One of the fundamental questions astronomers are trying to answer is: What is the Universe made of? Numerous lines of evidence show that the Universe is about 73 percent "dark energy," 23 percent "dark matter," and only 4 percent normal matter. Yet this answer raises further questions, including: Where is all the normal matter?

Astronomers call this dilemma the "missing mass" problem. They can see normal, baryonic matter -- protons, electrons, and neutrons -- when it forms luminous stars, or when it blocks starlight as huge, dark molecular clouds. And what they see totals only a fraction of the normal matter they know is out there.

Now, astronomer Fabrizio Nicastro of the Harvard-Smithsonian Center for Astrophysics (CfA) and colleagues have found evidence for the existence of a large reservoir of baryons in our Local Group of galaxies. This baryonic matter forms a warm fog surrounding and enveloping the Milky Way and its neighbors.

"Our research shows that this warm fog may hold as much as two-thirds of the normal matter within the neighborhood of the Milky Way," says Nicastro.

Finding the missing mass
This warm intergalactic fog is a challenge to find. Astronomers cannot see it directly because it is too diffuse, despite its temperature of 100,000 to 10 million Kelvin (10^5 -- 10^7 K), which causes it to shine faintly in X-rays. Instead, they detect the fog using the shadow it casts. Nicastro and his team looked at ultraviolet and X-ray wavelengths where the intergalactic fog absorbed light from distant sources like quasars and active galactic nuclei. They culled data from the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite to identify about 50 clouds, or fog banks, surrounding our galaxy in every direction.

Atoms in individual clouds absorb light at specific wavelengths, creating dark lines in the spectra of background light sources. The motion of a cloud shifts the wavelength of its spectral line due to the Doppler effect. Nicastro's team used these spectral shifts to derive radial (line-of-sight) velocities for the clouds, giving clues to the clouds' locations and origins. Those studies showed that the warm clouds were almost certainly part of the Local Group of galaxies, which is comprised of the Milky Way and Andromeda spirals, along with about 30 smaller galaxies.

Given the amount of material they detected using FUSE and NASA's Chandra X-ray Observatory, Nicastro and his associates infer that the warm fog in the Local Group contains as much mass as a million million (10^12) Suns. This result shows remarkable agreement with the amount of matter needed to gravitationally bind together the galaxies within the Local Group.

A relic of galaxy formation
"Given the fact that this warm fog exists, it raises the question of where this matter came from," says Nicastro. "Most likely, it is material left over from the galaxy formation process, a relic from the early history of the Universe."

Theories indicate that the early Universe was filled with a nearly homogeneous mix of hydrogen and helium gas. Clumps of dark matter within this primordial soup acted as seeds for galaxy formation. Over several hundred million years of time, the force of gravity pulled together some of the Universe's normal matter to form galaxies holding billions of stars.

However, only about one-third of the Universe's baryonic matter was consumed. Much of it still floats between the galaxies, invisible except for the shadow it casts.

"Finding this leftover material provides further evidence that our theories of galaxy formation are correct and offers clues to the history of our own Milky Way galaxy," says Nicastro. "This discovery, combined with future research, also may help track dark matter because the intergalactic filaments of baryonic matter should connect the dark matter clumps."

This research was reported in the February 12, 2003, issue of the scientific journal Nature in a paper authored by Fabrizio Nicastro (CfA); Andreas Zezas and Martin Elvis (CfA); Smita Mathur (Ohio State University); Fabrizio Fiore (Osservatorio Astronomico di Monteporzio); Cesare Cecchi-Pestellini, Douglas Burke, Jeremy Drake, and Piergiorgio Casella (CfA).

Headquartered in Cambridge, Massachusetts, the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists organized into six research divisions study the origin, evolution, and ultimate fate of the universe.