Gaia BH3 might be the first exciting discovery to come out of Gaia DR4, but it certainly won’t be the last. The new-and-improved DR4 NSSP is expected to uncover dozens of binary systems with dormant black hole companions. Astronomers are already hard at work testing different theories for Gaia BH3’s formation, and each additional system we discover will be a new chance to put those theories to the test. To see just how many black holes Gaia finds, mark your calendar for DR4’s release at the end of 2025!

Astrobite edited by Ivey Davis.

Featured image credit: ESO/L. Calçada/Space Engine (spaceengine.org) (binary system); ESA/ATG medialab (satellite); eyes added by A. Masegian