Disk galaxy formation in a λ cold dark matter universe

We describe hydrodynamic simulations of galaxy formation in a λ cold dark matter cosmology, performed using a subresolution model for star formation and feedback in a multiphase interstellar medium (ISM). In particular, we demonstrate the formation of a well-resolved disk galaxy. The surface brightness profile of the galaxy is exponential, with a B-band central surface brightness of 21.0 mag arcsec^-2 and a scale length of R_d = 2.0 h ^-1 kpc. We find no evidence for a significant bulge component. The simulated galaxy falls within the I-band Tully-Fisher relation, with an absolute magnitude of I = -21.2 and a peak stellar rotation velocity of V_rot = 121.3 km s^-1. While the total specific angular momentum of the stars in the galaxy agrees with observations, the angular momentum in the inner regions appears to be low by a factor of ∼2. The star formation rate of the galaxy peaks at ∼7 M⊙ yr^-1 between redshifts z = 2 and 4, with the mean stellar age decreasing from ∼10 Gyr in the outer regions of the disk to ∼7.5 Gyr in the center, indicating that the disk did not simply form inside-out. The stars exhibit a metallicity gradient from 0.7 Z⊙ at the edge of the disk to 1.3 Z⊙ in the center. Using a suite of idealized galaxy formation simulations with different models for the ISM, we show that the effective pressure support provided by star formation and feedback in our multiphase model is instrumental in allowing the formation of large, stable disk galaxies. If ISM gas is instead modeled with an isothermal equation of state, or if star formation is suppressed entirely, growing gaseous disks quickly violate the Toomre stability criterion and undergo catastrophic fragmentation.