Dynamical decomposition of galaxies reveals the unquiet life of disks

The motions of stars in galaxies are the result of their formation processes, of the internal dynamical evolution of galaxies, and of the episodes of accretion and merger that galaxies may experience along their life. Hence analyzing how stars at different locations and with different ages or chemical compositions orbit within galaxies provide invaluable information to understand how galaxies formed and evolve. This task, however, is extremely challenging and requires advanced instrumentation and observational techniques to map the kinematics and the properties of stars, and utterly complex analysis methods to model the physical state and evolution of the systems.

A group of scientists, led by Ling Zhu of the Max Planck Institute for Astronomy in Heidelberg and including Stefano Zibetti from the INAF-Arcetri Astrophysical Observatory, has made a big step in this direction with the results presented in a paper published on January 1st, 2018 in Nature Astronomy, "The stellar orbit distribution in present-day galaxies inferred from the CALIFA survey" (also available here). They have analyzed the statistical distribution of the orbits of stars for the first time in a representative sample of present-day galaxies. The team has exploited the wealth of exquisite Integral-Field Spectroscopic data (i.e. spatially resolved spectroscopy) collected by the CALIFA (Calar Alto Legacy Integral Field Area) survey to obtain detailed kinematic maps of 300 galaxies. The observations were modeled using the Schwarzschild orbit-superposition method to decompose the stellar population of each galaxy into three different families of orbits: "cold" orbits, representing the ordered circular motions of stars in galactic disks; "hot" orbits, characteristic of chaotic motions occurring in spheroidal components, such as the bulges and the haloes; "warm" orbits, with a significant rotational component, yet with non-negligible random motions, typical of thick disks and "pseudo-bulges" (see Figure 1).

Figure 1: Models of stellar orbits (left) are matched to the observed images and maps of stellar velocity and velocity dispersion measured from the CALIFA spectra (right; example for the galaxy NGC001 in the sample). Credit: Instituto de Astrofisica de Andalucia (IAA-CSIC). Adapted from figure 1 of Zhu et al, 2018.

The statistical distribution of stars in these three orbital families, as a function of galaxy properties, is a key benchmark and will be of fundamental importance to constrain galaxy evolution models and simulations of the Universe (see Figure 2). Interestingly, this study finds that no galaxy is fully dominated by ordered circular motions. Even those galaxies that appear structurally dominated by a thin disk, actually have a majority of stars in warm orbits and a fraction on hot ones. This supports the idea that, although stars are born mainly in thin purely rotating disks, the so-called dynamical "secular evolution" driven by instabilities (spiral arms, bars) and minor interactions with external galaxies plays a major role in reshaping the orbital distribution of stars and in "warming up" the galactic disks.

Figure 2: Average fractional contribution of each of the three orbital components, plus counter-rotating (CR) orbits, as a function of the galaxy stellar mass. Source: figure 3 of Zhu et al, 2018.

For furher info, see "Quando le stelle perdono la bussola" on Media INAF.

Edited by S. Zibetti and A. Gallazzi