Poster Abstract

P.4 Sven De Rijcke (UGent)

Instabilities in disc galaxies: from noise to grooves to spirals

Despite decades of research, the detailed dynamics underlying spiral structure in disc galaxies are not yet known and many open questions remain. Are spiral patterns short-lived and transient amplified noise? Are they self-excited modes that can persist for many revolutions? Spirals formed in N-body simulations tend to be time-dependent and transient. However, detailed analysis of recent simulations (Sellwood & Carlberg 2019) seems to indicate that this continuous variability could be due to the superposition of multiple rotating spiral patterns and that spiral activity persists globally through the appearance of successive generations of such patterns.
This then begs the question of what causes these patterns. I present an investigation of how grooves carved in the phase space of a half-mass Mestel disc can trigger the vigorous growth of two-armed spiral eigenmodes (De Rijcke et al. 2019). Such grooves result from the self-induced dynamics of a disc subject to finite-N shot noise, as swing-amplified noise patterns push stars towards lower angular momentum orbits at their inner Lindblad radius. Thus, it is possible for an isolated, linearly stable stellar disc to spontaneously become linearly unstable to such groove modes via the self-induced formation of phase-space grooves through finite-N dynamics. This stability analysis has now been extended to also include a gas disc (De Rijcke in prep.). These results help explain the growth and maintenance of spiral patterns in simulated and real disc galaxies. This ongoing investigation is part of the SEGAL (Secular Evolution of GALaxies) project (PI: C. Pichon, IAP, France).