Optical imaging of IRAS galaxies : the evolution of infrared- bright galaxies.

Abstract

Gravitational interactions play an important role in galaxy evolution, both in causing rapid structural changes in individual galaxies, and in changing the overall properties of galaxies over the age of the Universe. Galaxy interactions have also been linked to high far-infrared luminosities and Seyfert activity. In this thesis, the relationship between far-infrared luminosity and interactions is explored by means of an I-band CCD imaging survey of a 60 $\mu$m flux-limited sample of 275 galaxies. The galaxies in this sample are classified as interacting or non-interacting based on the information in these images. The definition of an interacting pair used here is: the companion galaxy must have at least 1/4 the I-band luminosity of the infrared galaxy, the separation between the two must be less than three times the larger radius, and the velocity difference for the two galaxies must be less than 500 km/s. It is found that 56 of these galaxies are interacting, 198 are non-interacting, and 21 are ambiguous. The interacting galaxies have an average 60 $\mu$m luminosity of $\sim$6 times that of the non-interacting galaxies, consistent with numerical models of interacting galaxies. The 60 $\mu$m luminosity functions $\phi$(L) of interacting galaxies and of non-interacting galaxies are then derived. Non-interacting galaxies dominate the luminosity function at low luminosities, while interacting dominate at high luminosities. The luminosity function of non-interacting galaxies drops off fairly steeply at {\rm L \u3e 10\sp{10}L\sb{o}(\phi(L) \propto L\sp{-2.1})}, while that of interacting galaxies is flatter {\rm (\phi(L) \propto L\sp{-1.2})}. There are $\sim$5 times as many non-interacting galaxies as interacting galaxies having L(60) $\u3e$ L(MILKY WAY), and $\sim$100 times more having L(60) $\u3e$ 2 $\times$ 10\sp8L\sb{\rm o}. The derived luminosity functions of interacting and non-interacting galaxies are used to predict 60 $\mu$m source counts in deeper surveys. If only interacting galaxies are assumed to evolve, the predicted source counts are 60-80% those determined if all galaxies evolve. Assuming the I-band light ratio approximates the mass ratio, the 60 $\mu$m luminosity is compared with mass ratio and with pair separation. It is found that the mean luminosity of pairs with separation greater than 3 times the radius is similar to that of galaxies without bound companions, suggesting that encounters between galaxies with separations greater than three times the radius do not greatly enhance the star formation rate. Additionally, low mass companions (m\sb1/m\sb2) are not found to greatly enhance the far-infrared luminosity