HUT program H02 is the premiere science program for Astro-2, and embodies the heart of the original science goal envisioned for the telescope when it was first proposed back in 1978. The goal of this program is grand indeed--to study the vast but exceedingly tenuous regions of space between the galaxies using one or more distant, high redshift quasars as a background source of light, to better understand the conditions in the medium from which all of the structure in the universe subsequently formed. This intergalactic medium (IGM) is thought to consist almost entirely of hydrogen and helium that was the first material to form in the cooling phase after the Big Bang that created our expanding universe.
This medium has proven very difficult to detect or study. No evidence has been found for the expected hydrogen, leading to speculation that the conditions must be such that the hydrogen is all ionized. Helium atoms are harder to ionize; partially ionized helium would reveal itself as a telltale absorption at certain extreme UV wavelengths, not only confirming the presence of the IGM, but also providing information on its temperature and density. As we look at high enough redshifts, these EUV absorptions will get redshifted into the far-ultraviolet portion of the spectrum above 912 angstroms where they can be observed with HUT. The same measurement can (in principle) by made by the Hubble Space Telescope, but only for higher redshift objects that shift the helium absorption above about 1216 angstroms.
The problem then becomes finding an appropriate high redshift quasar to use as a background source against which to measure this helium absorption. Even though a large number of quasars with appropriate redshifts for this study are known, almost all of these fade to invisibility before reaching the wavelengths where the helium absorption could be observed. It is a testament to the incredible distances of these high redshift objects that their light gets completely absorbed by an intervening medium whose density is many orders of magnitude less than the best vacuum we can create on Earth! The higher redshift (and hence much more distant) objects needed to make this observation with HST also, then, stand a considerably higher probability of being completely absorbed at the appropriate wavelengths. The shorter UV wavelengths available to HUT could make a crucial difference in detecting the IGM.
There are only a couple of quasars known with redshifts that place
the expected helium absorption in the HUT primary 900 - 1200
wavelength range AND that still have detectable UV background light
at the shortest UV wavelengths observed to date (as observed by HST
or the IUE satellite). The brighter of these, an object called
HS1700+64, is the primary target of this program. Even so, it is
expected to be very near the faint detection limit of HUT and will
require multiple pointings during the mission to gather sufficient
data to make this measurement. Although a fledgling attempt to
perform this observation was made during Astro-1, the project had
to be abandoned because of the pointing problems and other
difficulties encountered on that mission. With HUT's improved
sensitivity, hopes are high that data from Astro-2 will provide a
definitive measurement of this elusive component of the early
universe.
