HUT programs H03 - H06 all deal with understanding various aspects of the active galaxy phenomenon. It is widely believed today that quasars and active galaxies are powered by supermassive black holes which produce copious luminosity even as they gobble their prey. Quasars (or QSOs) are thought to be the most extreme (both in distance and luminosity) form of the phenomenon. Some quasars are observed to have associated "fuzz" which is believed by some to be light from a surrounding galaxy or proto-galaxy. Lower redshift objects sometimes show clear galactic structures surrounding very bright central regions, leading to the term active galactic nuclei, or AGN. This leads to a picture whereby AGN may be a closer and less luminous manifestation of the same phenomenon occurring in quasars, and could even represent an evolutionary step to normal galaxies.
The picture developed for these objects includes the idea that a disk of accreting material forms around the central black hole, an idea that is supported by some tantalizing glimpses of the outer portions of such structures provided by the Hubble Space Telescope. It is the goal of program H03 to test this picture for distant AGN and QSOs by searching for the spectral shapes predicted by theories of these "accretion disks". Results from HUT on Astro-1 have already provided some evidence for accretion disks in a few key objects, and HUT scientists have actively pursued the generation of more sophisticated computer models to help interpret such data. Because each object is unique, observations of additional objects on Astro-2 will permit these ideas to be tested more thoroughly.
Programs H04 and H05 study two additional kinds of active galaxies called Seyfert 1 and Seyfert 2 galaxies. The numbers are indicative of an observational classification scheme, and some scientists think that the two types are basically the same except for the line-of- sight geometry to the central black hole. These galaxies are relatively nearby, which permits the black hole/accretion disk paradigm to be studied at a more detailed level than for the more distant examples. For instance, some Seyfert galaxies show evidence for cones of radiation which are thought to be collimated by an opaque torus of cool gas that surrounds the black hole and the accretion disk.
Astro-1 HUT observations were obtained for the brightest members of the Seyfert 1 and 2 classes, NGC 4151 and NGC 1068, respectively. The spectrum of NGC 4151 showed a broad line of O VI never before observed for such a low redshift Seyfert, and showed evidence for absorption in the far-UV spectrum thought to arise in the obscuring torus itself. This galaxy will be re-observed on Astro-2, using multiple pointings spread through the mission to study the variability of the UV emission on timescales of days to weeks. The spectrum of NGC 1068 showed many narrow emission lines, including several in the 900 - 1200 A range that had not been observed previously. These new lines, in conjunction with the data above 1200 A, implied a significant contribution from heating by shock waves a process once proposed to be important but which had fallen out of favor as the leading cause of the emission. During Astro-2, pointings at several positions near the active nucleus of NGC 1068 are planned to try and isolate the region responsible for this emission. One additional member of each class is slated for observation, although even these "next best" candidates are considerably fainter and more difficult to observe.
BL Lacertae objects (H06), named after the prototype of the class, are thought to be active galaxies where our line-of-sight is nearly aligned with a relativistic jet of particles streaming out of the nucleus. HUT astronomers hope to observe the brightest available member of this class to round out their sampling of active galaxies.
Program G13 involves HUT observations of yet another class of active galaxy, although the motivation of this program is somewhat different. Starburst galaxies is a term used to describe a class of galaxies that show evidence for very intense on-going star formation activity. The HUT spectra of these objects will be useful in comparison with HUT spectra of other AGN and normal galaxies to understand the differences in the types of stars in each class of object. However, the primary motivation is more closely associated with the H02 IGM program.
The IGM should consist almost entirely of hydrogen and helium that were the first elements formed after the Big Bang. The main way the hydrogen in the IGM could have escaped detection is if it is highly ionized . If this is the case, then some source (or sources) of high energy light must have been available in the early universe that caused this ionization to occur. The source of this energy has been debated. One source might be the light from the quasars themselves. However, another substantial source might have been the UV light from bursts of star formation as galaxies were forming early in the universe, IF some fraction of this light could escape from the local region of star formation and into the IGM.
It is the primary goal of program G13 to test this idea by looking at
(relatively) nearby starburst galaxies. Photons of light capable of
ionizing hydrogen must have wavelengths shorter than 912
angstroms, which is a region of the spectrum normally unobservable
due to absorption by hydrogen gas in our own galaxy. However, the
galaxies in program G13 are sufficiently redshifted that the region
below 912 angstroms in the rest frame of these galaxies is
observable in the HUT spectrum above 912 angstroms. Hence, the HUT
spectra will be searched carefully to investigate whether a
substantial fraction of the light from the hot stars in these galaxies
can "leak out" at short enough wavelengths to ionize hydrogen. If so,
and if these relatively nearby galaxies are similar to the starburst
galaxies in the early universe, they could have been substantial
contributors to the ionization of the IGM.
