Galaxy structure and evolution at the end of the spiral sequence

I have used multiwavelength observations to examine the global, structural, and kinematic properties of nearby extreme late-type (ELT) spiral galaxies. These are examples of some of the smallest and faintest disk galaxies that exhibit regular optical structures.;ELT spirals possess properties intermediate between giant spirals and irregular galaxies. They exhibit a diverse range of disk morphologies in an otherwise similar physical parameter space. Asymmetries are found frequently in their rotation curves, stellar disks, and global HI profiles.;Many ELT spirals are barred, indicating bars readily form and subsist in some low surface brightness galaxies. Compact nuclei are also a common feature of ELT spirals, even though these galaxies lack strong central potentials. I show these are true compact star cluster nuclei analogous to those in giant galaxies.;ELT spirals include some of the least evolved nearby galaxies in a dynamical and in a star-formation sense. Some have very blue colors B-V<0.4 and/or large gas fractions MHI LV∼3 - 9 . Mean optical surface brightness correlates with MHILV in the sense that the faintest ELT spirals have the highest fraction of their visible matter in the form of HI gas. Steep radial color gradients are found in some ELT spirals, implying significant ongoing disk evolution.;Many ELT spirals exhibit sharp radial surface brightness gradients and cannot be characterized by a single exponential brightness distribution. Radial disk reddening also occurs in some cases. These characteristics appear to be unique among the smallest late-type disk galaxies, and may indicate that viscous evolution has been inefficient. Certain ELT spirals have disk scale length-to-scale height ratios as high as 20, implying their disks are extremely dynamically cold.;Dark matter becomes increasingly important in some of the smallest and faintest rotationally-supported disk galaxies. The dynamically coldest ELT spirals must require massive dark halos for stabilization. ELT spirals often exhibit rotation curves that rise slowly with respect to the stellar disk, and visible matter cannot account for the observed rotational velocities, even within the inner galaxy. Some ELT spirals are underluminous relative to the standard Tully-Fisher relation, also suggesting their dynamics are highly influenced by dark matter.