Gas and dust in the magellanic clouds a thesis submitted for the award of the degree of doctor of philosophy in physics, Mangalore University Ananta Charan Pradhan [Ph.D Thesis]

Thesis Supervisor Prof. Jayant Murthy

Doctor of Philosophy Mangalore University, Karnataka 2011

The Magellanic Clouds (MCs) are nearby irregular dwarf galaxies where the gas and dust is known to be different from the Milky Way due to a low metallicity and high gas- to-dust ratio. We have presented the first observations of the far ultraviolet diffuse radiation in the Magellanic Clouds and have shown that it is predominantly due to scattering of starlight from interstellar dust grains. We have also presented a survey of OVI absorption in the Large Magellanic Cloud (LMC), a tracer of hot gas in the interstellar medium (ISM). We have used serendipitous observations made with the Far Ultraviolet Spectroscopic Explorer (FUSE) to measure the diffuse FUV (1000 – 1150°A) radiations from the MCs. We have analyzed a complete set of FUSE data in and around the MCs and obtained 81 suitable FUV diffuse observations in different parts of the LMC and 30 observations in the Small Magellanic Cloud (SMC). These observations are bright, patchy and cover important regions of the MCs with intensities that range from around 103 photons cm−2 s−1 sr−1 °A−1 to as high as 3 × 105 photons cm−2 s−1 sr−1 °A−1. We also found
that the diffuse light in some regions is due to light coming from distant stars scattered by local dust. A strong correlation between FUSE diffuse radiation and the diffuse radiation mea- sured from Ultraviolet Imaging Telescope (UIT) was obtained and, based on this, we estimated the contribution of FUV diffuse radiation to the total integrated flux in the MCs. The fraction of the diffuse emission is typically 5%–20% of the total at 1100 °A in the LMC and 34% to 44% in the SMC. In all cases, the behaviour of the diffuse fraction is almost the same, rising by same factor as that of the albedo over the wave- length range of 1000 – 1615 °A. We also found a good correlation between the FUV diffuse emission and Hα emission in the H II regions of the SMC. UV absorption lines produced by collisional ionization are used to study the hot ISM. O VI is one such ion which is produced at a temperature of 106 degrees. We have used high resolution absorption spectra obtained by FUSE for 70 lines of sight and presented a wide survey of O VI column density measurements for the LMC. The column density varies from a minimum of log N(O VI) = 13.72 atoms cm−2 to a maximum value of log N(O VI) = 14.57 atoms cm−2. We found a high abundance of O VI in both active (superbubbles) and inactive regions of the LMC. The abundance and properties of OVI absorption are similar in the LMC and the Milky Way (MW) despite the fact that the LMC has lower metallicity than the MW. O VI absorption in the LMC does not correlate with Hα (warm gas) or X-ray (hot gas) but correlates with X-ray emission in the 30 Doradus region, a star forming region of the LMC and decreases with increase in angular distance from the star cluster R136 suggesting that the strength of the stellar wind from the central cluster decreases outwards.