A Direct Black Hole

How did supermassive black holes form in the early epochs of the universe? More importantly, how did they have enough time to grow as large as they did? The answer requires a very different universe.  And back then, conditions were much different than they are now.  There was a lot of gas, little dust, no stars, and a plethora of dark matter.

Direct collapse simulation. Credit: Aaron Smith/TACC/UT-Austin

Astronomers have spent decades observing early quasars, massive active galaxies powered by huge black holes feeding on surrounding gas.  But these galaxies are seen so early in the universe’s history, one starts to wonder how a black hole finds sufficient time to grow large enough and power a quasar?

The general idea is that a black hole forms from the collapse of a star and slowly grows by accreting (sucking up) gas in its environment.  But this process takes a very long time, much longer than these early black holes had to work with.  The other problem is that star formation is detrimental to black hole growth.  If the black hole started from the collapse of a massive star, and there were other stars around, there wouldn’t be much gas or dust to feed the black hole.  This is because stars are very efficient at blowing away leftover gas and dust in their birth clouds.

But in the distant past, conditions were perfect for a long-theorized event.  The formation of a direct-collapse black hole.  In this case, a massive clump of gas in the early universe begins to collapse rapidly, with streams of gas flowing inward from all directions.  In the modern epoch, the gas would cool as it collapses and fragment into clumps that would form individual massive stars.   But in the early universe, a soup of ultraviolet photons prevented the cloud from fragmenting, instead keeping it massive and dense, and allowing it to collapse directly into a supermassive black hole.

This was all theoretical when it was proposed in 2003.  But as of today – there’s evidence!

Hubble data had found an ancient source called CR7 that seemed to have unique features.  When follow up observations were performed with some of the larger Earth-based telescopes, they showed that the source must be very hot and have very high energy in UV wavelengths.  At this time in the history of the universe this could only be a dense cluster of early stars or a supernova likely formed by direct collapse. Supercomputer simulations showed that a cluster of protostars was not possible based on the observations, and so the most plausible explanation is for the existence of a direct-collapse black hole.

Of course, follow up observations and a lot more simulations are needed to confirm the results, but it is exciting to see elusive evidence emerge for a good theory.  This is what happens when we push the limits of our understanding.  We require the most powerful telescopes and the most advanced simulation code running on powerful supercomputers.  Every step requires us to dig deeper, see further back in time, and learn new ways of finding the gems of wisdom in a plethora of data.

 

 

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