In 1916, Karl Schwarzchild theorized the existence of black holes as a resolution of Einstein’s field equations for his theory of general relativity. In the mid-twentieth century, astronomers began to detect black holes for the first time using indirect methods, which involved observing their effects on surrounding objects and space. Since the 1980s, scientists have studied supermassive black holes (SMBHs), which reside at the center of most massive galaxies in the Universe. And in April 2019, the Event Horizon Telescope (EHT) collaboration released the first ever image of an SMBH.
These observations provide an opportunity to test the laws of physics under the most extreme conditions and provide insight into the forces that have shaped the Universe. According to a recent study, an international research team relied on ESA data Gaia Observatory to observe a Sun-like star with strange orbital characteristics. Due to the nature of its orbit, the team concluded that it must be part of a black hole binary system. This makes it the closest black hole to our solar system and implies the existence of a large population of dormant black holes in our galaxy.
The research was led by Kareem El-Badry, a Harvard Society Fellow astrophysicist from the Harvard-Smithsonian Center for Astrophysics (CfA) and the Max Planck Institute for Astronomy (MPIA). He was joined by researchers from CfA, MPIA, Caltech, UC Berkely, Flatiron Institute’s Center for Computational Astrophysics (CCA), Weizmann Institute of Science, Paris Observatory, Kavli Institute of astrophysics and space research from MIT and several universities. The article describing their findings will be published in the Royal Astronomical Society Monthly Notices.
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As El-Badry explained to Universe Today via email, these observations were part of a larger campaign to identify dormant companions of black holes to normal stars in the Milky Way. “I have searched for dormant black holes over the past four years using a wide range of datasets and methods,” he said. “My previous attempts have revealed a diverse menagerie of binaries masquerading as black holes, but this was the first time the search bore fruit.”
For the purposes of this study, El-Badry and his colleagues relied on data obtained by the European Space Agency (ESA) Gaia Observatory. This mission spent nearly a decade measuring the positions, distances and proper motions of nearly a billion astronomical objects, such as stars, planets, comets, asteroids and galaxies. By tracking the motion of objects orbiting the center of the Milky Way (a technique known as astrometry), the Gaia mission aims to build the most accurate 3D spatial catalog ever created.
For their purposes, El-Badry and his colleagues examined the 168,065 stars in Gaia Data Release 3 (GDR3) that appeared to have two-body orbits. Their analysis found one particularly promising candidate, a G-type (yellow star) designated Gaia DR3 4373465352415301632 – for their purposes, the team designated it Gaia BH1. Based on its observed orbital solution, El-Badry and his colleagues determined that this star must have a binary black hole companion. Said El Badry:
“Data from Gaia constrains how the star moves across the sky, tracing an ellipse in orbit around the black hole. The size of the orbit and its period give us a constraint on the mass of its unseen companion – approximately 10 solar masses. In order to confirm that Gaia’s solution is correct and to rule out alternatives other than black holes, we observed the star spectroscopically with several other telescopes. This tightened our constraints on the mass of the companion and proved that it is really “dark”.
To confirm their observations, the team analyzed Gaia BH1’s radial velocity measurements from multiple telescopes. This included the High Resolution Scale Spectrometer (HIRES) at the WM Keck Observatory, the Long Range Optical Spectrograph (FEROS) at the MPG/ESO Telescope, the X-Shooter Spectrograph at the Very Large Telescope (VLT), the Multi-Object Spectrographs Gemini. (GMOS), the Magellan Echellette Spectrograph (MagE) and the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST).
Similar to the method used for exoplanet hunting (Doppler spectroscopy), the spectra provided by these instruments allowed the team to observe and measure the gravitational forces influencing its orbit. These follow-up observations confirmed the orbital solution of Gaia BH1 and that a companion of about ten solar masses was co-orbiting with it. As El-Badry pointed out, these findings could be the first black hole in the Milky Way that hasn’t been observed based on its X-ray emissions or other energetic releases:
“Models predict that the Milky Way contains about 100 million black holes. But we only observed about twenty of them. All the precedents we’ve observed are in “X-ray binaries”: the black hole eats a companion star, and it glows brightly in X-rays as the gravitational potential energy of that material is transformed into light. But these represent only the tip of the iceberg: a much larger population may be lurking, hidden in more widely separated binaries. The discovery of Gaia BH1 sheds light on this population.
If confirmed, these findings could mean that there is a robust population of dormant black holes in the Milky Way. This refers to black holes that are not evident from bright disks, bursts of radiation, or hypervelocity jets emanating from their poles (as is often the case with quasars). If these objects are ubiquitous in our galaxy, the implications for stellar and galactic evolution could be profound. However, it is possible that this particular dormant black hole is an outlier and not indicative of a larger population.
To verify their findings, El-Badry and his colleagues are eagerly awaiting Gaia Data Release 4 (GDR 4), the date of which is yet to be determined, which will include all data collected during the nominal five-year mission (GDR 4). . This release will include the most up-to-date astrometric, photometric, and radial velocity catalogs for all observed stars, binaries, galaxies, and exoplanets. The fifth and final version (GDR 5) will include nominal and extended mission data (the full ten years).
“Based on the occurrence rate of the companion BH implicated by Gaia BH1, we estimated that the next release of Gaia data will allow the discovery of dozens of similar systems,” El-Badry said. “With just one object, it’s hard to know exactly what it’s implying about the population (it could just be a quirk, a fluke). We’re excited about the demographic studies we can do with larger sample sizes. . »
Further reading: arXiv
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