A principal scientific goal for HUT on the Astro-2 Mission will be an attempt to detect the primordial intergalactic medium, a tenuous gas believed to pervade the vast reaches of space between the galaxies. This gas - composed almost entirely of hydrogen and helium, the two lightest elements - is the material that was formed during the first few minutes after the Big Bang, some 10 to 20 billion years ago. As the Universe expanded, the gas eventually cooled enough to allow individual protons and electrons to combine to form ordinary hydrogen atoms. Meanwhile, alpha particles captured two electrons each to form atomic helium. All of the galaxies that exist today (and everything in them) are thought to have condensed from this primordial gas sometime later.
There is at least one key thing that has long been missing in this remarkable picture of the origin and evolution of the Universe, however. No one has ever directly observed the primordial intergalactic medium - until now.
The best method to find such gas is to observe very distant quasars, objects so luminous that they can be seen across billions of light-years of intergalactic space. The intervening gas should reveal itself by producing certain tell-tale absorption features in the spectra of distant quasars. Discrete clouds of hydrogen have in fact been discovered in this way, but astronomers have searched for the expected absorption from diffuse hydrogen for the past 30 years and have not yet found it. The extremely low limits that have been placed on the amount of intergalactic atomic hydrogen have led most astronomers to conclude that the primordial hydrogen must have become very highly ionized again, rendering it invisible. When a hydrogen atom loses its single electron through ionization it can no longer absorb radiation effectively.
How could the intergalactic gas have become re-ionized? It is widely believed that the energy radiated by the quasars themselves, perhaps along with radiation from the first generation of galaxies and stars, was responsible for re-ionizing the intergalactic hydrogen gas. If so, however, it might still be possible to observe absorption produced by primordial intergalactic helium. This is because a helium atom has two electrons that are more tightly bound to its nucleus, and it is therefore more difficult to ionize completely. Even a trace amount of singly-ionized helium, consisting of a helium nucleus (an alpha particle) that retains one of its two electrons, could produce detectable absorption of the light of a distant quasar. The expected absorption feature would occur in the extreme ultraviolet part of the spectrum, and therefore can only be detected by an ultraviolet telescope orbiting high above Earth's atmosphere. The exact wavelength that must be studied depends on the redshift of the quasar, which is related to its distance from us.
A report that appeared in the British journal Nature in July 1994 indicates that intergalactic helium may have been detected at last. The evidence came from ultraviolet observations of a quasar made with the Hubble Space Telescope. The article was authored by a group led by Dr. Peter Jakobsen of the European Space Agency, who was a visitor in the Johns Hopkins Center for Astrophysical Sciences last Spring when the work was carried out. At this point the result is still somewhat tentative, however, as other explanations for the absorption they detected are possible. Observations of two dozen other quasars by Jakobsen's group have not yielded results that can either confirm or refute this single successful observation, because the ultraviolet spectra of these quasars are heavily contaminated with absorption by foreground galaxies that happen to lie along the line of sight to each of them. The quasars observed with Hubble are so far away that it was expected that most or perhaps all of them would suffer from this problem.
The Hopkins Ultraviolet Telescope is well-suited to the task of observing intergalactic helium. In fact, it was originally conceived with this goal in mind. HUT has a unique design that enables it to observe shorter ultraviolet wavelengths than those that can be studied with Hubble and previous UV telescopes. As a result, it can be used to search for helium absorption in quasars whose redshifts (and hence distances) are somewhat smaller than those where Hubble may be used. Such quasars are generally brighter, but more importantly, they are much less likely to suffer foreground absorption by intervening galaxies. They are therefore much more likely to present a clear, unambiguous case for (or against) the existence of intergalactic helium.
HUT's prime target will be a quasar known as HS1700+64. Observations of this quasar were actually attempted with HUT on the Astro-1 mission. They were unsuccessful, however, due to difficulties experienced with the instrument pointing system on the shuttle, which could not be aimed precisely enough to find the faint quasar. Improvements to the pointing system should allow the necessary observations to be made on Astro-2. Furthermore, improvements made to HUT since its first flight should enable it to study such faint objects more readily.
We thus have high hopes that HUT will be able to establish firmly the existence of the long-sought primordial intergalactic medium. HUT may also be able to measure the density and degree of ionization of such gas, and perhaps study its evolution with time, since smaller redshifts correspond to more recent epochs in the history of the Universe. Such observations will place extremely important constraints on cosmological theory and help to elucidate the missing links between the Big Bang and the formation of the galaxies we see today.
At last, we may be on the verge of seeing the stuff from which we, and everything else in our Universe, were made. Astro-2 promises to be an exciting mission indeed.
Arthur F. Davidsen