November 5, 2007
Not only has a large chunk of the universe that was thought to have been found in 2002 gone missing, but it is taking some friends with it, according to new research at the University of Alabama in Huntsville (UAH).
The UAH group has discovered that some X-rays thought to have come from intergalactic clouds of warm gas are instead probably caused by lightweight electrons.
If the source of so much X-ray energy is tiny electrons instead of hefty atoms, it is as if billions of lights thought to be from billions of aircraft carriers were instead found to be from billions of extremely bright fireflies.
"This means the mass of these X-ray emitting clouds is much less than we initially thought it was," said Dr. Max Bonamente, an assistant professor in UAH's Physics Department. "A significant portion of what we thought was missing mass turns out to be these relativistic electrons."
Traveling at almost the speed of light, these featherweight electrons collide with photons from the cosmic microwave background. Energy from the collisions converts the photons from low-energy microwaves to high-energy X-rays.
The discovery was made while trying to analyze the makeup of warm, X-ray emitting gas at the center of galaxy clusters — the largest cosmological structures in the universe. In 2002, the UAH team reported finding large amounts of extra soft, relatively low-energy, X-rays coming from the vast space in the middle of galaxy clusters. This was in addition to previously discovered hot gas in that space, which emits hard X-rays with higher energy.
Although the soft X-ray-emitting atoms were thought to be spread less than one atom per cubit meter through space, they would have filled billions of billions of cubic light years. The cumulative mass of the atoms was thought to account for as much as ten percent of the mass and gravity needed to hold together galaxies, galaxy clusters and perhaps the universe itself.
When Bonamente and his associates looked at data gathered by several satellite instruments from a galaxy cluster in the southern sky, they found that energy from those additional soft X-rays doesn't look like it should.
"We have never been able to detect spectral emission lines associated with those detections," Bonamente explained. "If this bump in the data were due to cooler gas, it would have emission lines."
The best, most logical explanation seems to be that a large fraction of the energy comes from electrons smashing into photons instead of from warm atoms and ions, which would have recognizable spectral emission lines.
Finding these electrons, however, is like finding "the tip of the iceberg," said Bonamente, because they would not be limited to emitting only the soft X-ray signal. The signal from these electrons would also make up part of the previously observed harder X-rays, which would reduce the amount of mass thought to make up the hot gas at the center of galaxy clusters.
To further complicate the issue, the energy from these electrons might also puff up the cluster. Previously, astrophysicists used the energy coming from inside these clusters to calculate how much mass is needed to reach the equilibrium seen there. If there was too much mass, the cloud would collapse. Too little and the hot gas cloud would expand.
Since the energy coming from these hot clouds can be accurately measured, it was thought the mass could be calculated with reasonable confidence for astrophysics.
Instead, said Bonamente, if a significant portion of the total X-ray energy comes from fast electrons, "that could trick us into thinking there is more gas than is actually there."
If part of the hard X-ray energy comes from electrons and photons, it might also shift what we think is the mix of elements in the universe. Outside of the excess soft X-rays, the X-ray energy coming from galaxy clusters has emission lines, which are especially prominent around iron and other metals.
Non-thermal X-rays from electrons colliding with photons might mask those emission lines, like thick snow can mask the height of fence posts.
"This is also telling us there is fractionally more iron and other metals than we previously thought," said Bonamente. "Less mass but more metals."