Unveiling the counter-rotating nature of the kinematically distinct core in NGC 5813 with MUSE

Abstract

MUSE observations of NGC5813 reveal a complex structure in the velocity\ndispersion map, previously hinted by SAURON observations. The structure is\nreminiscent of velocity dispersion maps of galaxies comprising two\ncounter-rotating discs, and may explain the existence of the kinematically\ndistinct core (KDC). Further evidence for two counter-rotating components comes\nfrom the analysis of the higher moments of the stellar line-of-sight velocity\ndistributions and fitting MUSE spectra with two separate Gaussian line-of-sight\nvelocity distributions. The emission-line kinematics show evidence of being\nlinked to the present cooling flows and the buoyant cavities seen in X-rays. We\ndetect ionised gas in a nuclear disc-like structure, oriented like the KDC,\nwhich is, however, not directly related to the KDC. We build an axisymmetric\nSchwarzschild dynamical model, which shows that the MUSE kinematics can be\nreproduced well with two counter-rotating orbit families, characterised by\nrelatively low angular momentum components, but clearly separated in integral\nphase space and with radially varying contributions. The model indicates that\nthe counter-rotating components in NGC5813 are not thin discs, but dynamically\nhot structures. Our findings give further evidence that KDCs in massive\ngalaxies should not necessarily be considered as structurally or dynamically\ndecoupled regions, but as the outcomes of the mixing of different orbital\nfamilies, where the balance in the distribution of mass of the orbital families\nis crucial. We discuss the formation of the KDC in NGC5813 within the framework\nof gas accretion, binary mergers and formation of turbulent thick discs from\ncold streams at high redshift.\n