Project FIRE Uses Star Feedback To Explain Less Massive Galaxies

A puzzling contradiction has challenged astrophysicists for decades: many galactic wind models—which are simulations of how matter is distributed in the universe—predict that most of the “normal” matter exists in stars at the center of galaxies. However, these stars actually only account for less than 10 percent of the matter in the universe. Now, new simulations have revealed insight into this mismatch between models and reality, highlighting that the energy released by individual stars within galaxies can have a substantial effect on where matter is located in the universe.

As the result of a multiyear, multi-university effort, the Feedback in Realistic Environments (FIRE) project simulates the evolution of galaxies from shortly after the Big Bang through today. FIRE is the first simulation to factor in the realistic effects of stars on their galaxies, suggesting that the radiation from stars is powerful enough to push matter out of galaxies.

Philip Hopkins, assistant professor of theoretical astrophysics at the California Institute of Technology (CalTech), says that the push is enough to account for the “missing” galactic mass in previous calculations. The findings were published in a recent issue of Monthly Notices of the Royal Astronomical Society.

“People have guessed for a long time that the ‘missing physics’ in these models was what we call feedback from stars,” Hopkins says. “When stars form, they should have a dramatic impact on the galaxies in which they arise, through the radiation they emit, the winds they blow off of their surfaces, and their explosions as supernovae. Previously, it has not been possible to directly follow any of these processes within a galaxy, so the earlier models simply estimated—indirectly—the impact of these effects.”

Hopkins and his team incorporated the data of the individual stars into whole-galaxy models, which enabled them to look at the actual effects of star feedback—specifically, how radiation from stars “pushes” on galactic matter—on each of the galaxies they observed. The team will be able to focus their model on specific galaxies, using what are called zoom-in simulations, thanks to new and improved computer codes.

“Zoom-in simulations allow you to ‘cut out’ and study just the region of the universe—a few million light-years across, for example—around what’s going to become the galaxy you care about,” he says. “It would be crazy expensive to run simulations of the entire universe—about 50 billion light-years across—all at once, so you just pick one galaxy at a time, and you concentrate all of your resolution there.”

The researchers used the zoomed-in view of evolving stars within galaxies to see the radiation from stars and supernovae explosions blowing large amounts of material out of those galaxies. When the team calculated the amount of matter lost from the galaxies during these events, they found that the stars’ feedback in the simulation accurately accounts for the low masses that have been actually observed in real galaxies.

“The big thing that we are able to explain is that real galaxies are much less massive than they would be if these feedback processes weren’t operating,” he says. “So if you care about the structure of a galaxy, you really need to care about star formation and supernovae—and the effect of their feedback on the galaxy.”

The team questioned where the matter goes after it is pushed out of the galaxy by the stars. They hope that by combining their simulations with new observations in the coming months, they will begin to find answers.

“Stars and supernovae seem to produce these galactic superwinds that blow material out into what we call the circum- and intergalactic medium—the space around and between galaxies. It’s really timely for us because there are a lot of new observations of the gas in this intergalactic medium right now, many of them coming from Caltech,” Hopkins says.

“For example, people have recently found that there are more heavy elements floating around a couple hundred thousand light-years away from a galaxy than are actually inside the galaxy itself. You can track the lost matter by finding these heavy elements; we know they are only made in the fusion in stars, so they had to be inside a galaxy at some point. This fits in with our picture and we can now actually start to map out where this stuff is going.”

The FIRE simulations are able to accurately account for the low mass of small- to average-size galaxies. The physics included, as in the previous models, is unable to explain all the missing mass in very large galaxies like those larger than our own Milky Way galaxy. The team’s theory is that black holes at the centers of these large galaxies might release enough energy to push out the rest of the matter not blown out by stars. “The next step for the simulations is accounting for the energy from black holes that we’ve mostly ignored for now,” he says.

The FIRE simulation data reveals that feedback from stars can alter the growth and history of galaxies in a much more dramatic way than anyone had previously anticipated. Hopkins said, We’ve just begun to explore these new surprises, but we hope that these new tools will enable us to study a whole host of open questions in the field.”