Forming Realistic Late-Type Spirals in a LCDM Universe: The Eris
  Simulation
  Simulations of the formation of late-type spiral galaxies in a cold dark
matter LCDM universe have traditionally failed to yield realistic candidates.
Here we report a new cosmological N-body/SPH simulation of extreme dynamic
range in which a close analog of a Milky Way disk galaxy arises naturally.
Termed Eris, the simulation follows the assembly of a galaxy halo of mass
Mvir=7.9x10^11 Msun with a total of N=18.6 million particles (gas + dark matter
+ stars) within the final virial radius, and a force resolution of 120 pc. It
includes radiative cooling, heating from a cosmic UV field and supernova
explosions, a star formation recipe based on a high gas density threshold
(nSF=5 atoms cm^-3 rather than the canonical nSF=0.1 atoms cm^-3), and neglects
AGN feedback. At the present epoch, the simulated galaxy has an extended
rotationally-supported disk with a radial scale length Rd=2.5 kpc, a gently
falling rotation curve with circular velocity at 2.2 disk scale lenghts of
V2.2=214 km/s, a bulge-to-disk ratio B/D=0.35, and a baryonic mass fraction
that is 30% below the cosmic value. The disk is thin, is forming stars in the
region of the Sigma_SFR - Sigma_HI plane occupied by spiral galaxies, and falls
on the photometric Tully-Fisher and the stellar mass-halo virial mass
relations. Hot (T>3x10^5 K), X-ray luminous halo gas makes only 26% of the
universal baryon fraction and follows a flattened density profile proportional
to r^-1.13 out to r=100 kpc. Eris appears then to be the first cosmological
hydrodynamic simulation in which the galaxy structural properties, the mass
budget in the various components, and the scaling relations between mass and
luminosity are all consistent with a host of observational constraints.
(Abridged)
