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In the beginning, the Universe was all primordial gas. Somehow, some of it was swept up into supermassive black holes (SMBHs), the gargantuan singularities that reside at the heart of galaxies. The details of how that happened and how SMBHs accumulate mass are some of astrophysics\u2019 biggest questions. <\/p>\n
<\/p>\n Black hole science took a big step in 2019 when the Event Horizon Telescope captured the first image of a black hole. That SMBH was in Messier 87, a supergiant elliptical galaxy over 50 million light-years from Earth. As fascinating an accomplishment as that was, it didn\u2019t answer our longstanding questions about how these objects become so massive. <\/p>\n Scientists know that two main processes govern SMBH growth: They accrete cold gas from their host galaxy, and they merge during galaxy collisions. <\/p>\n But there are some mysterious, unanswered questions. One concerns their origins. We can see SMBHs accreting matter, but the speed at which they acquire mass can\u2019t really explain their size. Some of them are billions of times more massive than the Sun. Did SMBHs have some type of growth spurt in the Universe\u2019s early ages?<\/p>\n What about intermediate-mass black holes (IMBHs.) Are these elusive objects, which may reside in the center of globular clusters, stepping stones to SMBHs?<\/p>\n Black hole jets are also mysterious. These jets are extremely powerful and accelerate matter to extreme speeds. Astrophysicists understand the basics of how SMBHs create these jets. But these jets can reach relativistic speeds and how they do that is unclear. <\/p>\n Since SMBHs are so difficult to observe in detail, scientists rely on theories to explain them. Over time, they try to refine their theories. But sometimes, as our observing power increases, our theories don\u2019t match our observations. This is true of the accretion disks around SMBHs. While theory says these disks should be flat like pancakes, observations show that they\u2019re puffy.<\/p>\n This is where simulations come in.<\/p>\n Detailed simulations are one of astrophysicists\u2019 best tools for understanding SMBHs. New research published in The Open Journal of Astrophysics examines the accretion disks around SMBHs with simulations. These disks are the reservoirs of gas that feed SMBH growth. The research is \u201cFORGE\u2019d in FIRE: Resolving the End of Star Formation and Structure of AGN Accretion Disks from Cosmological Initial Conditions.\u201d The lead author is Philip Hopkins, a professor of Theoretical Astrophysics at Caltech. <\/p>\n \u201cOur new simulation marks the culmination of several years of work from two large collaborations started here at Caltech,\u201d said lead author Hopkins in a press release.<\/p>\n Hopkins is talking about FIRE (Feedback in Realistic Environments) and STARFORGE (Star Formation in Gaseous Environments.) STARFORGE is a small-scale simulator that focuses on how individual stars form in clouds of gas called molecular clouds. FIRE focuses on galaxy formation, including things like black hole feedback and quenching. <\/p>\n FIRE and STARFORGE are on opposite ends of a scale, and the new work fills in the gap between the two.<\/p>\n \u201cBut there was this big gap between the two,\u201d Hopkins explains. \u201cNow, for the first time, we have bridged that gap.\u201d <\/p>\n \u201cIt has recently become possible to zoom in from cosmological to sub-pc scales in galaxy simulations to follow accretion onto supermassive black holes (SMBHs),\u201d the authors write in their research. \u201cHowever, at some point, the approximations used on ISM <interstellar medium> scales (e.g. optically-thin cooling and stellar-population-integrated star formation [SF] and feedback [FB]) break down.\u201d<\/p>\n The physics driving small-scale accretion is different from the physics driving large-scale accretion. \u201cIt is by no means clear what physically occurs when the different physics most relevant on different scales intersect,\u201d the researchers write. <\/p>\n Large-scale simulations are based on things like the collective effects of entire star populations and the initial mass function. Small-scale simulations are based on things like the formation of individual protostars and stellar winds from individual stars. At an even smaller scale, simulations focus on individual aspects of accretion disks around SMBHs. <\/p>\n \u201cAs a result, there have not been simulations that can span all three of these regimes simultaneously and self-consistently,\u201d Hopkins and his co-authors explain. <\/p>\n Bridging the gap wasn\u2019t a simple matter. Hopkins and his fellow researchers needed a simulation with much higher resolution. The resolution had to be over 1,000 times greater than the previous best simulator. <\/p>\n \u201cThis allows us to span scales from ~100 Mpc down to <100 au (~300 Schwarzschild radii) around an SMBH at a time where it accretes as a bright quasar in a single simulation,\u201d the researchers explain in their paper. <\/p>\n Their simulations had a surprise in store. They show that magnetic forces play a larger role in SMBH accretion disks than thought. <\/p>\n Theory shows that the rotating accretion disks around SMBHs should be flat like pancakes. This is due to the conservation of angular momentum and viscous forces in the disk that distribute momentum, keeping the disk flat. But our theories don\u2019t line up with observations. <\/p>\n \u201cOur theories told us the disks should be flat like crepes,\u201d Hopkins says. \u201cBut we knew this wasn\u2019t right because astronomical observations reveal that the disks are actually fluffy\u2014more like an angel cake. Our simulation helped us understand that magnetic fields are propping up the disk material, making it fluffier.\u201d<\/p>\n \n