Stellar sources of dust in the high-redshift Universe

With the aim of investigating whether stellar sources can account for the >= 10(8) M-circle dot 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 M-circle dot and 0-1 Z(circle dot), we show that the role of asymptotic giant branch (AGB) stars in cosmic dust evolution at high redshift might have been overlooked. In particular, we find that (i) for a stellar population forming according to a present-day Larson initial mass function (IMF) with m(ch) = 0.35 M-circle dot, the characteristic time-scale at which AGB stars dominate dust production ranges between 150 and 500 Myr, depending both on the assumed star formation history and on the initial stellar metallicity; (ii) this result is only moderately dependent on the adopted stellar lifetimes, but it is significantly affected by variations of the IMF: for a m(ch) = 5 M-circle dot, dust from AGB starts to dominate only on time-scales larger than 1 Gyr and SNe are found to dominate dust evolution when m(ch) >= 10 M-circle dot. 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) the star formation history obtained in a numerical simulation by Li et al. which predicts that a large stellar bulge is already formed at z = 6.4, and (ii) a constant star formation rate of 1000 M-circle dot yr(-1), as suggested by the observations if most of the far-infrared luminosity is due to young stars. The total mass of dust predicted at z = 6.4 by the first model is 2 x 10(8) M-circle dot, within the range of values inferred by observations, with a substantial contribution (similar to 80 per cent) of AGB dust. When a constant star formation rate is adopted, the contribution of AGB dust decreases to similar to 50 per cent but the total mass of dust formed is a factor of 2 smaller. Both models predict a rapid enrichment of the interstellar medium 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.