NGC 1399: A complex dynamical case

¹ Osservatorio Astronomico di Capodimonte, via Moiariello 16, 80131 Napoli, Italy
² Dipartimento di Scienze Fisiche, Università di Napoli Federico II, complesso Monte S. Angelo, via Cintia, 80126 Napoli, Italy

Corresponding author: N. R. Napolitano, napolitano@na.astro.it

Received: 14 September 2001
Accepted: 17 December 2001

Abstract

Evidence for a disturbed velocity structure in the outer regions of the galaxy NGC 1399 comes from a re-analysis of the planetary nebulae data from Arnaboldi et al. (1994). We find a strong rotation along a for which is followed by a rapid drop off at larger radii, where the velocity dispersion starts to increase. This kinematical behavior can be interpreted as an indication for an interaction scenario. Interaction is advocated in previous analysis of the halo regions of this system, based on different dynamical tracers such as globular clusters and X-rays, but in all these studies the mass distribution is derived under the equilibrium hypothesis, which is not appropriate when an interaction takes place. Here we attempt a non-equilibrium dynamical analysis of NGC 1399: with a simple model and under the impulse approximation, we show that the observed kinematics is consistent with an energy injection caused by a flyby encounter of NGC 1399 with the nearby system NGC 1404. In this approach, we find a mass-to-light ratio, within , which is about half of that requested when equilibrium is assumed, i.e. .