Simulating Galactic Outflows
Friday, December 20, 2019
Science Update - A look at CfA discoveries from recent journals

Astronomers have known for decades that massive outflows of gas are being ejected from galaxies. These fast-moving, bipolar streams act to slow down the rate of star formation and inhibit the gravitational collapse of the galaxy, and they help to counterbalance the inflow of material from the intergalactic medium. Two physical mechanisms power these outflows, supernovae explosions in star-forming regions and winds produced in the vicinities of the central supermassive blackholes as they accrete material. Understanding these processes is essential to understanding how galaxies develop, but attempts using numerical simulations have been stymied for decades because both star formation and black hole accretion operate at small scales,roughly ten billion times smaller than the scale of the whole galaxy and its host environment. It is computationally very challenging to model both large-scale and small-scale processes with the same code. As a result, cosmological simulations of galaxy evolution developed over the years have not been able to be compared directly to observations of outflows.

The Illustris project is an international collaboration that has been producing simulated galaxy evolution scenarios for over five years. The smallest sizes in its simulations are about 2300 light-years and, to describe processes occurring in volumes smaller than that,the code invokes a generic algorithm rather than perform detailed calculations.The project has been extremely successful in being able to reproduce the vast cosmological web of galaxies that developed after the big bang. IllustrisTNG ("the next generation") is a new version of the Illustris simulation project that partially addresses the scale problem by focusing on detailed consideration of selected small volumes while still capturing the essential large-scale processes. The IllustrisTNG50 simulation, the third and final version in this series, simulates activity in dimensions as small as hundreds of light-years in an overall volume fifty million parsecs (163 million light-years) on a side, offering a unique combination of both large volume and fine resolution.

CfA astronomers Rainer Weinberger and Lars Hernquist are members of the TNG50 team that has just published its first results. As galaxies become more massive, the outflow rate compared to the star formation rate decreases. However, in moderately large systems, this trend reverses because of the increased influence of the supermassive black hole winds. The scientists also report finding that the outflows are naturally collimated into bipolar shapes, and that the wind velocities increase with galaxy mass to speeds in excess of three thousand kilometers per second. Not least, although galaxies undergoing more active star formation drive faster winds in general, in high mass galaxies in which star formation has been suppressed the winds remain strong because of the activity of the accreting black holes.


"First Results from the TNG50 Simulation: Galactic Outflows Driven by Supernovae and Black Hole Feedback," Dylan Nelson, Annalisa Pillepich, Volker Springel, Rudiger Pakmor, Rainer Weinberger, Shy Genel, Paul Torrey, Mark Vogelsberger, Federico Marinacci, and Lars Hernquist, MNRAS 490, 3234, 2019.