Gas Rich Mergers in Disk Formation

In order to explain disk galaxy formation within the hierarchical structure for- mation, it seems that gas rich mergers must play an important role. We review here our previous studies which have shown the importance of mergers at high redshift being gas rich, in the formation of both the stellar halo and thick disk components of disk galaxies. Regulation of star formation in the building blocks of our galaxy is required to form a low mass low metallicity stellar halo. This regulation results in high redshift, gas rich mergers durin g which the thick disk forms. In these proceedings, we categorise stars from our simulated disk galaxy into thin and thick disk components by using the Toomre diagram. Rotation velocity, metallicity and age histograms of the two populations are presented, along with alpha element abundances (oxygen, silicone, magnesium), age-height above the plane, age-radius, metallicity-height, and metalicity-radius gradients. The overcooling problem (White & Rees 1978), coupled with the ineffec- tiveness of implemented feedback algorithms in regulating star formation, Katz & Gunn (1991), have been central to the difficulties of simula ting disk galaxies in a cold dark matter (CDM) Universe. Without sufficient feed back, stars form rapidly in the earliest collapsing dark matter halos, which subsequently merge; angular momentum is transferred to the dark matter halo, and the resultant disk is significantly smaller (in scalelength), and has less angular momentum, than those observed. Feedback from supernovae, stellar winds, quasars etc, has proven to be difficult to simulate, yet the effects of feedbac k are essential in the formation of disk galaxies. Recent models incorporate these processes with- out attempting to detail the complex multiphase gas physics involved. This has highlighted the importance of regulating star formation in the "building blocks" of galaxies. (note: We use the term "building blocks" when refering to early forming small galaxies which will have merged or been accreted to the final galaxy at redshift 0. By contrast, the term "satellites " is reserved for those small galaxies which are found orbiting the central galaxy at z=0).