Chandra telescope finds missing baryonic matter

February 4, 2005

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.