Super-Eddington Accretion and Feedback from the First Massive Seed Black Holes (1811.04953v2)

Abstract: Super-Eddington accretion onto massive black hole seeds may be commonplace in the early Universe, where the conditions exist for rapid accretion. Direct collapse black holes are often invoked as a possible solution to the observation of super massive black holes (SMBHs) in the pre-reionisation Universe. We investigate here how feedback, mainly in the form of bipolar jets, from super-Eddington accreting seed black holes will affect their subsequent growth. We find that, nearly independent of the mass loading of the bipolar jets, the violent outflows generated by the jets evacuate a region of approximately 0.1 pc surrounding the black hole seed. However, the jet outflows are unable to break free of the halo and their impact is limited to the immediate vicinity of the black hole. The outflows suppress any accretion for approximately a dynamical time. The gas then cools, recombines and falls back to the centre where high accretion rates are again observed. The overall effect is to create an effective accretion rate with values of between 0.1 and 0.5 times the Eddington rate. If this episodic accretion rate is maintained for order 500 million years then the black hole will increase in mass by a factor of between 3 and 300 but far short of the factor of $10^4$ required for the seeds to become the SMBHs observed at $z>6$. Therefore, direct collapse black holes born into atomic cooling haloes and which experience strong negative mechanical feedback will require external influences (e.g. rapid major mergers with other haloes) to promote efficient accretion and reach SMBH masses within a few hundred million years.

Super-Eddington Accretion and Feedback from the First Massive Seed Black Holes

The paper presented explores the astrophysical phenomenon of super-Eddington accretion onto massive black hole seeds, particularly in the nascent Universe. Black holes, formed from fluctuating high-density regions during the Universe's formation, potentially exhibit super-Eddington growth when conditions conducive to rapid mass accumulation exist. The authors focus on direct collapse black holes (DCBHs), which are hypothesized to transition into supermassive black holes (SMBHs) through super-Eddington phases.

Overview of Simulation Approach and Findings

In this paper, simulations are conducted using the Enzo adaptive mesh refinement code, where the accretion process onto a black hole following the collapse of a supermassive star (SMS) is explored. The paper describes a realistic cosmological setting where direct collapse black hole seeds form within atomic cooling haloes, with the environmental conditions suppressing H$_2$ formation and promoting rapid accretion.

The simulations make use of a self-consistent 3D cosmological setup, accounting for feedback predominantly in the form of bipolar jets resulting from super-Eddington accretion rates. The feedback loop created by these jets and their impact on the black hole's growth is a central focus.

Numerical Results

One of the paper's pivotal discoveries is the capacity for super-Eddington accretive episodes to produce significant feedback via bipolar jets. These jets, while intensely disrupting local accretion dynamics, could not escape the halo confines and therefore had limited overall efficacy at affecting grand-scale halo dynamics. Despite the possibility of large initial mass gain rates stemming from super-Eddington phases, ongoing jet activity detracts from efficient mass growth, resulting in intermittency and lower overall accretion efficiencies.

The paper quantifies accretion rates, finding them suppressed to approximately 0.1 to 0.5 times the Eddington rate when accounting for feedback mechanisms. Over extended cosmic timescales (order of 500 million years), black hole mass increases by factors ranging from 3 to 300, yet still far short of the factor required for DCBHs to evolve into SMBHs observed at high redshifts ($z > 6$). Thus, the paper posits an ongoing need for external influences—such as rapid major mergers with other haloes—to stimulate efficient mass accretion overcoming mechanical feedback impediments.

Implications and Speculation

The research has broad implications, especially regarding the evolving understanding of black hole physics in early cosmic epochs. Mechanical and radiative feedback from early black hole seeds are critical elements influencing the mass accumulation efficiency necessary for SMBH formation. The authors highlight that despite the simulated seed's suboptimal growth rates resulting from mechanical feedback, external cosmic interactions, notably halo mergers, could provide pathways to bridge mass growth gaps.

Future investigations into black hole accretion dynamics can build on this work by incorporating more complex radiative feedback models and exploring the effects of more detailed merger histories and environments through enhanced resolution simulations. Moreover, understanding the interplay between feedback mechanisms and cosmic environment changes will illuminate theoretical paths to reconcile simulation results with astronomical observations.

Conclusion

The paper presents thorough insights into how feedback from super-Eddington accretion influences the trajectory of black hole growth under cosmological conditions. It accentuates the importance of feedback processes, suggesting they hold substantial sway over how initial black hole seeds transition into the SMBHs detected in the early Universe. The work provides a foundation for nuanced studies integrating both radiative and mechanical feedback on black hole growth with the ambition that future models better predict the evolution of the Universe's first massive seed black holes.