  With the aim of investigating whether stellar sources can account for the
>10^8 Msun dust masses inferred from mm/sub-mm observations of samples of
5<z<6.4 quasars,we develop a chemical evolution model which follows the
evolution of metals and dust on the stellar characteristic lifetimes, taking
into account dust destruction mechanisms.Using a grid of stellar dust yields as
a function of the initial mass and metallicity over the range 1-40 Msun and 0-1
Zsun,we show that the role of AGB stars in cosmic dust evolution at high
redshift might have been over-looked.We apply the chemical evolution model with
dust to the host galaxy of the most distant quasar at z=6.4, SDSS
J1148+5251.Given the current uncertainties on the star formation history of the
host galaxy, we have considered two models: (i) a star formation history
obtained in a numerical simulation by Li et al.(2007) which predicts that a
large stellar bulge is already formed at z=6.4,and (ii) a constant star
formation rate of 1000 Msun/yr, as suggested by the observations if most of the
FIR luminosity is due to young stars.The total mass of dust predicted at z=6.4
by the first model is 2x10^8Msun,within the range of values inferred by
observations,with a substantial contribution (80%) of AGB-dust.When a constant
star formation rate is adopted,the contribution of AGB-dust decreases to 50%
but the total mass of dust formed is a factor 2 smaller.Both models predict a
rapid enrichment of the ISM with metals and a relatively mild evolution of the
carbon abundance,in agreement with observational constraints. This supports the
idea that stellar sources can account for the dust observed but show that the
contribution of AGB stars to dust production cannot be neglected, even at the
most extreme redshifts currently accessible to observations.
