A Census of the LyC Photons that Form the UV Background During
  Reionization
  We present a new, on-the-fly photon flux and absorption tracer algorithm
designed to directly measure the contribution of different source populations
to the metagalactic UV background and to the ionisation fraction of gas in the
Universe. We use a suite of multifrequency radiation hydrodynamics simulations
that are carefully calibrated to reproduce a realistic reionization history and
galaxy properties at $z \ge 6$, to disentangle the contribution of photons
emitted by different mass haloes and by stars with different metallicities and
ages to the UV background during reionization. While at very early cosmic times
low mass, metal poor haloes provide most of the LyC photons, their contribution
decreases steadily with time. At $z = 6$ it is the photons emitted by massive
systems (${\rm M_{halo}}/{\rm M_\odot} > 10^{10} \, {\rm h ^{-1}}$) and by the
metal enriched stars ($10^{-3} < Z/Z_{\rm \odot} < 10^{-1.5}$) that provide the
largest contribution to the ionising UV background. We demonstrate that there
are large variations in the escape fraction depending on the source, with the
escape fraction being highest ($\sim 45-60\%$) for photons emitted by the
oldest stars that penetrate into the IGM via low opacity channels carved by the
ionising photons and supernova from younger stars. Before HII regions begin to
overlap, the photoionisation rate strongly fluctuates between different,
isolated HII bubbles, depending on the embedded ionising source, which we
suggest may result in spatial variations in the properties of dwarf galaxies.
