Chandra telescope finds missing baryonic matter

Scientists studying data from the

Chandra X-ray telescope have

found strong evidence for the missing half of ordinary matter

in the universe. Two immense intergalactic clouds confirm the

existence of a cosmic web of hot gas, predicted by computer

simulations, containing baryons, the material making

up visible objects in the Universe.

"An inventory of all the baryons in stars and gas inside and outside

of galaxies accounts for just over half the baryons that

existed shortly after the Big Bang," Fabrizio Nicastro of the

Harvard-Smithsonian Center for Astrophysics. "Now we have found the

likely hiding place of the missing baryons." Nicastro is lead author

of a paper in the February 3, 2005 issue of

Nature describing the recent research.

Various measurements give a good estimate of the mass-density of

baryons - the neutrons and protons

that make up the nuclei of atoms and ions - in the

Universe 10 billion years ago. However, sometime

during the last 10 billion years a large fraction of the baryons,

commonly referred to as "ordinary matter" to distinguish them

from dark matter and dark

energy, have gone missing.

Using the computer predictions, Nicastro and colleagues

searched specifically for the diffuse system of gas clouds

galaxies and galaxy clusters form within. Evidence of the

clouds had been previously detected around our own galaxy and in

the Local Group, but the temperature range (a few

hundred thousand to a million degrees Celsius) and low density

have prevented detection outside the immediate neighborhood of

galaxies and the reliable estimation of baryon density for the

entire universe.

The clouds were detected using Chandra observations when the

galaxy Mkn 421 brightened in X-ray

emissions. The X-ray spectral data showed two hot gas clouds,

150 million and 370 million light years distant, absorbing the

X-rays from Mkn 421. The data shows the presence of carbon,

nitrogen, oxygen, and neon present in the clouds. Combined with

other data the research team determined the thickness and mass of

the clouds.

Assuming that the size and distribution of the clouds are

representative, Nicastro and colleagues made the first reliable

estimate of average density of baryons in such clouds throughout

the Universe, finding it consistent with the mass density of the

missing baryons.

Mkn 421 was observed three times with Chandra's Low-Energy

Transmission Grating (LETG), twice in conjunction with the High

Resolution Camera (May 2000 and July 2003) and once with the

Advanced CCD Imaging Spectrometer (October 2002). The distance to

Mkn 421 is 400 million light years.