Polarimetry of the crab nebula and NGC 1569

Abstract

This thesis contains a description of" the technique used to measure the optical linear polarisation of the Crab Nebula, and the irregular galaxy NGC 1569, and the results obtained from these measurements. This work was undertaken while the author was a member of the Durham University Astronomy Group. The first chapter is an outline of the differing states of polarisation and the method used to describe these states via the Stokes Parameters. A short review of the mechanisms producing linear polarisation of the optical radiation from astronomical objects is included. Chapter 2 contains a brief account of the design and operation of the Durham University polarimeter used to detect and measure optical linear polarisation, with a short description of the electronographic camera used as a recording device. The technique for the reduction of the data, from its initial electronograph form to the final extraction of the Stokes parameters for over 1000 points in the object observed is also described. The following three chapters contain a review of the present information known about the Crab Nebula, an account of the technique employed to measure the optical linear polarisation of the nebula, together with a final map of the polarisation, and a comparison of the results obtained by the present method and the previous photographic measurements of Woltjer made in 1957.The final chapter is devoted to the study of the irregular galaxy NGC 1569. A short review of the known parameters of the galaxy is included and the results of the present polarisation measurements are shown. An interpretation of the results follows in which it is suggested that the polarisation detected is not intrinsic to the N&C 1369 but produced as the light from the galaxy passes through our interstellar medium. An attempt is made to remove the interstellar polarisation leaving NGC I569 apparently unpolarised.