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Growth of Spirals: Secular or Driven by Mergers ?

The physical phenomena contributing to the galaxy growth can be tested all the way to z= 1. Galaxy mass, extinction, star formation and gas metal abundance can be measured in a robust way, as well as the distribution of the galaxy morphologies. I discuss here the observational methods and their accuracy. Physical quantities can be evaluated with uncertainties much lower than 0.3 dex, if they are based on 2 sets of independent measurements. For example, at a given IMF, the star formation rate is well estimated by combining flux measurements of the extinction corrected Balmer line and of the mid-IR continuum. Spiral mass growth had occurred from gas accretion and from merging. Gas accretion can explain at most half of the spiral mass growth: at moderate redshift, the numerous population of compact, merger and irregular galaxies requires another origin. A spiral rebuilding scenario is able to reproduce all the evolutionary trends observed since z~1, and could be at the origin of the present-day, numerous population of early type spirals.