Has ten percent of the mass of the universe disappeared? Not really, but it’s true to say that our assessment of that mass has to be reconsidered, given recent findings on the nature of x-rays emitted from the vast spaces at the heart of galaxy clusters. How we interpret the x-ray data has a great bearing on how we calculate the mass of gases in the galactic clusters, and the mass of the clusters themselves.
The story begins in 2002, when a University of Alabama in Huntsville team studying warm, x-ray emitting gas in galactic clusters reported that it had found large amounts of comparatively low-energy x-rays in addition to higher energy ‘hard’ x-rays. The so-called ‘soft’ x-ray emitting atoms were assumed to exist at a density of one atom per cubic meter, but their cumulative mass was thought to amount to as much as ten percent of that needed to hold galactic clusters together.
But a closer look at data provided by the Chandra X-Ray Observatory, among other instruments, found no emission lines associated with the soft x-rays. Instead of flagging the presence of cooler gas, the energy is now thought to come from the collision of electrons with photons from the cosmic microwave background, converting the photons from low-energy microwaves to high-energy x-rays.
And that’s a problem, for the signal from such electrons would also make up part of the mass of the already observed hard x-rays. Thus our measurements of the gas in galactic clusters may be significantly off. Max Bonamente (UAH) nails the problem:
…the mass of these x-ray emitting clouds is much less than we initially thought it was. A significant portion of what we thought was missing mass turns out to be these ‘relativistic’ electrons.”
In addition, x-rays produced by electrons colliding with photons might mask the emission lines of the hard x-ray energy coming from galaxy clusters, lines which are prominent in iron and other metals. “This is also telling us there is fractionally more iron and other metals than we previously thought,” says Bonamente. “Less mass but more metals.”
The paper is Bonamente, Nevalainen et al., “Soft and Hard X-Ray Excess Emission in Abell 3112 Observed with Chandra,” Astrophysical Journal 668 (October 20, 2007), pp. 796-805 (abstract). So much for what was thought to be ‘missing mass,’ another reason for keeping research into just what holds the universe together on the front line of modern physics.