The stars and gas in outer parts of galaxy disks: Extended or truncated -- flat or warped?

Abstract

I review observations of truncations of stellar disks and models for their origin, compare observations of truncations in moderately inclined galaxies to those in edge-on systems and discuss the relation between truncations and HI-warps and their systematics and origin. Truncations are a common feature in edge-on stellar disks, but the relation of truncations in face-on to those in edge-on galaxies needs further clarification. The origin of truncations is most likely related to a maximum in the specific angular momentum in the material that formed the stellar disks, but this model does probably require some redistribution of angular momentum. HI-warps start just beyond the truncation radius and disks and warps appear distinct components. This suggests that inner disks form initially and settle as rigid, very flat structures, while HI-warps result from later infall of gas with a different orientation of the angular momentum.