Analysis of Resonances in Grand Design Spiral Galaxies

We have searched for corotation resonances (CRs) in three southern grand design spiral galaxies: NGC 1365, NGC 1566, and NGC 2997. We have also introduced a method of quantifying errors in the phase diagram used to detect CRs. We established the m = 2 pattern CR at 12.1, 9.4, and 7 kpc for NGC 1365, NGC 1566, and NGC 2997, respectively. By using published rotation curves, we could determine spiral pattern angular speeds of 25.0, 12.2, and 17.6 km s-1 kpc-1, respectively. A three-armed component has been detected in NGC 2997, with the CR placed at 8.7 kpc with a pattern angular speed Ω = 12.7 km s-1 kpc-1. An m = 1 component was detected in NGC 1566. We warily locate the CR at 7.1 kpc, with a pattern angular speed ΩCR ≃ 16.6 km s-1 kpc-1. This pattern does not present inner Lindblad resonance. Ages have been determined by studying the radial density profile of the m = 2 Fourier components in g (newly formed stars) and i (perturbing spiral density wave supported by the disk of old stars), aided by the global aspect of the real spiral pattern in comparison with numerical simulations. The pattern is ~1200 Myr old in NGC 1365, ~800 Myr old in NGC 1566, and younger than 80 Myr in NGC 2997.