Galactic ionising photon budget during the Epoch of Reionisation in the
  Cosmic Dawn II simulation
  Cosmic Dawn ("CoDa") II yields the first statistically-meaningful
determination of the relative contribution to reionization by galaxies of
different halo mass, from a fully-coupled radiation-hydrodynamics simulation of
the epoch of reionization large enough ($\sim$ 100 Mpc) to model global
reionization while resolving the formation of all galactic halos above $\sim
10^8 M_\odot$. Cell transmission inside high-mass haloes is bi-modal -- ionized
cells are transparent, while neutral cells absorb the photons their stars
produce - and the halo escape fraction $f_{esc}$ reflects the balance of star
formation rate ("SFR") between these modes. The latter is increasingly
prevalent at higher halo mass, driving down $f_{esc}$ (we provide analytical
fits to our results), whereas halo escape luminosity, proportional to $f_{esc}
\times$SFR, increases with mass. Haloes with dark matter masses within
$6.10^{8} M_\odot < M_h < 3.10^{10} M_\odot$ produce $\sim 80$% of the escaping
photons at z=7, when the Universe is 50% ionized, making them the main drivers
of cosmic reionization. Less massive haloes, though more numerous, have low
SFRs and contribute less than 10% of the photon budget then, despite their high
$f_{esc}$. High mass haloes are too few and too opaque, contributing $<10$%
despite their high SFRs. The dominant mass range is lower (higher) at higher
(lower) redshift, as mass function and reionization advance together (e.g. at
z$=8.5$, x$_{\rm HI}=0.9$, $M_h < 5.10^9 M_\odot$ haloes contributed
$\sim$80%). Galaxies with UV magnitudes $M_{AB1600}$ between $-12$ and $-19$
dominated reionization between z$=6$ and 8.
