A team of researchers, including astronomers from Michigan State University, has uncovered a process that may explain how the universe's largest elliptical galaxies continue making stars long after their peak years of star birth.
Images from NASA's Hubble Space Telescope allowed the astronomers to see brilliant knots of hot, blue stars forming along the jets of active black holes found in the centers of these galaxies.
Combining the Hubble data with observations from ground-based and space telescopes, two research teams found that the black holes, jets and newborn stars are all parts of a self-regulating cycle. High-energy jets shooting from the black hole heat a halo of surrounding gas, controlling the rate at which the gas cools and falls into the galaxy.
Think of the gas surrounding a galaxy as an atmosphere, said Megan Donahue, an MSU physics and astronomy professor who led one of the studies.
"That atmosphere can contain material in different states, just like our own atmosphere has gas, clouds and rain,” she said. “What we are seeing is a process like a thunderstorm. As the jets propel gas outward from the center of the galaxy, some of that gas cools and precipitates into cold clumps that fall back toward the galaxy's center like raindrops."
The "raindrops" eventually cool enough to become star-forming clouds of cold molecular gas.
But what should be a monsoon of raining gas is reduced to a mere drizzle by the black hole. While some outwardly flowing gas will cool, the black hole heats the rest of the gas around a galaxy, which prevents the whole gaseous envelope from cooling more quickly.
This discovery explains the mystery of why many elliptical galaxies in the present-day universe are not ablaze with a higher rate of star birth. For many years, the question has persisted of why galaxies awash in gas don't turn all of that gas into stars. Theoretical models of galaxy evolution predict that present-day galaxies more massive than the Milky Way should be bursting with star formation, but that is not the case.
Now scientists understand this is simply arrested development, where a cycle of heating and cooling keeps star birth in check.
The study led by Donahue looked at far-ultraviolet light from a variety of massive elliptical galaxies found in the Cluster Lensing and Supernova Survey with Hubble, or CLASH, which contains elliptical galaxies in the distant universe. These included galaxies that are raining and forming stars, and others that are not.
The researchers were aided by a new set of computer simulations of the hydrodynamics of the gas flows developed by Yuan Li of the University of Michigan and a team of collaborators including MSU physics and astronomy professors Mark Voit and Brian O'Shea.
"These are the first simulations to show in detail how black-hole jets can trigger showers of star formation," Voit said. "And when we compare these new models to the Hubble data, there's a stunning similarity between the star-forming showers we observe and ones that occur in our simulations.”