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The SWS and LWS will be used to measure the brightest cooling lines of shocked
neutral gas in the prototypical ultraluminous infrared galaxies (ULIRGs)
Arp220 and NGC6240. In ground-based observations, we have found that the H2
emission peaks between the two remnant stellar nuclei of the merging galaxies.
This is a result of dissipation in shocks in the gas, which makes the gas
components merge before the dissipationless stellar components merge. The
shock-excited line emission is the direct signature of this dissipation and
can be used to trace the flow of molecular gas towards the bottom of the
potential well. ULIRGs are suspected to harbour dust-embedded quasars, formed
in their dusty centers. The immense concentrations of dissipative material, in
a deep potential well, may form ideal birth places for active nuclei, by
building up a massive black hole, or by fueling a pre-existing seed black
hole. Aim of the SWS/LWS observations is a determination of the total energy
radiated away in line emission by the shocks, thereby measuring the rate at
which mechanical energy is being dissipated. In addition the shock velocities
involved will be determined. This will allow us to measure the rate at which
the molecular gas flows towards the bottom of the potential well. The line
cooling is dominated by H2, CO, H2O and OH rotational lines and H2 vibrational
lines. Of these, only the vibrational lines can be measured from the ground.
Our proposal is to measure the brightest rotational lines of the 4 species
listed above with ISO. Since the central regions of ULIRGs will form the birth
places of extreme starbursts or luminous active nuclei, the programme will
have implications on the formation of active nuclei and extreme starbursts in
general.