The dust mass in z > 6 normal star forming galaxies
  We interpret recent ALMA observations of z > 6 normal star forming galaxies
by means of a semi-numerical method, which couples the output of a cosmological
hydrodynamical simulation with a chemical evolution model which accounts for
the contribution to dust enrichment from supernovae, asymptotic giant branch
stars and grain growth in the interstellar medium. We find that while stellar
sources dominate the dust mass of small galaxies, the higher level of metal
enrichment experienced by galaxies with Mstar > 10^9 Msun allows efficient
grain growth, which provides the dominant contribution to the dust mass. Even
assuming maximally efficient supernova dust production, the observed dust mass
of the z = 7.5 galaxy A1689-zD1 requires very efficient grain growth. This, in
turn, implies that in this galaxy the average density of the cold and dense
gas, where grain growth occurs, is comparable to that inferred from
observations of QSO host galaxies at similar redshifts. Although plausible, the
upper limits on the dust continuum emission of galaxies at 6.5 < z < 7.5 show
that these conditions must not apply to the bulk of the high redshift galaxy
population
