How black holes shape the cosmos

Visualization of the intensity of shock waves in the cosmic gas (blue) around collapsed dark matter structures (orange/white). Similar to a sonic boom, the gas in these shock waves is accelerated with a jolt when impacting on the cosmic filaments and galaxies.

Every galaxy harbours a supermassive black hole at its center. A new computer model now shows how these gravity monsters influence the large-scale structure of our universe. The research team includes scientists from the Heidelberg Institute for Theoretical Studies (HITS), Heidelberg University, the Max-Planck-Institutes for Astronomy (MPIA, Heidelberg) and for Astrophysics (MPA, Garching), US universities Harvard and the Massachusetts Institute of Technology (MIT), as well as the Center for Computational Astrophysics in New York. The project, "Illustris -- The Next Generation" (IllustrisTNG), is the most complete simulation of its kind to date. Based on the basic laws of physics, the simulation shows how our cosmos evolved since the Big Bang. Adding to the predecessor Illustris project, IllustrisTNG includes some of the physical processes which play a crucial role in this evolution for the very first time in such an extensive simulation. First results of the IllustrisTNG project have now been published in three articles in the journal Monthly Notices of the Royal Astronomical Society. These findings should help to answer fundamental questions in cosmology.

A realistic universe out of the computer

At its intersection points, the cosmic web of gas and dark matter predicted by IllustrisTNG hosts galaxies quite similar to the shape and size of real galaxies. For the first time, hydrodynamical simulations could directly compute the detailed clustering pattern of galaxies in space. Comparison with observational data -- including newest large surveys -- demonstrate the high degree of realism of IllustrisTNG. In addition, the simulations predict how the cosmic web changes over time, in particular in relation to the underlying "back bone" of the dark matter cosmos. "It is particularly fascinating that we can accurately predict the influence of supermassive black holes on the distribution of matter out to large scales," says principal investigator Prof. Volker Springel (HITS, MPA, Heidelberg University). "This is crucial for reliably interpreting forthcoming cosmological measurements."

The most important transformation in the life cycle of galaxies

In another study, Dr. Dylan Nelson (MPA) was able to demonstrate the important impact of black holes on galaxies. Star-forming galaxies shine brightly in the blue light of their young stars until a sudden evolutionary shift ends the star formation, such that the galaxy becomes dominated by old, red stars, and joins a graveyard full of "red and dead" galaxies. "The only physical entity capable of extinguishing the star formation in our large elliptical galaxies are the supermassive black holes at their centers," explains Nelson. "The ultrafast outflows of these gravity traps reach velocities up to 10 percent of the speed of light and affect giant stellar systems that are billions of times larger than the comparably small black hole itself."

Where the stars sparkle: New findings for the structures of galaxies

IllustrisTNG also improves researchers´ understanding of the hierarchical structure formation of galaxies. Theorists argue that small galaxies should form first, and then merge into ever larger objects, driven by the relentless pull of gravity. The numerous galaxy collisions literally tear some galaxies apart and scatter their stars onto wide orbits around the newly created large galaxies, which should give them a faint background glow of stellar light. These predicted pale stellar halos are very difficult to observe due to their low surface brightness, but IllustrisTNG was able to simulate exactly what astronomers should be looking for in their data. "Our predictions can now be systematically checked by observers," Dr. Annalisa Pillepich (MPIA) points out, who led a further IllustrisTNG study. "This yields a critical test for the theoretical model of hierarchical galaxy formation."

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