Year

2003

Degree Name

Doctor of Philosophy (PhD)

Department

Department of Engineering Physics - Faculty of Engineering

Abstract

The Magellanic Bridge is a region of predominantly neutral hydrogen, connecting the Small and Large Magellanic Clouds (SMC and LMC respectively). We present here widefield observations of this region in the 21cm transition of HI. These observations are made using the ATCA and Parkes telescopes. The resulting dataset encompasses a spatial range of 98? � 7(degrees) and are at a velocity resolution of ~1.63 km s(superscript �1). This dataset has formed the basis for a series of analyses which are the subject of this thesis. The HI in the Bridge is seen to have a complex and filamentary structure across the entire range of observed spatial and velocity scales. A census of the expanding shell population, which were parameterised using a strict selection criteria, has resulted in the detection of a shell population with a mean kinematical age of 6.2 Myr. This is in general agreement with the shell population of the SMC, although is shown that the strict shell selection criteria was insensitive to fragmenting or incomplete shells. A strict selection criteria led to the detection of a region in the Bridge where an apparently older shell population exists. The HI of this region, found at Right Ascensions greater than approximately 2(superscript h)20, appears relatively quiescent. It appears that shells in this region may left to develop for a longer time before they are ruptured and distorted by ambient turbulence. A study of the spatial power spectrum has shown that distribution of structure in the Bridge is quite inhomogeneous. Two apparently different morphological regions are detected in the Bridge: the northern part is at a higher velocity and shows very little small scale variation and the southern, lower velocity and significantly brighter part shows an approximately Kolmogorov power spectrum. The power law index of the southern part is consistent with power indices derived from HI in the SMC. The power spectrum of both the northern and southern part are well fit using a single component, and show no indication for a dominant scale. Visually, the southern part of the Bridge appears to suggest a east-west elongation in the structure. This is not apparent in a quantitative test of the Fourier transform of the image data. A smaller-scale analysis using the Spectral Correlation Function, which provides a measurement of the variation of spectra as a function of spatial lag, suggests that the spectra do in fact vary more slowly in the East-West direction, i.e. approximately along the line joining the SMC and LMC. Several large scale features found in the Bridge show evidence for large energy deposition events. Part of the HI in the Bridge appears to be distinctly offset in velocity and position and a comparison of numerical N-Body simulations of the Magellanic System, where the SMC is modelled as a spiral galaxy, shows excellent agreement with the observed HI distribution, both in position and in velocity. From these simulations, it appears that the velocity offset part of the Bridge is in fact a distant arm of the SMC while the closer arm comprises a link between the LMC-SMC. The observed filamentary nature of the lower velocity component is consistent with severe turbulent mixing which may be generated by the tidal interaction of the SMC and LMC. The brighter, lower velocity part of the Bridge is shown to have a bimodal spectral profile. The bimodality is contiguous in position and in velocity with a shell complex in the SMC. The observed bimodality in the Bridge indicates that the Bridge formed after the formation of the SMC shell complex, however the kinematical ages for the shell complex are a factor of ~15 younger than the predicted age of the Bridge structure. A large radii (~1.6 kpc) loop filament is observed off the North-Eastern corner of the SMC. The velocity structure of the loop is reminiscent of an expanding region, although the energy requirements for this shell are so extreme that no current hole or shell formation theories can satisfactorily account for its development. Such a feature would require an unseen massive stellar association to generate the necessary energy output. To form the observed feature by a high velocity cloud collision would require the impinging mass to have velocities and densities which are entirely inconsistent with the observed HVC population. Finally, the idea that the loop occurs at the first gravitational Lagrange point of the LMC-SMC system is inconsistent with Numerical simulations in that the simulations suggest that the loop is in fact located in the distant spiral arm of the SMC and therefore not in the correct location of L1 point. Furthermore, the simulations do not show any evidence for the formation of this feature as observed. As such, the formation mechanism for this significant, large-scale loop is still uncertain. A survey of H? emission regions throughout the Bridge, using a new and untested H� dataset, has isolated several new H? emission regions. In general, these H? regions appear to be spatially associated with UV sources from the FAUST catalogue, although in some cases the H� emission region show a morphology which is characteristic of formation by collisional heating. Some of the H� features which show a ring-like morphology are compared with the HI dataset. In this way, an estimate of the age of the shell can be made. Due to calibration difficulties, and a lack of continuum information, this part of the thesis comprises only a list of positions of the H? features and a discussion of possible ionisation mechanisms. A search for (superscript 12)CO(1-0) emissions regions in the Bridge has resulted in the first detection of this molecule in the Magellanic Brodge. The CO emission region appears to be embedded in an HI cloud and has a S(subscript 60)/S(subscript 100) which is similar to those found in the SMC. The marrow line width of the Bridge CO detection is consistent with a low metallicity, indicating that the HI in the Bridge is less evolved than that of the SMC or the Magellanic Stream. From empirical models based on analyses of CO emission regions in the SMC, an upper limit of the radius of this region is estimated at ~16 pc. [Note: this abstract contained scientific formulae that would not come across on this form. Please see the 01Front files abstract for the full details.]

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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.