The Case of the "Missing Baryons

About 10% of the galaxies we see around us appear to be caught in the act of a "starburst." These starbursts sometimes happen in the centers of otherwise fairly normal spiral galaxies, similar to our own Milky Way, and sometimes in galaxies that have no well-defined structure. For some reason, some galaxies or regions within galaxies suddenly begin to form tens or hundreds of suns per year. The closest example of such a galaxy is Messier 82 in the constellation of Ursa Major, visible through a small telescope on a dark winter night.

While the nature of these strange galaxies remains an enigma, the focus of the HUT observations with Astro-2 is not so much on the galaxies themselves, but on the possibility that these galaxies may help to solve one of the great puzzles of cosmology.

If the standard Big Bang model is correct, "baryonic" matter makes up between 1% and 10% of the matter needed to gradually stop the expansion of the universe. Baryonic matter is the familiar protons, neutrons, and electrons that make up the atoms of all the materials on earth and in the stars. Other types of matter, for example neutrinos, or "weakly interacting massive particles" (WIMPS), may provide the remaining mass ("dark matter") necessary to halt the expansion. Or they may not. In either case, astronomers have a problem even accounting for all the baryons. The best estimates are that all the stars, gas, and dust within galaxies constitute at most 40% of the baryons predicted by the Big Bang. Where are the rest?

The most likely place for the rest of the baryons to be hiding is in diffuse gas between the galaxies: the intergalactic medium. Astronomers can estimate the amount of gas in the intergalactic medium by essentially counting up the number of atoms that absorb the light from distant quasars. This number once again falls well short of that required by the Big Bang theory. If the gas is there, many of the atoms must be ionized, that is stripped of some of their electrons, so that they cannot absorb the radiation.

Such observations have sparked a raging debate over what could be ionizing the intergalactic medium. Some astronomers contend that the quasars themselves can ionize most of the material in the universe, while others estimate that the total amount of quasar radiation falls short, or that it does do not have the required spectrum.

This is where starburst galaxies come back into the picture. If all the radiation from the first generations of stars that formed in galaxies were to escape, it could easily ionize the intergalactic gas. However, the dust and gas within these protogalaxies may absorb nearly all the ionizing radiation before it gets out. Whether it actually does depends on how thick the dust is, and how it is distributed among the stars, something that is extremely difficult to calculate from first principles.

In the absence of solid theory, astronomers turn to observations. Because it reaches to short ultraviolet wavelengths, HUT can detect directly the ionizing radiation that escapes from bright, relatively nearby starburst galaxies. The key to the HUT observations is the fact that even the relatively small redshifts of the target galaxies are enough to make the "Lyman limit" of hydrogen visible through the absorbing dust and gas of our own galaxy. Photons that escape from below the Lyman limit, at 912 angstroms, are capable of ionizing hydrogen, the most abundant gas in the intergalactic medium. If even 10% of the ionizing radiation released by the stars in starburst galaxies escapes into the intergalactic medium, it could be a significant contributor to the overall background of ionizing radiation.

By themselves, the HUT observations of starburst galaxies will not solve the problem of the ionization of the intergalactic medium. Nevertheless, by providing a direct measurement of the radiation escaping from starburst galaxies, the observations will help to remove one of the largest uncertainties in trying to understand the physical conditions in the intergalactic medium, putting us one step closer to solving the case of the missing baryons.

Henry C. Ferguson