While approximately 80 percent of all matter in the Universe is in the form of dark matter, a substantial fraction of ordinary matter is also "missing." The total amount of matter produced in the Big Bang is a known quantity from a variety of observations and calculations, but it doesn't seem to reside in the stars, gas, and dust that form galaxies. The most widely accepted models postulate that it either surrounds galaxies in a hot cloud, or was ejected into the intergalactic medium (IGM)—the regions of space between galaxies. Previous observations of distant galaxies have backed up the second assertion.

A new set of observations performed with the orbiting Chandra X-ray Observatory may have revealed the presence of a large cloud of hot gas surrounding the Milky Way. A. Gupta, S. Mathur, Y. Krongold, F. Nicastro, and M. Galeazzi measured absorption features from the region beyond the densest parts of our galaxy and found a signal consistent with ionized oxygen atoms with temperatures around a million degrees. Based on the thickness of the gas cloud, the researchers estimated its mass to be equivalent to 10 billion Suns—roughly the same amount as all the ordinary matter in the galaxy's disk.

A variety of observations, including precision measurements of the cosmic microwave background left over from the Big Bang, have led to a fairly complete picture of the contents of the Universe. In particular, we know how much ordinary—or baryonic—matter there is. Since those atoms could not have disappeared, they must be present in the modern Universe as well, but they don't seem to be part of the disk or bulge that make up the luminous regions of galaxies. Figuring out what happened to them has become known as the "missing baryon problem."

Theoretical models predicted that these atoms may have been ejected from the central parts of galaxies into the circumgalactic medium (CGM), a region that's outside the luminous parts, but still within the galaxy's gravitational pull. Alternately, they could have been thrown into the IGM. In either case, the atoms would have relatively high temperature but low density, which makes them more difficult to find. Attempts to spot emission and absorption from atoms at temperatures around 100,000 degrees (105°C) failed to locate the missing baryons.

Spiral Galaxy Structure

Spiral galaxies like the Milky Way are comprised of three major regions. The disk is the bit containing the spiral arms, most of the brightest stars, and the spare gas and dust. The bulge has the central supermassive black hole and older stars. The halo surrounds the galaxy and contains most of the galaxy's mass in the form of dark matter. While there are many stars in the halo, they are distributed over a huge volume. The circumgalactic medium (CGM) described in the main text is part of the halo.



The present study focused on higher temperatures. At these temperatures (106 to 107°C), atoms such as oxygen or silicon can lose several electrons due to their high-energy collisions. This places the light they emit and absorb strongly into the X-ray portion of the spectrum. For this reason, the researchers used data from Chandra observations of quasars: extremely bright jets of light from supermassive black holes in distant galaxies. These objects are intense X-ray sources, so any hot, ionized gas between the quasar and Earth would create absorption features in the quasar's spectrum.

The researchers looked specifically at absorption by oxygen atoms missing six or seven electrons, known as O VII and O VIII. (The Roman numeral is always one greater than the number of missing electrons. Neutral oxygen, with all its electrons, is known as O I; if it loses a single electron, it is known as O II; and so on.) They found clear absorption features in 21 out of 29 quasars where the data were sufficiently clean.

A major part of the analysis involved showing the absorption was from atoms near the Milky Way, as opposed to more distant gas clouds. After all, these observations don't show the gas itself, but its spectral shadow, so the researchers had to reconstruct its approximate distance from Earth and the thickness of the gas cloud. Their results indicated the oxygen was quite close to the Milky Way and very spread out.

If the atoms were uniformly distributed around the galaxy, then they would make a sphere over 600 thousand light-years across, about six times the diameter of the Milky Way's disk. Since the atoms are almost certainly not in a uniform cloud, the actual size of the cloud can't be known, so the researchers weren't able to determine whether the oxygen was in the CGM or the IGM. Either way, it is consistent with theoretical predictions.

Using the density of the cloud (again based on absorption) and assuming its shape is approximately spherical, the researchers calculated its mass to be at least 12 billion times the mass of the Sun. That's comparable to the mass of all the gas and stars in the Milky Way's disk—and similar to the mass of the "missing" baryons.

To determine whether these oxygen atoms are truly part of the missing baryons, the full shape and size of the cloud will need to be determined. That would provide a better estimate of the mass. However, the existence of a hot cloud surrounding the Milky Way is a significant discovery, and could very well help resolve the case of the missing atoms—a long-standing problem in astrophysics.

Astrophysical Journal Letters, 2012. DOI: 10.1088/2041-8205/756/1/L8 (About DOIs).