by Kirk Barrow
Last year the astrophysics community celebrated the first direct image of a supermassive black hole (SMBH) by the Event Horizon Telescope. We know that the black hole they studied, the one at the center of the galaxy Messier 87 (or M87) in the Virgo Galaxy Cluster, is around three and a half billion times as massive as our own Sun, but how did it accumulate so much mass and, more generally, how and when did these massive black holes form?
Supermassive mysteries
The earliest SMBH discovered powers a bright quasar and was observed a redshift of about z=7.1 (about 760 million years after the Big Bang) during a period of the Universe’s 13.75-billion-year history called the Epoch of Reionization, or Cosmic Dawn. By that time it already weighed in at about 2 billion solar masses (Mortlock, et al., 2011). Most theories predict that stellar remnants serve as the initial seeds for massive black hole formation, but the Mortlock quasar accumulated so much mass so early in the lifetime of the Universe that, according to the classical Eddington theory of accretion, there would not have been enough time for the remnant of a normal star to become so large. Specifically, the theory states that some of the kinetic energy of the gas that falls into a black hole is converted into radiation which pushes back against the infalling gas, thus establishing an Eddington Limit for the maximum accretion rate—a rate far exceeded by the SMBH at the center of M87.
Accretion conundrum
More than one possible answer to this accretion conundrum has been proposed. For example, one possibility is that the original stellar somehow remnant accreted mass more quickly than the Eddington limit allows.
A second possibility, which I and my collaborators investigated, is that the progenitor star was significantly more massive than those we see in the local Universe.
My collaborators and I investigated this second "massive star" scenario with a series of cosmological simulations. We then made predictions about how the formation and evolution of a massive black hole seed would progress and how it might look if seen through the forthcoming James Webb Space Telescope (JWST).
Stars form from clouds of gravitationally collapsing gas as the clouds gradually cool by radiating away their heat. The slower the cooling rate, the larger the mass of gas that ends up in each stellar core (Jeans mass). Lone hydrogen atoms, aka atomic hydrogen, cool very inefficiently so the size of stellar cores is usually modulated by the presence or absence of molecular hydrogen (H2) and what astronomers call metals: elements with an atomic number greater than helium. These "atomic cooling haloes" cool much more efficiently because more complex molecules and atoms have more complex electron shell structures, giving a wider range of quantized energy levels that electrons can jump through, radiating energy via emitted photons in the process.
However, our simulations show that in rare configurations, in metal-free regions of the early Universe, starlight from a nearby galaxy can break up molecular hydrogen back into atomic hydrogen. This leaves a much more homogeneous gas cloud to collapse uniformly into one large central clump without fragmenting into smaller proto-stars. Instead, the central clump coalesces into a single transitory massive star, which, once it burns rapidly through its life, quickly collapses into a black hole seed with a mass of up to 20,000 times the mass of our Sun. While this is a surprising scenario that could not happen in our present day metal-rich Universe, simulations show that it is possible for such a massive “quasi-star" to form during the Cosmic Dawn, fuse hydrogen normally for a short period of time, and then disperse, “leaving behind the naked seed [massive black hole] with a mass of thousands to hundred thousands of solar masses.” (From Volonteri and Bellovary, 2012, Section 2.2).
Radiation from the accretion disk around the black hole then quickly ionizes the surrounding gas, which actually promotes the production of molecular hydrogen in the still-collapsing gas around the black hole (Abel et al. 1997). Our simulation showed for the first time that this gas fragments and forms a large cluster of black hole-induced metal-poor stars which surround it (Figure 1, below).
Searching for confirmation with the JWST
The combination of a massive black hole seed and a metal-free star cluster produces a unique, bright signature that astronomers can search for using the JWST (Figure 2). This provides an enticing opportunity for theory and simulation to join with observation to finally confirm the origins of SMBHs—an opportunity that should arise in the very near future with the advent of the JWST. Going forward, we hope to explore this scenario further and try to understand how likely these objects are to form and if they do, what effect they have on their galactic neighborhood.
Ultimately, we hope to shed some light on the formation of the most ravenous monsters in the Universe—the huge SMBHs at the centers of galaxies like M87.
Related reading / watching
Observational signatures of massive black hole formation in the early universe (Barrow, et al., 2018. arXiv link.)
Article metrics for above article as published in Nature
First Light: exploring the Spectra of High-Redshift Galaxies in the Renaissance Simulations (Barrow, et al., 2017. arXiv link.)
Black Holes in the Early Universe (Volonteri and Bellovary, 2012. arXiv link.)
Short clip about the Renaissance Simulations (2017)
