The First Supermassive Black Holes

MNRAS 000,1–6 (2017) Preprint 29 June 2017 Compiled using MNRAS LATEX style file v3.0 The first supermassive black holes Aaron Smith,1 Volker Bromm1 and Abraham Loeb2 1Department of Astronomy, The University of Texas at Austin, Austin, TX 78712, USA 2Department of Astronomy, Harvard University, 60 Garden Street, Cambridge, MA 02138, USA The existence of a supermassive black hole (SMBH) at the by balancing the attractive gravitational force against the build-up centre of almost every galaxy provides strong evidence for a of pressure from the radiation emitted by the infalling gas as it generic growth mechanism alongside that of the host galaxy (Ko- encounters viscous heating. For typical radiative efficiencies, the rmendy & Ho 2013). Nuclear black holes typically comprise very maximal growth rate – known as the Eddington limit – barely al- much less than 1% of the available baryonic mass in the galaxy lows growth from stellar-mass black hole progenitors in the limited and are therefore gravitationally dominant only locally. But their time available since the Big Bang. As an analogy, we consider the efficient conversion of rest mass to radiation plays an important exponential growth of a bank account accruing compound interest. arXiv:1703.03083v2 [astro-ph.GA] 27 Jun 2017 role in the evolution of galaxies as a powerful source of feedback A lifetime of steady investments makes the burden of saving for that drives winds, suppresses star formation and contributes to the retirement relatively straightforward and secure. However, if the reionization of the universe. Understanding the physical processes growth has to occur in less than a decade then there is a timing cri- leading to the formation and growth of SMBHs is crucial in unrav- sis: the unfortunate individual must either save money at exorbitant eling fundamental questions about the galactic building blocks of rates, experience a miracle akin to winning the lottery – or delay the universe. While the largest-scale structures took several billion their retirement. years to form, supermassive black holes up to ∼ 1010 M (quasars) are observed very early in the history of the universe, around a cosmological redshift of z ≈ 7, at less than 5% of the current age of Timing Matters the universe (Fan et al. 2006; Mortlock et al. 2011; Wu et al. 2015). How and when did the first supermassive black holes form? Al- This presents a timing problem under conventional scenarios for though there may not be a universal pathway, rapid seeding or the growth of these objects by accretion. growth is unavoidable within our current understanding of the most The rate of black hole growth via gas consumption is regulated distant quasars. The first SMBHs were likely to have co-assembled c 2017 The Authors 2 A. Smith et al. near-extremal Kerr black hole if cooling does not initiate fragmen- 1010 Growing the First tation (Rees 1984). ) Supermassive ¯ n Early simulations of collapsing primordial gas clouds showed etio M Black Holes ccr ( 108 ic A that most of the gas does indeed fragment into dense stellar clumps isod Observed s Ep s which eventually virialize into a spheroidal galactic bulge (Loeb & Direct Quasars a 6 Collapse Rasio 1994). The emergence of a SMBH would thus be forestalled. M 10 6 e On the other hand, if a central seed black hole of mass & 10 M l o Dense were to form during the collapse then it could quickly grow by H 4 Eddington-limited 10 Cluster steady accretion to a quasar-size black hole (Li et al. 2007). Other- k c Hyper-Eddington a wise dynamical instabilities in the inner region of the disc would in- l 2 Inefficient Growth B 10 Stellar hibit the accretion required for such growth. The crucial bottleneck Remnant is the formation of the initial, million solar-mass, seed. Either way, 0 250 500 750 realistic modeling of massive black hole inception is highly com- Time after Big Bang (Myr) plex and ultimately relies on understanding both small- and large- scale phenomena. For example, low angular momentum configura- tions would help facilitate runaway collapse because the centrifugal Figure 1. Illustrating the timing problem for supermassive black hole barrier is significantly lower; in this context, quasar seeds could be 9 growth in the early universe. The observed quasars with a mass M• ≈ 10 – a natural consequence of the initial collapse of regions with un- 10 ≈ − 10 M at redshift z 6 7 could have originated from massive seeds or usually small rotation (Eisenstein & Loeb 1995). Such low-spin from low-mass stellar progenitors with hyper-Eddington growth rates. cosmological perturbations might provide environments in which SMBH formation is an extreme manifestation of the ΛCDM model. within the first galaxies a few hundred million years after the Big Bang. Remarkably, primordial galaxies that could delay the onset Forming the first massive black holes of star formation seem to facilitate a mechanism to produce mas- 4 6 The main contenders for the earliest quasar seeds