INTERNAL EXTINCTION OF DISC GALAXIES

Abstract

ABSTRACT Many techniques to correct for the extinction of light by dust in our own Galaxy have been developed but for many years it was believed that the extinction of starlight by dust in the interstellar media of external galaxies was negligible. Over the last ten years the evidence for low extinctions in galaxies has become very controversial and numerous papers indicating both high and low extinction have been published, throwing the situation into confusion. This has triggered a new interest in the subject as many other areas of astrophysics depend on its resolution. In this thesis a new technique for mapping the extinction of spiral galaxies at high resolution is presented. The technique combines a low resolution energy balance measurement of the extinction with an optical/near infrared colour map that shows relative extinction at high resolution. When combined correctly, an extinction map that has both high resolution and absolute normalisation can be obtained. An extinction map is computed for the nearby, face-on, Sc galaxy NGC 6946. The total extinction in NGC 6946 is found to be AB=0.45, considerably higher than previous estimates for Sc galaxies (AB=0.2). The high resolution map shows the extinction to be extremely patchy and to severely affect the appearance of the galaxy. When the extinction map is used to correct the observed image of NGC 6946 for the effects of dust it displays a markedly different morphology, the amplitude of the spiral arms is decreased and the bulge regions become more prominent. As well as the extinction map, a number of other related results were found during the analysis. The mass of dust in NGC 6946 was found to be ~ 4-10 times that estimated by previous measurements, bringing the gas-to-dust mass ratio in agreement with that of the Galaxy. Contrary to previous beliefs, the distribution of the dust was found to have a longer scale length than the stars, - cold dust therefore dominates the far infrared energy output at large radial distances. A comparison of optical and far infrar\\ ed luminosity functions is used to show that, on average, the B band light from\\ galaxies arises from regions with tauB ~ 1. Modelling of the extinction as a function of wavelength suggests that absorption at near infrared wavelengths may produce the largest fraction of the far infrared emission.