Evolution of Galaxies in the Young Universe

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This study aimed for to investigating the spectral energy distribution (SED) of starburst galaxies to establish a way to discover efficiently primeval galaxies at cosmological distances. First we built up an evolutionary synthesis model for the SED evolution of galaxies that solves radiation field by dust and stars as well as chemical enrichment of gas-stellar system under a certain scheme of star formation history. Applying the model to UV-selected (UVSBGs) and ultra-luminous IR (ULIRGs) galaxies, we have found that although the SEDs of these galaxies show a rather wide variety, this is not due to a spread in the burst age, as was supposed before, but due to a difference in the compactness of star forming region. More active starburst regions tend to be more compact and be obscured by denser dust. We also find that starburst regions are not entirely covered by the dust, instead photons are leaking from the deep inside and influence significantly the apparent feature of SEDs. Without precise measurements of these effects, it is rather meaningless to derive age, optical depth, and size of the star forming regions. This study suggests that starbursts are bimodal. Weaker bursts should be induced by gravitational instability of gaseous disk, while very intensive bursts are likely to be triggered by external dynamical disturbances such as galaxy-galaxy mergings. It is very much plausible that starbursts in the young universe are also bimodal; thus a caution is required when we analyze the SEDs of primeval galaxies at very large redshifts. The present study will provide a fundamental insight, once observational data of high-z galaxies become available with new generation space telescopes and far-infrared, submm interferometers.