Unveiling the counter-rotating nature of the kinematically distinct core
  in NGC5813 with MUSE
  MUSE observations of NGC5813 reveal a complex structure in the velocity
dispersion map, previously hinted by SAURON observations. The structure is
reminiscent of velocity dispersion maps of galaxies comprising two
counter-rotating discs, and may explain the existence of the kinematically
distinct core (KDC). Further evidence for two counter-rotating components comes
from the analysis of the higher moments of the stellar line-of-sight velocity
distributions and fitting MUSE spectra with two separate Gaussian line-of-sight
velocity distributions. The emission-line kinematics show evidence of being
linked to the present cooling flows and the buoyant cavities seen in X-rays. We
detect ionised gas in a nuclear disc-like structure, oriented like the KDC,
which is, however, not directly related to the KDC. We build an axisymmetric
Schwarzschild dynamical model, which shows that the MUSE kinematics can be
reproduced well with two counter-rotating orbit families, characterised by
relatively low angular momentum components, but clearly separated in integral
phase space and with radially varying contributions. The model indicates that
the counter-rotating components in NGC5813 are not thin discs, but dynamically
hot structures. Our findings give further evidence that KDCs in massive
galaxies should not necessarily be considered as structurally or dynamically
decoupled regions, but as the outcomes of the mixing of different orbital
families, where the balance in the distribution of mass of the orbital families
is crucial. We discuss the formation of the KDC in NGC5813 within the framework
of gas accretion, binary mergers and formation of turbulent thick discs from
cold streams at high redshift.
