Title:  An icy view of ULIRGs 

Henrik Spoon
Kapteyn Institute, Groningen, The Netherlands


Abstract:
 
Ultra-Luminous InfraRed Galaxies (ULIRGs) are mergers of gas rich disk
galaxies, with infrared luminosities exceeding 10^12 solar luminosities.
Responsible for this huge luminosity is a combination of vigorous star 
formation and AGN activity, both of which are fueled by an enormous 
concentration of molecular gas in the central region. The question as to
what power source dominates, on average, the luminosity  of ULIRGs has
been a matter of intense debate. Depending on the wavelength  range used
to probe the nuclear power sources, a different conclusion  may be
reached. With the advent of ISO, powerful mid-infrared spectroscopic 
tools have become available to probe the conditions of dust and gas in 
these nuclei. These conditions are distinctly different for both
environments, leading to spectral characteristics which can be easily
discerned.  A mid-infrared starburst spectrum is dominated by PAH
emission features and shows low-ionization fine structure lines on a
continuum which rises steeply beyond 10 microns. In contrast, a
mid-infrared AGN spectrum shows  high-ionization fine structure lines
and, depending on whether there is a  clear line of sight to AGN-heated
hot dust, a strong, slowly rising, hot  dust continuum. These
mid-infrared spectral characteristics have been exploited in several
studies to determine the dominant power source in  ULIRGs. 

Recently, we have detected the 6.0 micron absorption band of water ice
in the ISO spectra of 18 (ultra-)luminous infrared galaxies. The detection 
of water ice confirms that cold molecular gas is able to survive in these 
highly active power houses. Among the three main galaxy types in my galaxy 
sample, the incidence of water ice is highest in ULIRGs. 
We also obtained high resolution M-band spectra of the nucleus of the nearby 
starburst/seyfert-2 galaxy NGC4945 and detected `XCN' and CO ice, for the 
first time outside our Galaxy. The presence of `XCN' ice and the shape of
the CO ice profile provide a tell-tale story of the interaction of newly 
formed stars with their environment.