The Birth of a Galaxy. II. The Role of Radiation Pressure
  Massive stars provide feedback that shapes the interstellar medium of
galaxies at all redshifts and their resulting stellar populations. Here we
present three adaptive mesh refinement radiation hydrodynamics simulations that
illustrate the impact of momentum transfer from ionising radiation to the
absorbing gas on star formation in high-redshift dwarf galaxies. Momentum
transfer is calculated by solving the radiative transfer equation with a ray
tracing algorithm that is adaptive in spatial and angular coordinates. We find
that momentum input partially affects star formation by increasing the
turbulent support to a three-dimensional rms velocity equal to the circular
velocity of early haloes. Compared to a calculation that neglects radiation
pressure, the star formation rate is decreased by a factor of five to 1.8 x
10^{-2} Msun/yr in a dwarf galaxy with a dark matter and stellar mass of 2.0 x
10^8 and 4.5 x 10^5 solar masses, respectively, when radiation pressure is
included. Its mean metallicity of 10^{-2.1} Z_sun is consistent with the
observed dwarf galaxy luminosity-metallicity relation. However, what one may
naively expect from the calculation without radiation pressure, the central
region of the galaxy overcools and produces a compact, metal-rich stellar
population with an average metallicity of 0.3 Z_sun, indicative of an incorrect
physical recipe. In addition to photo-heating in HII regions, radiation
pressure further drives dense gas from star forming regions, so supernovae
feedback occurs in a warmer and more diffuse medium, launching metal-rich
outflows. Capturing this aspect and a temporal separation between the start of
radiative and supernova feedback are numerically important in the modeling of
galaxies to avoid the "overcooling problem". We estimate that dust in early
low-mass galaxies is unlikely to aid in momentum transfer from radiation to the
gas.
