An article published in the journal “Astronomy and Astrophysics Letters” reports the identification of a stellar black hole with a mass estimated to be approximately 33 times the Sun’s that was cataloged as Gaia BH3. A team of researchers used data collected by ESA’s Gaia space probe to find this black hole in the Milky Way’s halo, less than two thousand light-years from Earth. Its mass is remarkable for a stellar black hole and it has a companion, a very ancient star, as its age is estimated to be around 11 billion years.
The image (ESA/Gaia/DPAC. CC BY-SA 3.0 IGO or ESA Standard License) shows the position of the three stellar black holes discovered so far in the Milky Way, represented in projection, thanks to the Gaia mission.
The Gaia space probe is a telescope specialized for mapping the sky with billions of objects of various types. Occasionally, it allowed to find a black hole but it’s a rare event, especially when it concerns a so-called dormant object, that is, one that isn’t detected through electromagnetic emissions generated by materials around it. The presence of a companion can be crucial in finding a dormant black hole.
Among the various teams of researchers who are preparing the data collected in the Gaia mission for the publication of Data Release 4 (DR4), there is one who noticed a star about 1926 light-years from Earth whose trajectory was showing a gravitational influence. By examining the oscillation of this star, it was possible to reconstruct its movement and the influence on it by an object that must be a dormant black hole with a mass about 33 times the Sun’s.
This black hole, cataloged as Gaia BH3, and its companion move around a center of gravity in an orbit of about 11.6 years. The data collected by the Gaia space probe processed so far includes about 5.5 years of observations and this was crucial in this research.
The mass of the Gaia BH3 black hole is truly remarkable for a stellar black hole, that is, an object born from a stellar core left over after a supernova. Explaining a core that is colossal to the point of generating a black hole with 33 solar masses is very difficult. The researchers also examined its companion and obtained information that may help find answers.
Gaia BH3’s companion was also examined in follow-up observations conducted with various instruments that confirmed data obtained by the Gaia space probe on its orbit and offered new information. Its age was estimated at around 11 billion years, and this means that it’s a truly ancient star, part of the so-called Population II. Its examination indicates that it wasn’t contaminated by the explosion of Gaia BH3’s progenitor star, suggesting that these two objects became gravitationally bound only sometime after the supernova.
According to current cosmological models, very massive stars born from almost pure hydrogen, in almost the absence of elements heavier than helium, could leave very massive cores and therefore black holes like Gaia BH3. This would mean that both objects have a very ancient origin, in some globular cluster whose stars spread across the Milky Way over time.
Doubts connected to the origin of the black hole Gaia BH3 and its companion will be the subject of a future follow-up study by the team that discovered it. Other researchers could use other instruments to investigate the characteristics of this unusual black hole.
It’s important to have different studies on such objects, also to avoid problems such as the one that emerged from the discovery of the black hole cataloged as LB-1, which initially had an estimated mass of about 70 times the Sun’s. Subsequent studies significantly reduced that estimate, also based on data collected by the Gaia space probe but “only” on Data Release 2 (DR2) published on April 25, 2018. Repeating the observations increases the precision of the data collected by the Gaia space probe and that’s why each new Data Release is important for astronomers.
