Near infrared surface photometry of spiral galaxies
Stewart, James Malcolm
Very little surface photometry of normal galaxies has yet been performed in the near infrared. The likely sources of infraredemission in galaxies are discussed paying particular regard to mechanisms which could cause variations in the observed colours. Late type giants and, in the particular galaxies selected,extinction from dust are shown to be the major contributors to the appearance of the galaxies in the near infrared.The problems of measuring accurate surface colours in the presence of large brightness gradients are discussed. It is shown that simultaneous measurements at two wavelengths are necessary, requiring the construction of a special infrared photometer with two detectors, a beamsplitter and a common focal plane aperture. The design criteria of the system are discussed and the performance of the system in the laboratory and on the telescope is analysed.Techniques for mapping galaxies are discussed and it is shown that beamswitching may be replaced by measurements on the sky. Good instrument stability and performance of the chopping secondary mirror have been shown to be necessary.Five normal spiral galaxies were observed and the colour variations analysed. The effect on the colours of the distribution of dust and stars are discussed. The near infrared extinction properties of dust in these galaxies have been shown to be similar to those of dust in the Milky Way.The distribution of reddening is related to the distribution ofinterstellar gas, predominantly molecular, in NGC 7331. A centraldeficiency of molecular gas in NGC 7331 has been deduced, analagous to the "molecular ring" in the Milky Way. At least 2 percent of the mass in the inner 9 kpc is in the interstellar medium (ISM). The advantage is demonstrated of higher spatial resolution than is currently available in CO data. The surface density of the ISM is shown to be relatively constant between 3 and 9 kpc, similar to the distribution in NGC 2841.