Supermassive black holes in the early universe

ABSTRACT

The emergence of the first black holes during the first billion years of cosmic history marks a key event in cosmology. Their formation is part of the overall process of ending the cosmic dark ages, when the first stars appeared in low-mass dark matter haloes about a few 100 million years after the Big Bang. The first stars, galaxies, and black holes transformed the Universe from its simple initial state into one of ever increasing complexity. We review recent progress on our emerging theoretical picture of how the first black holes appeared on the cosmic scene, and how they impacted the subsequent history of the Universe. Our focus is on supermassive black holes, in particular assessing possible pathways to the formation of the billion-solar-mass black holes inferred to power the luminous quasars at high redshifts. We conclude with a discussion of upcoming empirical probes, such as the James Webb Space Telescope (JWST), and the Laser Interferometer Space Antenna (LISA), further ahead in time.

Keywords:

• Black holes
• galaxies: formation
• galaxies: evolution
• galaxies: high-redshift

Acknowledgments

We thank Kohei Inayoshi, Anna Schauer, and the anonymous referees for very helpful comments on the draft.

Disclosure statement

No potential conflict of interest was reported by the authors.

ORCID

Aaron Smith http://orcid.org/0000-0002-2838-9033

Notes

1 The first stars, formed out of primordial hydrogen and helium gas, are often called Population III (Pop III) stars. This terminology extends the traditional sequence of astronomy, where metal-rich stars, like our Sun, are Population I stars, and the metal-poor stars in the outer regions of the Milky Way constitute Population II.

2 In astronomy, the mass of the Sun is denoted with ${\mathrm{M}}_{\odot }\approx 2\times {10}^{30}$ kg.

3 Intuitively, $m_{\mathrm{P}}$ is the mass of a black hole that is just massive enough to gravitationally hold on to its mass, before quantum-tunnelling is able to return matter to the outside of the black hole event horizon. Classically, nothing is allowed to do that.

4 Standard analysis of the primordial chemistry indicates that the dark matter has to be ‘non-baryonic’, made up of a new kind of elementary particle.

5 These photons reside in the so-called Lyman-Werner (LW) wavelength band, giving rise to the term LW radiation.

6 Hawking radiation, named after the British theoretical physicist Stephen Hawking, is black-body radiation predicted to be released by black holes due to quantum effects near the event horizon.

7 Astronomers often use ‘erg’ for units of energy, which translates to SI units as $1\mathrm{e}\mathrm{r}\mathrm{g}/\mathrm{s}={10}^{-7}\mathrm{W}$.

8 ‘Optical depth’ is a term that describes the (logarithmic) transmission of light through a material, and is calculated along ray segments as ${\tau }_e={\int }_0^L{n}_e{\sigma }_{\mathrm{T}}\mathrm{d}\ell$, where $n_e$ denotes the number of free electrons per unit volume.

9 The Jeans instability causes gas clouds to compress and fragment when the internal pressure is not strong enough to prevent gravitational collapse. This occurs when the free-fall time, $t_{\mathrm{f}\mathrm{f}}\approx 1/\sqrt{G\rho }$, is shorter than the sound-crossing time, $t_s\approx R/c_s$, with G the universal gravitational constant, ρ the density, R the radius, and $c_s$ the sound speed.

Additional information

Funding

Support for Program number HST-HF2-51421.001-A was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555. VB acknowledges support from National Science Foundation (NSF) grant AST-1413501.

Notes on contributors

Aaron Smith

Aaron Smith is currently a NASA Einstein Fellow at the Kavli Institute for Astrophysics and Space Research at the Massachusetts Institute of Technology. Smith recently received his PhD from the University of Texas at Austin where he held a National Science Foundation Graduate Research Fellowship. He primarily works in galaxy formation theory and numerical radiation hydrodynamics.

Volker Bromm

Volker Bromm is a professor of Astronomy at The University of Texas at Austin. Bromm was educated at Yale University before undertaking postdoctoral research at Cambridge University and the Harvard-Smithsonian Center for Astrophysics. He has worked on numerous problems in the areas of the first stars and galaxies, dark matter theory, and stellar archaeology, and has published several reviews on these topics.