There are supermassive black holes. There are ultramassive black holes. How big can these strange objects grow? Well, there could be something even bigger than ultramassive: great large black holes, according to the latest research.
These hypothetical black holes (more than 100 billion times the mass of the Sun) have been explored in a new document called SLAB, an acronym meaning “Stupendously LArge Black holeS.”
“We already know that black holes exist in a wide range of masses, with a supermassive black hole of 4 million solar masses residing in the center of our own galaxy,” said astronomer Bernard Carr of Queen Mary University London. .
“While there is currently no evidence of the existence of SLABs, it is conceivable that they may exist and may also reside outside galaxies in intergalactic space, with interesting observational consequences.”
Black holes only have a few broad mass categories. There are black holes of stellar mass; these are black holes that are around the mass of a star, up to about 100 solar masses. The next ascending category is black holes of intermediate mass and the degree of size that seems to depend on who you are talking to. Some say 1,000 solar masses, others say 100,000 and others say 1 million; whatever the upper limit, they seem to be quite rare.
Supermassive black holes (SMBH) are much, much larger, on the order of millions to billions of solar masses. These include the SMBH in the heart of the Milky Way, Sagittarius A *, with 4 million solar masses, and the most photogenic SMBH in the Universe, M87 *, with 6.5 million solar masses.
The largest black holes we have detected are ultramassive solar masses, with more than 10 billion (but less than 100 billion). These include an absolute beast that records 40 billion solar masses at the center of a galaxy called Holmberg 15A.
“However, surprisingly, the idea of SLABs has been neglected so far,” Carr said.
“We have proposed options on how these SLABs could be formed and we hope that our work will begin to motivate discussions among the community.”
The fact is that scientists do not know very well how large black holes form and grow. One possibility is that they form in their host galaxy and then grow thicker and thicker, releasing a lot of stars, gas, and dust, and colliding with other black holes as the galaxies merge.
This model has an upper limit of about 50 billion solar masses: this is the limit at which the prodigious mass of the object would require a disk of accretion so massive that it would fragment under its own gravity. But there is also an important problem: supermassive black holes have been found in the early Universe at masses too high to have grown by this relatively slow process in time since the Big Bang.
Another possibility is something called primordial black holes, first proposed in 1966. The theory is that the variable density of the early universe could have produced pockets so dense that they collapsed into black holes. These would not be subject to the size restrictions of the black holes in collapsed stars and could be extremely small or, wonderfully large.
The extremely small ones, if they ever existed, would probably have evaporated due to Hawking radiation. But the much older ones could have survived.
Thus, based on the primordial model of the black hole, the team calculated exactly the magnitude that these black holes could have, between 100 billion and 1 quintillion (i.e., 18 zeros) of solar masses.
The researchers said the purpose of the work was to consider the effect of these black holes on the space around them. We may not be able to see SLABs directly (black holes that do not accumulate material are invisible, as light cannot escape their gravity), but massive invisible objects can still be detected depending on the behavior of space in the around him.
Gravity, for example, curves space-time, which causes light traveling through these regions to follow a curved path as well; this is called a gravitational lens and the effect could be used to detect SLABs in intergalactic space, the team said.
Huge objects would also have implications for the detection of dark matter, the invisible mass that injects more gravity into the Universe than it should have, based on what we can actually detect directly.
A hypothetical candidate for dark matter, weakly interacting massive particles (WIMPs), would accumulate in the region around a SLAB due to the immense gravity, at such concentrations that they would collide and annihilate each other, creating a gamma radiation halo.
And primordial black holes are also candidates for dark matter.
“The SLABs themselves could not provide the dark matter,” Carr said. “But if they exist, it would have important implications for the early universe and make it plausible that lighter primordial black holes could do that.”
Also, we could not resist calculating the size of a black hole of 1 quintillion of solar mass. The horizon of events would end up more than 620,000 light-years in diameter. Uh. Great.
The team’s research has been published in Monthly notices from the Royal Astronomical Society.