Lyman alpha emission from the first galaxies: Implications of UV
  backgrounds and the formation of molecules
  The Lyman alpha line is a robust tracer of high redshift galaxies. We present
estimates of Lyman alpha emission from a protogalactic halo illuminated by UV
background radiation fields with various intensities. For this purpose, we
performed cosmological hydrodynamics simulations with the adaptive mesh
refinement code FLASH, including a detailed network for primordial
chemistry,comprising the formation of primordial molecules, a multi-level model
for the hydrogen atom as well as the photo-ionization and photo-dissociation
processes in a UV background. We find that the presence of a background
radiation field J_21 excites the emission of Lyman alpha photons, increasing
the Lyman alpha luminosity up to two orders of magnitude. For a halo of \sim
10^10 M_sun, we find that a maximum flux of 5 \times 10^-15 erg cm^-2 s^-1 is
obtained for J21 \times f_esc = 0.1, where f_esc is the escape fraction of the
ionizing radiation. Depending on the environmental conditions, the flux may
vary by three orders of magnitude. For J_21 \times f_esc > 0.1 the Lyman alpha
luminosity decreases as the atomic hydrogen abundance becomes rather small. The
fluxes derived here can be probed using Subaru and the upcoming James Webb
Space Telescope. The emission of Lyman alpha photons is extended and comes from
the envelope of the halo rather than its core. In the center of the halo, line
trapping becomes effective above columns of 10^22 cm^-2 and suppresses the
emission of Lyman alpha. In addition, cooling by primordial molecules may
decrease the gas temperature in the central region, which further reduces Lyman
alpha emission. In the central core, H_2 is photo-dissociated for a background
flux of J_21 \geq 1000. For weaker radiation fields, i.e. J_21 < 0.1, H_2 and
HD cooling are particularly strong in the center of the halo, leading to gas
temperatures as low as \sim 100 K.
