Evolutionary Transition of Galaxies

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An evolutionary transition of galaxies is defined as a sudden decline of star forming activity in young galaxies. We have built up an evolutionary model of galaxy spectra and have applied it to elliptical galaxies in clusters. We find that color-magnitude (CM) relation of elipticals is due to a systematic variation of mean stellar metallicity as a function of galaxy luminosity which could be best explained by a galactic wind model. We find that evolution of the CM relation strongly depends on an epoch of major star formation. A detailed comparison of our model CM relation with those of elliptical galaxies in clusters at different redshifts strongly suggests that these galaxies should have formed at zィイD2FィエD2 > 2.5 - 4.We measured equibalent widths of several metallic lines in high redshift (z 〜 3 - 4) quasars and found very strong iron emission there. This suggests that at least interstellar matter of host galaxies of these quasars was already enriched by ejecta of type Ia supernovae. … More Thus, galaxies at these redshifts are older than SNIa progenitors, a typical lifetime of which is around 1.5 Gyr. This means the host galaxies of these quasars, very likely elliptical galaxies, formed at zィイD2FィエD2 > 5 - 9.Simple calculation of CO luminosity of cluster ellipticals, based on our CM relation analysis, indicates that CO luminosity during actively star forming phase must be high enough to be detected by radio telescope. This reasonablly explains why we can detect CO in a quasar at z = 4.6, if the host galaxy is an elliptical before the onset of evolutionary transition.From these results, we conclude that the evolutionary transition in elliptical galaxies occurred at z 〜 4 - 5, possibly due to a galactic wind.With a hope that in near future submillimeter observations will become more easier and we will be able to get more spectra of galaxies in these wavelength region, we have newly built up an evolutionary model of galaxy spectra in which effects of dust absorption and re-emission are fully taken into account. We have applied the model to nearby star burst galaxies and are successful in desentagling age and dutiness from the observed optical-nearinfrared and farinfrared colours. Since primeval galaxies at high redshifts would be very similar to those nearby starburst galaxies, we believe that we have established a way to derive the age of these high redshift galaxies without any uncertainties. This would eventually lead us to estimate the epoch of formation of these primival galaxies. Less