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

TL;DR

This review discusses the occurrence and origins of stellar disk truncations and HI-warps in galaxy disks, highlighting their observational features, possible formation mechanisms, and the relationship between these phenomena.

Contribution

It synthesizes observational data and models to clarify the connection between disk truncations and HI-warps, proposing that truncations relate to angular momentum limits and warps result from later gas infall.

Findings

Truncations are common in edge-on stellar disks.

HI-warps start just beyond the truncation radius.

Disks and warps are distinct components with different formation histories.

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.