Figuring Out Gas & Galaxies in Enzo (FOGGIE). I. Resolving Simulated
  Circumgalactic Absorption at 2 < z < 2.5
  We present simulations from the new "Figuring Out Gas & Galaxies in Enzo"
(FOGGIE) project. In contrast to most extant simulations of galaxy formation,
which concentrate computational resources on galactic disks and spheroids with
fluid and particle elements of fixed mass, the FOGGIE simulations focus on
extreme spatial and mass resolution in the circumgalactic medium (CGM)
surrounding galaxies. Using the Enzo code and a new refinement scheme, FOGGIE
reaches spatial resolutions of 381 comoving $h^{-1}$ pc and resolves extremely
low masses ($\lesssim 1$--$100$ Msun out to 100 comoving $h^{-1}$ kpc from the
central halo. At these resolutions, cloud and filament-like structures giving
rise to simulated absorption are smaller, and better resolved, than the same
structures simulated with standard density-dependent refinement. Most of the
simulated absorption arises in identifiable and well-resolved structures with
masses $\lesssim 10^4$ Msun, well below the mass resolution of typical zoom
simulations. However, integrated quantities such as mass surface density and
ionic covering fractions change at only the $\lesssim 30$% level as resolution
is varied. This relatively small changes in projected quantities---even when
the sizes and distribution of absorbing clouds change dramatically---indicate
that commonly used observables provide only weak constraints on the physical
structure of the underlying gas. Comparing the simulated absorption features to
the KODIAQ (Keck Observatory Database of Ionized Absorption toward Quasars)
survey of $z \sim2$--$3.5$ Lyman limit systems, we show that high-resolution
FOGGIE runs better resolve the internal kinematic structure of detected
absorption, and better match the observed distribution of absorber properties.
These results indicate that CGM resolution is key in properly testing
simulations of galaxy evolution with circumgalactic observations.
