Stars forming within powerful outflows of material blasted out from supermassive black holes at the center of galaxies with a mass that can be billions of times that of the sun. These are the first confirmed observations of stars forming in this kind of extreme environment. Researchers said their discovery could help us understand how galaxies evolve from scratch.
By using two of the world-leading VLT spectroscopic instruments, MUSE and X-shooter, the group of European astronomers researchers could carry out a very detailed study of the properties of the emitted light to determine its source.
The researchers study an ongoing collision between two galaxies, that lie around 600 million light-years from Earth. The group observed the colossal winds of material — or outflows — that originate near the supermassive black hole at the heart of the pair’s southern galaxy, and have found the first clear evidence that stars are being born within them.
the astronomers say that stars totalling around 30 times the mass of the Sun are being created every year. This accounts for over a quarter of the total star formation in the entire merging galaxy system.
Such galactic outflows are driven by the huge energy output from the active and turbulent centers of galaxies. Supermassive black holes lurk in the cores of most galaxies, and when they gobble up matter they also heat the surrounding gas and expel it from the host galaxy in powerful, dense winds.
The expulsion of gas through galactic outflows leads to a gas-poor environment within the galaxy, which could be why some galaxies cease forming new stars as they age. Although these outflows are most likely to be driven by massive central black holes, it is also possible that the winds are powered by supernovae in a starburst nucleus undergoing vigorous star formation.
“Astronomers have thought for a while that conditions within these outflows could be right for star formation, but no one has seen it actually happening as it’s a very difficult observation,” comments team leader Roberto Maiolino from the University of Cambridge. “Our results are exciting because they show unambiguously that stars are being created inside these outflows.”
Radiation from young stars is known to cause nearby gas clouds to glow in a particular way. The extreme sensitivity of X-shooter allowed the team to rule out other possible causes of this illumination, including gas shocks or the active nucleus of the galaxy.
The group then made an unmistakable direct detection of an infant stellar population in the outflow. These stars are thought to be less than a few tens of millions of years old, and preliminary analysis suggests that they are hotter and brighter than stars formed in less extreme environments such as the galactic disc.
As further evidence, the astronomers also determined the motion and velocity of these stars. The light from most of the region’s stars indicates that they are travelling at very large velocities away from the galaxy center — as would make sense for objects caught in a stream of fast-moving material.
Co-author Helen Russell, Cambridge, UK) expands: “The stars that form in the wind close to the galaxy center might slow down and even start heading back inwards, but the stars that form further out in the flow experience less deceleration and can even fly off out of the galaxy altogether.”
The discovery provides new and exciting information that could better our understanding of some astrophysics, including how certain galaxies obtain their shapes; how intergalactic space becomes enriched with heavy elements; and even from where unexplained cosmic infrared background radiation may arise.
Maiolino is excited for the future: “If star formation is really occurring in most galactic outflows, as some theories predict, then this would provide a completely new scenario for our understanding of galaxy evolution.”