The Morphology of Low Surface Brightness Disk Galaxies

We present $UBVI$ and H$\alpha$ images of a sample of Low Surface Brightness (LSB) disk galaxies. These galaxies are generally late types, if they can be sensibly classified at all. However, they are not dwarfs, being intrinsically large and luminous. The morphology of LSB galaxies is discussed in terms of the physical interpretation of the Hubble sequence. Galaxies with high contrast relative to the sky background are subject to being more finely typed than those which appear merely as fuzzy blobs on photographic plates. This causes the stages of the Hubble sequence to be nonlinear in the sense that large morphological type distinctions are made between high surface brightness spirals when only small physical differences exist, and small morphological distinctions are made between low surface brightness galaxies even when large physical differences exist. Many LSB galaxies lack the old red disk conspicuous in higher surface brightness spirals. Their morphology is strikingly similar in all bands from $U$ to $I$, suggesting farily homogeneous stellar populations lacking a well developed giant branch. These properties, together with their very blue colors, suggest that LSB galaxies are relatively younger than their high surface brightness counterparts. A few of these LSB galaxies appear to be very young ($\simlt 1$~Gyr), and as such may represent local examples of protogalaxies.