"The Origin of Disks and Spheroids in Simulated Galaxies"

ABSTRACT

 

In the simplest scenario, disk galaxies form predominantly in halos with high angular momentum and quiet recent merging history, whereas spheroids are the slowly-rotating remnants of repeated merging episodes. We explore these assumptions using one hundred well-resolved Milky Way-sized systems identified in the series of gasdynamical simulations of large cosmological volumes GIMIC. At z=0, the simulated galaxies exhibit a wide variety of morphologies, from dispersion-dominated spheroids to pure disk galaxies. Surprisingly, these morphological features are very poorly correlated with halo properties. Also mergers seem to play only a minor role in forming the spheroids. We find that a better indicator of morphology is the 'coherent alignment in the angular momentum' of baryons that accrete over time to form a galaxy. In this scenario, accretion from a hot corona will thus favor disk formation, whereas gas that flows ``cold'', often along separate, misaligned filaments, favors the formation of spheroids. Since angular momentum is acquired largely at turnaround, our results indicate that morphology is imprinted early on by the interplay of the tidal field and the shape of the material destined to form the galaxy.