B. Moore on "Galaxy formation and transformation by environmental and secular processes"
Although galaxies are sometimes assumed to be "closed boxes", but in reality, they are not. Gas falls into them, it accretes, forms galacitc disks and there may be cooling flows. The latter have been proposed long ago, became unpopular for a while, because little evidence was found. Recently, they have been revived and Moore presents some evidence (recent papers by Kaufmann) for cooling flows.
In simulations, gas cools and forms a disk in an inside-out fashion. Unstable collapsing gas clounds form in the halo of surrounding hot gas and the study of angular momentum is an important method to distinguish different models.
When a small galaxy rich in neutral hydrogen moves through a region of hot gas, it can be stripped from its gas and leave a trail behind which falls onto the main galaxy. An example is the LMC circulating the Milky Way. Another one is spriral galaxies moving in the hot ICM of galaxy clusters.
If I understood correctly, he proposes that in less massive systems like polar ring galaxies, the main process is accretion of cold gas, while in more massive (L-star) ones, the gas is heated first and accrtetion by cooling is the more important.
He finished by "bashing" SPH simulations (quote: "they are pretty useless"), because they cannot capture certain instabilities that are important in a multi-phased medium. Grid-based simulations are preferable here.