Cain: Tau and Turbulence: Understanding reionization at the largest and smallest scales / Guo: Accretion and Feedback from Galaxies to Event Horizons

Speakers: Christopher Cain (ASU)/ Minghao Guo (Princeton University) In Person and zoom

Cain: Reionization is a fantastically multi-scale process. The timing of reionization sets the electron scattering optical depth to the CMB, measured from polarization data at the largest scales. Meanwhile, photo-heating from reionization drives pressure-smoothing of the diffuse intergalactic medium down to scales of a kpc or smaller. I will discuss two recent developments along these lines. The first is that, when combined with recent constraints on the end of reionization from the Lyman Alpha forest, the CMB optical depth becomes a probe of reionization's duration as well as its timing. This observation may challenge recent efforts to alleviate cosmological tensions by invoking an unexpectedly high value for the optical depth. The second is that reionization-driven pressure smoothing may fill the universe with turbulent eddies that could amplify intergalactic magnetic fields via the dynamo effect, possibly explaining lower limits on magnetic field strength from TeV blazars.

Guo: Accurately modeling supermassive black hole (SMBH) feeding and feedback from galactic to event horizon scales is a formidable task; the involved spatial scales span nearly nine orders of magnitude (from mpc to Mpc) and need to be resolved over an extended time period. We present a series of 3D (general relativistic) magnetohydrodynamic (GRMHD) simulations of the fueling of SMBHs on galactic scales, taking M87* as a typical case. The simulations reveal various accretion modes on different scales: magnetized filaments on scales ~0.03-3 kpc, a highly magnetized \beta ~ 10^-3 thick disk within ~ 30 pc, and a turbulent hot accretion flow within ~0.3 pc (10^3r_g) with strong outflows enough to balance the total cooling of the M87/Virgo hot halo out to ~ 50 kpc. Furthermore, we present a "cyclic zoom" method to capture the dynamics of accretion flows across a vast range of spatial and temporal scales. The method can accelerate GRMHD simulations by ~ 10^5 times for problems with large scale separation. As applications, we simulate Bondi and rotating torus accretion onto black holes from galactic scales, covering an extremely large dynamic range. In Bondi accretion, the accretion rate is suppressed relative to the Bondi rate by ~(10r_g/r_B)^{1/2} with a feedback efficiency of ~1% for vanishing spin, and ~ 10% for spin a ~ 0.9. Our new method likewise holds significant promise for applications to many other problems that need to cover vast spatial and temporal scales.