This thesis presents an investigation into the connection between
starburst activity and Active Galactic Nuclei (AGN) in warm
infrared galaxies. Previous work has found that some
infrared galaxies contain Active Galactic
Nuclei, while in others, the infrared source is intense
star formation. It is unknown whether there is an
evolutionary connection between the starburst and AGN activity in these
In order to study this issue, a large optical
spectroscopic survey of 285 IRAS galaxies was carried out in order to
obtain accurate optical classification.
Some infrared galaxies, particularly those with high infrared
luminosities, are known to contain AGN which are
obscured to the optical by large quantities of dust. To determine
the fraction of galaxies in our sample which contain obscured
AGN, the Parkes-Tidbinbilla interferometer was used to search for
compact radio cores in a subsample of ~60 objects. Of these,
37% of the starburst galaxies and 90% of the AGN type galaxies
contain compact radio cores. A bimodal core luminosity
distribution is observed, implying that the source of the compact radio
emission in many of the starburst galaxies is from complexes of compact
radio supernovae rather than from AGN. Deep VLBI imaging and X-ray
spectroscopy observations are planned to investigate this further.
Theoretical starburst and AGN modelling was carried out and involves
the use of stellar population
synthesis models (Starburst99 and Pegase) to produce the ionizing
radiation field which is then utilised by our shock and photoionization
modelling code MAPPINGs vIII.
These models were used to define a new classification scheme for
the optical diagnostic diagrams. This new scheme has been used to
classify the galaxies in our sample into AGN, starburst and LINER
types and compares
favourably against the previous semi-empirical scheme of Veilleux &
Osterbrock (1987), producing only 6% ambiguity in classifications
between the different diagnostic diagrams compared with 16% ambiguity
using the traditional method. A large fraction (70%) of the galaxies in
our sample are classified optically as starburst, 17% are Sy2, 4% are
Sy1, and 0.4% are LINERs.
Many galaxies in our sample lie on a mixing line between
starburst and AGN, and most likely contain both phenomena. Theoretical
models were used to define mixing lines on the diagnostic diagrams to
determine the AGN contribution to the optical emission for these galaxies.
The optical AGN contribution agrees well with that
determined from infrared studies, and it is concluded that dust extinction
is relatively unimportant for the majority of the objects in our sample.
To test whether an evolutionary scenario exists for the galaxies in
our sample, a subsample of galaxies was selected which are currently
undergoing interactions, as traced by tidal features (tails, plumes
and bridges). For these galaxies, the AGN contribution was compared with
three indicators of merger progress; morphological interaction class,
age of the old stellar population, and projected separation. There is
a distinct lack of optical AGN activity in the more evolved mergers, in
apparent contradiaction to the standard merger scenario.
This may tentatively be explained in terms of a combination of increased
dust extinction and circumnuclear star formation as gas and dust is
funnelled towards the centre of the merger in the later stages.