Title: The ASTRID simulation: the evolution of supermassive black holes
Authors: Yueying Ni, Tiziana Di Matteo, Simeon Bird, Rupert Croft, Yu Feng, Nianyi Chen, Michael Tremmel, Colin DeGraf, Yin Li
First Author’s Institution: Department of Physics, McWilliams Center for Cosmology, Carnegie Mellon University
Status: Published in MNRAS [open]
The Cosmic Seeds of Galaxies
Supermassive black holes (SMBHs) are found to reside at the centers of nearly all massive galaxies in the local Universe with compelling observational evidence. Their close connection with the large-scale properties of galaxies suggests a co-evolutionary process, where SMBHs grow by accreting gas while simultaneously regulating star formation. Despite their “universal” (wordplay acknowledged) nature, the origin of SMBHs is still a mystery. How did black holes form so rapidly, appearing less than a billion years after the Big Bang? And how did they become central to shaping galaxies into the structures we observe today?
The launch of cutting-edge observatories, such as the James Webb Space Telescope and the Nancy Grace Roman Space Telescope, is set to revolutionize our understanding of the Universe in its infancy. In order to meet a new generation of observational data, theoretical astrophysicists have developed sophisticated cosmological simulations—such as IllustrisTNG, EAGLE, Simba, and Magneticum—to model the evolution of star formation in galaxies, and explore different scenarios for the formation and growth of SMBHs.
ASTRID is a cosmological simulation of galaxy formation that focuses on high redshifts to study the early Universe (visualized in Figure 1). In addition to containing models to describe the reionization era, it incorporates an updated, specialized treatment of black holes.
Simulating the Growth of Supermassive Black Holes
Creating thousands of galaxies requires incredible amounts of computational power. Therefore, many cosmological simulations use “subgrid models” to capture physical processes that occur on scales smaller than the simulation’s resolution. Imagine you’re trying to model the movement of air in a room. Instead of tracking every tiny molecule, you can divide the room into small sections and describe how the air in each section moves and changes. This is similar to how cosmological simulations work. Instead of simulating each individual dark matter, gas, and star particle, they treat groups of particles as fluid-like cells, similar to how air flows. When gas cells become cool and dense enough, the simulation converts them into stars.
One theory of SMBHs is that they originated from the collapse of first-generation stellar remnants, metal-free gas, or nuclear star clusters, resulting in low mass “seed” black holes. In the ASTRID suite, a black hole “seed” particle is placed inside a galaxy whenever the galaxy becomes more massive than a certain threshold. Rather than a fixed initial mass, the galaxies can have varying black hole masses to start with, sampled from a power law distribution.
Compared to previous simulations, ASTRID does not artificially pin down the SMBH to the galaxy center, but allows the black hole particle to sink naturally relative to the gas and stars in its surroundings according to frictional and drag forces. As a result, ASTRID can simulate the coalescence of SMBHs during galaxy mergers more realistically, based on the relative distance and motion of the black hole pair (see this Astrobites article about SMBH mergers in ASTRID).
The black holes are allowed to grow through gaining mass from nearby gas cells at an accretion rate proportional to the black hole mass and surrounding density, and inversely related to the speed of the gas. The thermal energy of the SMBH heats up the medium around it, influencing the star formation of the galaxy.
From Cosmic Seeds to Galactic Giants
With its state-of-the-art black hole model, ASTRID can reproduce the population of SMBHs at high redshifts. The masses and luminosities (a function of accretion rate) of the SMBHs are consistent with observations, demonstrating the simulation’s predictive power (Figure 2).
Importantly, the SMBHs in the simulation agree with empirical constraints in terms of the distribution of black hole masses and X-ray luminosities over time. The positive scaling relations between MBH (black hole mass) and Mstar (stellar mass) as well as between ṀBH (black hole accretion rate) and SFR (star formation rate) in Figure 6 of the paper confirm that the evolution of SMBHs and galaxies are intertwined, with gas accretion being the dominant growth mechanism for black holes as opposed to mergers.
Combined with observations and other computer simulations, ASTRID can help astrophysicists explore how SMBHs form in galaxies: from seeds to giants!
Astrobite edited by Sparrow Roch and Katherine Lee.
Featured image credit: “The time evolution of a collision of two spiral galaxies with black holes at their center from a computer simulation,” by Tiziana Di Matteo.
