Effects of dust grains on early galaxy evolution

Abstract

Stars form out of molecular gas and supply dust grains during their last evolutionary stages; in turn hydrogen molecules (H2) are produced more efficiently on dust grains. Therefore, dust can drastically accelerate H2 formation, leading to an enhancement of star formation activity. In order to examine the first formation of stars and dust in galaxies, we model the evolution of galaxies in the redshift range of 5 < z < 20. In particular, we focus on the interplay between dust formation in Type II supernova ejecta and H2 production on dust grains. Such effect causes an enhancement of star formation rate by an order of magnitude on a time-scale (∼3– 5 galactic dynamical times) shorter than the Hubble time-scale. We also find that about half of the radiative energy from stars is reprocessed by dust grains and is finally radiated in the far-infrared (FIR). For example, the typical star formation rate, FIR and ultraviolet (UV) luminosity of a galaxy with Mvir= 1011.5 M⊙ (virial mass) and zvir= 5 (formation redshift) are 3 M⊙ yr−1, 4 × 109 L⊙ and 3 × 109 L⊙, respectively. This object will be detected by both the Atacama Large Millimeter Array (ALMA) and the Next Generation Space Telescope (NGST). Typical star formation rates and luminosities (FIR, UV and metal-line luminosities) are calculated for a large set of (Mvir, zvir). Using these results and the Press—Schechter formalism, we calculate galaxy number counts and integrated light from high-redshift (z > 5) galaxies in submillimetre and near-infrared bands. We find that: (i) ALMA can detect dust emission from several × 103 galaxies per square degree, and (ii) NGST can detect the stellar emission from 106 galaxies per square degree. Further observational checks of our predictions include the integrated flux of metal (oxygen and carbon) lines; these lines can be used to trace the chemical enrichment and the gas density in early galactic environments. We finally discuss possible colour selection strategies for high-redshift galaxy searches.