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The stellar orbit distribution in present-day galaxies inferred from the CALIFA survey

Author/s: L. Zhu, G. van de Ven, R. van den Bosch, H.-W. Rix, M. Lyubenova, J. Falcón-Barroso, M. Martig, S. Mao, D. Xu, Y. Jin, A. Obreja, R. J. J. Grand, A. A. Dutton, A. V. Maccio, F. A. Gomez, J. C. Walcher, R. García-Benito, S. Zibetti, S. F. Sánchez

Reference: 2018 Nature Astronomy 2 233 | Link

 The orbit-circularity λ<sub>z</sub> distribution for each of 300 CALIFA galaxies. Each thin slice vertically represents the λ<sub>z</sub> distribution of one galaxy within half-light-radius R<sub>e</sub>. Darker color indicates higher probability density as indicated by the color bar. The total orbit weights of each galaxy have been normalized to unity. From left to right, the galaxies are sorted with increasing total stellar mass M<sub>✸</sub>. The black box indicates the range of 9.7<log (M<sub>✸</sub>/M<sub>⦿</sub>)<11.4 where the CALIFA sample is representative. The right panel shows the volume-corrected average orbit-circularity distribution within this mass range. The cold, warm, hot and CR components are divided in λ<sub>z</sub> indicated by the three horizontal dashed lines.
The orbit-circularity λz distribution for each of 300 CALIFA galaxies. Each thin slice vertically represents the λz distribution of one galaxy within half-light-radius Re. Darker color indicates higher probability density as indicated by the color bar. The total orbit weights of each galaxy have been normalized to unity. From left to right, the galaxies are sorted with increasing total stellar mass M. The black box indicates the range of 9.7<log (M/M⦿)<11.4 where the CALIFA sample is representative. The right panel shows the volume-corrected average orbit-circularity distribution within this mass range. The cold, warm, hot and CR components are divided in λz indicated by the three horizontal dashed lines.

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 to the quiescence or violence of the galaxies' formation histories. Here we present such orbit distributions, derived from stellar kinematic maps via orbit-based modelling for a well-defined, large sample of 300 nearby galaxies. The sample, drawn from the CALIFA survey, includes the main morphological galaxy types and spans the total stellar mass range from 108.7 to 1011.9 solar masses. Our analysis derives the orbit-circularity distribution as a function of galaxy mass, pz|M), and its volume-averaged total distribution, pz). We find that across most of the considered mass range and across morphological types, there are more stars on `warm' orbits defined as 0.25 ≤ λz ≤ 0.8 than on either `cold' or `hot' orbits. This orbit-based "Hubble diagram" provides a benchmark for galaxy formation simulations in a cosmological context.

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