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Early phases of galaxy evolution

Final Report Summary - EARLY (Early phases of galaxy evolution)

The EARLY project is focused on characterizing the first phases of galaxy evolution using the VIMOS Ultra-Deep Survey (VUDS), the largest observational survey of the early universe based on spectroscopy conducted so far. VUDS sets to study a relatively poorly explored epoch in galaxy evolution from the end of reionisation (z~6, 13 billion years ago) to the peak in star formation activity in the universe (z~2, 10 billion years ago), an epoch of very active galaxy assembly. The main science goals are to identify the physical processes and associated timescales driving galaxy mass assembly and star formation activity, in the context of a comprehensive galaxy formation and evolution scenario.

EARLY with the VUDS survey does establish from direct observational evidence that galaxy formation proceeds along several different paths, and that galaxy evolution is the result of several important processes acting together. A number of important physical parameters are computed with unprecedented accuracy over a large sample of 10000 galaxies with 0
Combining deep spectroscopy and imaging (from HST), we identify a mode of galaxy formation where galaxies are born as extremely compact and massive clumps of star-forming gas. Some of these probably remain very compact for several billion years, growing mass by gas accretion and violent dissipative events like mergers. Elliptical galaxies observed in the local universe are their likely descendants. Another path of formation concerns galaxies born already as large disks of gas which fragment in large star-forming clumps. Some of these galaxies may end-up as the large spiral galaxies like our own Galaxy, the Milky Way, observed in the local universe.

Following these two modes of formation, the subsequent evolution takes complex routes. We find direct evidence for major physical processes at play: galaxies merge at a high rate at early times, their properties are linked to their environment, and star formation is rapidly quenched quite early in the evolution of some galaxies. We infer that gas accretion must be present, but only from indirect evidence. We show that galaxies start to be born very early on, as early as z~15, and that the formation of new galaxies is a continuous process at least until the peak of star formation in the universe at z~2. We identify some of the most distant proto-structures found today, and demonstrate that the evolution of galaxies is somehow accelerated in these rich environments. All these observed properties constitute a reference to galaxy formation and evolution models which have yet to reproduce the complexity of the real universe along cosmic time.

Overall EARLY brings a detailed insight into the process of galaxy formation and subsequent evolution based on robust observed properties. The data from the VUDS survey assembled within EARLY are made available for the benefit of the whole community and public at large.