Recently, the Gaia collaboration discovered the most massive black hole that originated from the collapse of a star in the Milky Way galaxy. It was identified through the orbital motion of its visible companion, a yellow giant star with an orbital period of 11.6 years.
This black hole, called Gaia BH3, has 33 times the mass of the Sun, in striking resemblance to the masses of the first black holes detected through gravitational waves by LIGO in 2015 involving a merger of 29 and 36 solar masses. Gaia BH3’s motion through the galaxy implies that it originated from a disrupted star cluster in the halo of the Milky Way where the oldest stars reside. Indeed, the iron-to-hydrogen ratio of its companion is 320 times smaller than the solar value, suggesting that Gaia BH3 is a relic of the early Universe where heavy elements were scarce.
Indeed, my research from three decades ago suggested that the first stars were much more massive than present-day stars because the gas clouds that birthed them contained primarily hydrogen and helium, with limited ability to cool and fragment into stars with less than ten solar masses. As a result, star-forming regions in the early universe were efficient factories of massive black holes, such as Gaia BH3. After formation, these black holes followed the collision-free dynamics of the dark matter and assembled into the extended halos of galaxies like the Milky Way.
On the other hand, the disk of the Milky Way contains younger stars because it formed later in cosmic history. The disk is estimated to contain about one black hole per thousand stars, yielding a population of about 100 million black holes. Given the local density of stars and their characteristic random motions, I calculated that over the age of the Solar system, one of the black holes entered the outer envelope of the Oort cloud at 20,000 times the Earth-Sun separation. This was likely the closest encounter ever of a black hole near Earth, lasting tens of thousands of years.
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