A dust emission model of Lyman-break galaxies

Abstract

Lyman-break galaxies (LBGs) contain a non-negligible amount of dust. Takeuchi (2003a, MNRAS, 343, 839) (T03) constructed a model of the infrared spectral energy distribution (SED) for very young galaxies by taking into account the dust size distribution in the early stage of galaxy evolution, which can be different from that of present-day evolved galaxies. We applied the T03 model to LBGs and constructed their expected SED. In order to examine the grain size distribution of dust, we calculated the SEDs based on two distinct types of the distribution models: a single-sized distribution and a power-law distribution with a slope of dN/da ∝ a −3.5 . We found that the single-sized and power-law dust size distributions yield a very similar detectability of LBGs at submillimetres (submm). We also found that galaxies with a power-law dust distribution have much less flux in the mid-infrared (MIR) than the other type. By making use of this fact we will be able to explore the dust grain size distribution in high-redshift galaxies through (observer-frame) FIR observations in future observations. We then applied the model to a gravitationally lensed LBG MS 1512−cB58 (cB58), a unique probe of the dust emission from LBGs. Observations by SCUBA suggest that the galaxy has hot dust. Our model well reproduced the hot dust temperature under natural physical assumptions for the star formation rate (SFR), starburst age, and the radius of the star forming region in this galaxy. We also examined the detectability of LBGs at submm wavelengths in an eight-hour deep survey by ALMA. The LBG population with an age >10 8 yr and a SFR > 10 Myr −1 can be detected in such a survey. By integrating over their redshifted SEDs with the observed luminosity functions, we obtained the contribution of LBGs to the cosmic infrared background radiation (CIRB). Although they have a non-negligible amount of dust, their contribution was found to be small, especially in the FIR ∼ 200 µm. Thus, we need a strongly obscured population of galaxies which contains a large amount of star formation, at some epoch in the history of the universe.