Degree Name

Doctor of Philosophy


Department of Engineering Physics - Faculty of Informatics


I present a detailed study of two Magellanic Irregular galaxies, NGC 4214 and NGC 1156. With relatively high star formation rates and plenty of neutral gas, they are interesting laboratories in which to study the organisational processes that lead to star formation. These galaxies have masses in the ransition region between dwarf irregular and spiral galaxies, and are close analogues of the Large Magellanic Cloud, so it is of interest make detailed observations for later comparative studies. The neutral Hydrogen observations are the highest angular resolution available to date for these galaxies (~ 800 FWHM), with fine velocity resolution (1.3 km.s‾¹). These show remarkable structure in both objects, including large holes and small clouds of gas (~ 107 Mסּ) with moderate velocity offsets from the general rotation. An large, incompletely evacuated hole (2.2 kpc diameter) dominates the western half of NGC 1156. Its origin is unclear. The velocity fields of both galaxies show most of the disk is rotating smoothly, but near the centre appear to have significant noncircular motions, probably induced by a stellar bar. Échelle spectroscopy at Hα shows the HII regions are following the same orbits as the neutral gas. The galaxies' optical structure and stellar populations are characterised through multicolour imaging, Hα emission-line imaging and densely-spaced longslit spectra. NGC 4214 exhibits two exponential components in its surface brightness profile, while NGC 1156 is consistent with a single exponential. In both objects the major axis of the bright bar lies along quite a dfferent position angle to the kinematic major axis. Colour maps reveal varied populations in the bright bars of both galaxies, and the outer disk of NGC 4214. I make detailed comparisons of the galaxies' colour-colour distribution with stellar population models, and employ absorption-line indices from the longslit spectra as cross-checks in a few locations. Taken together the data suggest that over a large fraction of the galaxies there is a young population some tens of Myr old that contributes a few percent of the total stellar mass, though the particular combination of mass fraction and age cannot be uniquely determined. Neither galaxy exhibits strong O/H abundance variations within the bright bar region. Both galaxies are consistent with a model in which a minimum HI surface density (10²¹‾²)is a necessary criterion for star formation. In NGC 1156 the radial distribution of star-forming regions is well predicted by the disk instability criterion discussed by Kennicutt (1989), while the criterion is less successful in NGC 4214. In a systematic survey of NGC 4214 I identifed 13 holes likely to be expanding or burst shells, roughly in accord with the number expected given the linear resolution and the HII-region population. In a majority of the holes, a star cluster capable of powering the expansion is visible within the hole boundary. However the holes are so young and that I expected all would contain a detectable cluster. In a few regions new star formation appears to have occurred in the walls of the identified holes, but the total number of HII regions near HI holes is consistent with that expected by chance.



Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.