are DCBHs, sive (10 –10 M ) black hole seeds, which form “in one go” from gas clouds with inefficient cooling. Such clouds thereby maintain super-Eddington accretion onto stellar remnant black holes, and high thermal pressure support, which in turn suppresses fragmen- runaway collisions in dense star clusters. Here we focus on DCBHs tation and thus star formation. These direct collapse black holes and highlight recent theoretical and observational evidence for this (DCBHs) circumvent the timing crisis. DCBHs can form only early new class of black hole seeds; we discuss the alternative scenarios in cosmic history when a specific set of rare conditions is satis- briefly. fied. At redshifts z & 10, they are still beyond the reach of current In typical galactic environments black hole accretion is telescopes. Progress will be made with next-generation observato- episodic because of self-regulating radiative feedback which yields ries using wide-field surveys and follow-up deep observations of accretion rates that are sub-Eddington when averaged over mul- select objects. Ultimately, this will provide compelling evidence tiple duty cycles (Johnson & Bromm 2007; Milosavljevic´ et al. for or against formation scenarios involving direct-collapse, per- 2009). However, maintaining super-Eddington accretion is possible sistent hyper-Eddington accretion, or other more exotic possibili- when the black hole is embedded within sufficiently dense gas; this ties (e.g. Dolgov & Silk 1993). In the sections below, we briefly renders the radiation pressure less effective (e.g. Wyithe & Loeb review the historical development of ideas about the first SMBH 2012; Pacucci et al. 2015a). Based on one-dimensional radiation- seeds, the physics of their formation and radiative feedback, re- hydrodynamics simulations, Inayoshi et al. (2016) find accretion ˙ 3 2 cent progress, and our outlook for the future. Figure1 provides a rates exceeding M• & 10 LEdd=c when the following condition is 5 −3 4 −1 4 3=2 schematic overview of the quasar-seed timing problem. satisfied: (n1=10 cm ) > (M•=10 M ) (T1=10 K) , where n1 and T1 are the density and temperature of the ambient gas. It remains an open question whether such growth rates are sustainable considering the violent assembly environments of the first galaxies Cosmological context where newly formed stars and supernovae blow away the surround- The standard cosmological model involves dark energy and cold ing gas. Other scenarios may also work, such as dense star clusters dark matter (ΛCDM). Both ingredients remain mysterious in terms that undergo runaway collapse. With a ubiquitous supply of cold of their underlying physics. However, the ΛCDM model matches gas effectively trapping accretion radiation, a ∼ 10 M black hole data across scales extending from large-scale structures to galaxy seed undergoing random motions through the cluster may initiate formation and evolution. Historically, supermassive black hole re- supra-exponential growth over a dynamical timescale (Alexander search predates the recent cosmological perspective. Summarizing & Natarajan 2014). these earlier insights, Rees (1984) discussed possible routes for On the other hand, forming a DCBH requires collapse with- runaway growth in active galactic nuclei. In isolated environments, out fragmentation, a cosmic “miracle” of sorts, that is naturally ex- possible formation pathways include: stellar-remnant black holes plained within the context of galaxy formation theory. The idea is after vigorous gas accretion; dense star clusters in which runaway that if a primordial gas cloud, devoid of any heavy chemical ele- collisions trigger the formation of a SMBH; or a cluster of post- ments (“metals” in astronomical terminology), is bathed in a sea of supernovae neutron stars or black holes coalescing in the dynami- ultraviolet radiation then it will be unable to cool and form stars cally unstable central core of the galaxy. In cosmology, SMBHs can (figure2). Specifically, according to the Jeans criterion for trig- grow over longer timescales via episodic galaxy mergers and ac- gering gravitational instability, high thermal pressure is required cretion from streams of cold gas along filaments of the cosmic web to prevent the gas cloud from fragmenting. In present-day star- (Mayer et al. 2010, 2015). Still, it was recognized that a gas cloud forming clouds, line cooling by heavy elements and dust radi- may conceivably bypass conventional star formation and yield a ates away thermal energy that would otherwise provide stability. MNRAS 000,1–6 (2017) The first supermassive black holes 3 T 104 K Supermassive Seed vir ≈ St rong LW e Flux r u g t n a i IGM t r a e e H2 + Metal p H c m i Cooling t e a b T a i d A Star Formation Figure 3. Illustration of the galactic outflow model for the Ly-α emitter Density CR7. A strong source of radiation drives an expanding shell of gas from the centre through the interstellar medium (ISM).

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