Title:

Nuclear Starbursts	

Abstract:

Investigations of some aspects of star formation in external galaxies
are presented. The role of tidal interactions in controlling the
star-formation rate of galaxies has been investigated using a large,
volume and luminosity limited, sample of galaxies drawn from the Sloan
Digital Sky Survey. Star-formation rates have been estimated from
aperture and extinction corrected \Halpha\ data, and, for a subset of
galaxies, from {\it IRAS} far-infrared observations.  Using the
\rband\ light distributions of galaxies, it was possible to take into
account the morphology of both of the galaxies involved in the
interaction in a way which is largely independent of star
formation. This statistical study is then extended by taking into
consideration the effect of density of environment; and, by
investigating the possibility that active galactic nuclei may also be
triggered by tidal interactions.

Being based on a \rband-selected volume-limited sample, the above
statistical study is fairly representative of the range star-formation
rates seen in the local universe. In order to investigate the highly
obscured intensely star-forming systems known to be increasingly
important in the younger Universe, observations and models of some
infrared-selected systems are presented. Emphasis is placed on
interpretation of mid-infrared observations, and as an important tool
for this, a comprehensive and physical model for dust emission from
nuclear starbursts is developed.
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It has been shown that galaxies with close companions have
star-formation rates which are, on average, enhanced compared to
isolated galaxies. Furthermore, their \rband\ light profiles and
\Halpha\ emission tend to be centrally concentrated, which is
interpreted as evidence that tidal interactions lead to nuclear star
formation. No evidence, however, for tidal triggering of AGN is found.
Also, a remarkable independence between the effects of density of
environment and galaxy interactions in the local universe is
demonstrated.

Making use of constraints from new high-resolution mid-infrared
imaging, and published far-infrared, radio, near-infrared
observations, mid-infrared emission from NGC 520 and Arp 220 is
modelled with considerable success. It is found that observations of
both of these galaxies are consistent with them being powered by
starbursts.  The mid-infrared models suggest that there is significant
processing of dust in the extreme radiation field of Arp 220, leading
to destruction of most, but not all, dust grains smaller than 30\,\AA.