The many phases of massive galaxies : a near-infrared spectroscopic study of galaxies in the early universe

A key issue in astronomy today is understanding the star-formation and assembly history of massive galaxies. Stellar population studies show that the bulk of the stars in low-redshift massive galaxies is formed at z~2 or even higher. Furthermore, there are strong indications that about 50% of the massive, quiescent galaxies in the local universe was already in a quiescent phase when the universe was only half its current age. This raises the question of whether quiescent galaxies can be found at even earlier epochs, and when massive galaxies formed and assembled their stellar mass. In order to address these questions we have pushed current studies to higher redshift and conducted a near-infrared spectroscopic survey of massive galaxies at z~2.5. In total we obtained deep spectra with a wavelength coverage of 1-2.5 micron for a sample of 36 galaxies. A surprising result of our survey is that a significant fraction of the massive galaxies at z~2.5 is already in a quiescent phase. Furthermore, several massive galaxies host active galactic nuclei, which may be responsible for the quenching of star formation. This thesis introduces the survey, presents these main results, and discusses the resulting constraints on the formation history of massive galaxies.