Ultraluminous Infrared Galaxies
The process of galaxy formation is a central element of the evolution of
the universe. It is now apparent that collisions between gas-rich
galaxies play a pivotal role in the star formation history of the
universe, and in determining the properties of the galaxies we see in the
local universe. When two galaxies collide, large-scale momentum exchange
occurs and substantial masses of gas and dust settle into the middle of
the gravitational potential well of the merging system. The resulting
dense nuclear concentration is an ideal environment for star formation,
and the resulting fireworks generate bolometric luminosities which rival
those of quasars. These galactic nuclei host frequent supernova
explosions and a bewilderingly complex mixture of molecular gas, dust,
high velocity outflows, and in many cases, supermassive black holes
revealed by the presence of a quasar nucleus.
Principal obstacles to the detailed study of these sysytems are the
extreme absorption and obscuration at most wavelengths, and the small
angular extent of the regions where all the action is occurring. By using
centimeter-wavelength radio astronomy, and VLBI, Haystack astronomers can
see through the dust and gas to probe conditions inside the maelstrom. In
the nearby merging galaxy system Arp 220, the radio signatures of 50
supernova explosions can be traced, and four new explosions have been
observed in only a 12-month period. The same observations can also reveal
the detailed properties of clouds of OH molecules, tracing the
distribution and dynamics of the molecular gas on parsec scales.
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