Gdds Highlights: Galaxy Evolution Revealed

The Gemini Deep Deep Survey, GDDS, produced several significant results relating to the evolution of galaxies. All of these results are consistent with the “downsizing” concept of galaxy formation and evolution, i.e., that the active periods of star formation moved progressively from very massive galaxies at high redshift to much lower mass galaxies at the present epoch. Spectra of massive red galaxies at z ∼ 1.7 demonstrates that they contain old stellar populations and hence must have formed their stars in the first ∼ 3Gyr of cosmic history; indicators of star formation activity show that the star formation rate in the most massive galaxies was much higher at z = 2 than today, that the activity in intermediate mass galaxies peaked near z ∼ 1.5, while, since z ∼ 1 the activity is primarily confined to lower mass galaxies. The GDDS also uncovered a relatively high percentage of post-starburst galaxies at z ∼ 1, a result that is anticipated given all the activity seen at higher redshifts. Measurements of the strengths of metal lines of a subsample of the GDDS and CFRS galaxies at z ∼ 0.7 reveal that, at a given mass, they had lower metallicities than at present. The evolution in the mass-metallicity relation is consistent with a model in which star formation lasts longest in less massive galaxies, again an expected result in the downsizing scenario.