![The stellar orbit distribution in present-day galaxies inferred from the CALIFA survey The orbit-circularity λz distribution for each of 300 CALIFA galaxies](/sites/default/files/styles/crop_square_2_2_to_320px/public/images/news/resultados198_214_0.jpg?h=c576f392&itok=yB--WWM8)
Galaxy formation entails the hierarchical assembly of mass, along with the condensation of baryons and the ensuing, self-regulating star formation. The stars form a collisionless system whose orbit distribution retains dynamical memory that can constrain a galaxy's formation history. The ordered-rotation dominated orbits with near maximum circularity λz≃1 and the random-motion dominated orbits with low circularity λz≃0 are called kinematically cold and kinematically hot, respectively. The fraction of stars on `cold' orbits, compared to the fraction of stars on `hot' orbits, speaks directly
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