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5. GALAXIES
The ratio of interstellar N 
/O as a function of distance from the
Galaxy's
center was originally studied from the KAO. By extending such studies across the
Galaxy and
looking at both the Large and Small Magellanic Clouds, Jean-Paul Baluteau found
a
decreasing nitrogen-to-oxygen ratio at increasing distances from the Galactic
Center. This
decline continues as we go to the LMC and SMC. Studies of this kind should
permit us to
obtain greater insight on the chemical evolution of galaxies on time scales of
billions of years.
A considerable body of work on active galactic nuclei (AGNs) and starburst
galaxies has been
carried out. Dieter Lutz showed us that the archetypical ultraluminous galaxy
Arp 220 has a
[NeII]/[OIV] ratio more similar to starbursts than to AGNs. In
contrast to these two
near infrared lines seen in emission, Jacqueline Fischer showed us a long
wavelength spectrum
of molecular lines all of which are seen in absorption. The sole emission line
in the far infrared
appeared to be [CII] at 158 
m. Her spectra would lead one to
conclude that
enormous opacities exist surrounding the sources of heating and a mid-infrared
optical depth
of order 20 at the wavelengths of [NeII] and [OIV]. It is difficult
to explain how
these lines can be seen strongly in emission while the [CII] line, which
lies at much
longer wavelengths where opacities are low, is so weak.
An equally difficult question to answer is why ultraluminous galaxies should in the first place contain so much dust when they appear to be merely two ordinary colliding galaxies? Where does all the dust come from? How is it distributed around the star forming regions? Where does it lie in relation to the molecular constituents?
There is much to learn.
Martin Harwit