Lightweight and Supermassive Black Holes Hidden in Galaxies

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In the past few days there have been two interesting astrophysics news about black holes. The first one is about a super-massive black hole discovered in an ultra-compact dwarf galaxy. Such galaxies are tiny (both in mass and size) when compared to our Milky Way. The galaxy M60-UCD1 (where UCD is the acronym of Ultra-Compact-Dwarf) is a lightweight object made by “only” 150 million solar masses, to be compared with our Milky Way with a mass of 1,250 billion solar masses. The galaxy has also a small radius, a few hundred light years across, whereas our galaxy extends for tens of thousands light years.

Despite its tiny size, this ultra-compact dwarf galaxy contains a super-massive black hole in its center, and a quite peculiar one: its mass is about 20 million solar masses, about 5 times bigger than the super-massive black hole in the center of the Milky Way. The mass of this black hole is not particularly remarkable, as we already know many supermassive black holes well more massive than this one (even a thousand times more massive). However, the discovery is quite extraordinary for two reasons. The first is that the black hole mass constitutes about 15% of the total galaxy mass, which is an absolute record-breaker. Again, for comparison, the super-massive black hole in our galaxy contains slightly more than 0.0003% of the total mass of the Milky Way, whereas in other galaxies the typical value is about 0.5%, still way below the 15% of the new record-holder. Second, this is the first ultra-compact dwarf galaxy discovered to contain a supermassive black hole, meaning that many other similar galaxies might contain one as well. This discovery doubles the current total estimate of supermassive black holes in the Universe.

The second news is also about black holes but not of the supermassive kind. The black hole in question was born after a supernova explosion of a massive star. An international team of researches has proposed that a previously known galactic X-ray binary, named Swift J1753.5–0127, might contain the lightest black hole known to date. According to the theory of General Relativity there is no true lower limit to the mass of a black hole. Indeed anything can turn into such an object when sufficiently squeezed (e.g., your body can turn into a black hole if you compress it down to a radius of, well… 0.0000000000000000000000001 meters). However, since astrophysical black holes form in massive stellar explosions, there is a specific channel of formation that allows only a certain mass range. E.g., a light black hole cannot be created because the gravitational pull will not be sufficient to overcome the nuclear forces emerging during the compression process. When this happens the gravitational collapse is stopped and a neutron star is formed in place of the black hole.

Theoretical calculations show that the minimum astrophysical black hole mass is about 2-3 solar masses, but the minimum mass ever measured for a black hole is approximately 5 solar masses. The reason for the existence of this mass gap between about 2 and 5 solar masses is not established yet, but it might have important implications for our understanding of supernova explosions. Indeed, in the past decade it has been proposed that the energy liberated in a supernova depends on the mass of the exploding star. Such energy might suddenly diminish when the star becomes sufficiently massive to form a black hole remnant. If the energy of the supernova is too low in massive stars, as this model suggests, then a large fraction of the stellar mass will not be expelled during the explosion and a minimum mass budget will always be present during the gravitational collapse. Such mass budget is set at the observed value of about 5 solar masses.

New observations of Swift J1753.5-0127, performed at various optical observatories, among which the Hubble Space Telescope, show that the stellar companion in the binary wobbles as if the black hole weight is below 5 solar masses. The estimated most probable mass is about 4 solar masses. If confirmed this might be the lightest black hole mass measured so far and would induce astronomers to reconsider the existence of a mass gap. This in turn might mean that either the energy of supernovae does not change much with the mass of the exploding star or that black holes in the mass-gap form in a way different than the gravitational collapse that follows supernovae explosions.

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