Galaxies typically contain within their nucleus a supermassive black hole, the mass of which is related to the mass of the galaxy by empirical law. For typical mass values of galaxies, the mass of supermassive black holes vary between 100,000 and 10 billion solar masses for the most massive galaxies. For example, the black hole in the Milky Way has a mass of 4 million solar masses.
The black hole of Holmberg 15A seems to be an exception to this empirical law. The first mass determinations made in the 2000s resulted in incredibly high values exceeding 300 billion solar masses, making it by far the heaviest black hole observed in the Universe. Studies in the following years have made it possible to significantly lower this value and today the most precise measure indicates a mass of 40 billion solar masses, which makes it remain one of the most massive black holes ever observed.
Whether this is the final value of the mass of the Holmberg 15A black hole is difficult to say. Today we are not able to directly study the mass of these objects and the only thing we can do is to use indirect methods that exploit the properties of the material that is in their vicinity (for example, the accretion disk). These methods obviously present a number of uncertainties, which are exacerbated by the fact that we study objects at long distances from the Earth.
In order to find out whether the Holmberg 15A black hole is exceptionally massive or whether it is due to measurement errors, we will have to wait for time until more complete measurements are available.
Credit: Matthias Kluge/USM/MPE.
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