The Aquila comparison Project: The Effects of Feedback and Numerical
  Methods on Simulations of Galaxy Formation
  We compare the results of thirteen cosmological gasdynamical codes used to
simulate the formation of a galaxy in the LCDM structure formation paradigm.
The various runs differ in their hydrodynamical treatment (SPH, moving-mesh and
AMR) but share the same initial conditions and adopt their latest published
model of cooling, star formation and feedback. Despite the common halo assembly
history, we find large code-to-code variations in the stellar mass, size,
morphology and gas content of the galaxy at z=0, due mainly to the different
implementations of feedback. Compared with observation, most codes tend to
produce an overly massive galaxy, smaller and less gas-rich than typical
spirals, with a massive bulge and a declining rotation curve. A stellar disk is
discernible in most simulations, though its prominence varies widely from code
to code. There is a well-defined trend between the effects of feedback and the
severity of the disagreement with observation. Models that are more effective
at limiting the baryonic mass of the galaxy come closer to matching observed
galaxy scaling laws, but often to the detriment of the disk component. Our
conclusions hold at two different numerical resolutions. Some differences can
also be traced to the numerical techniques: more gas seems able to cool and
become available for star formation in grid-based codes than in SPH. However,
this effect is small compared to the variations induced by different feedback
prescriptions. We conclude that state-of-the-art simulations cannot yet
uniquely predict the properties of the baryonic component of a galaxy, even
when the assembly history of its host halo is fully specified. Developing
feedback algorithms that can effectively regulate the mass of a galaxy without
hindering the formation of high-angular momentum stellar disks remains a
challenge.
