PROFESSOR S. SRIDHAR

The spiral patterns of disc galaxies like the Milky Way are some of the most beautiful structures formed by stars and gas in the universe. They are density enhancements, driving disc evolution through new star formation and angular momentum transport. The density-wave theory of the 1960s failed to produce long-lived spiral patterns because the wave is absorbed at Lindblad resonances. Further progress came through careful numerical experiments, which suggest that spiral structure could arise from transient modes that create conditions conducive for their regeneration. The speaker will present a theory of mode renewal in which a transient spiral mode leaves behind "resonant scars" in phase space. Dissipation is suppressed within scars, so they act as filters of the spectrum of noise-generated modes, promoting the renewal of a few select modes. Relic scars sustained by a galaxy disc, due to past tidal interaction with a passing companion, may still be active enablers of two-armed "grand design" spiral patterns.