Global Spiral Modes in NGC 1566: Observations and Theory

We present an observational and theoretical study of the spiral structure in galaxy NGC 1566. A digitized image of NGC 1566 in I-band, obtained with the Australian National University 30in telescope, was used for measurements of the radial dependence of amplitude variations in the spiral arms. The azimuthal variations of the surface brightness in the I-band are about ±7 % at 50 radius and increase up to 57 % at 100. We use the known velocity dispersion in the disk of NGC 1566, together with its rotation curve, to construct linear and 2D nonlinear simulations which are then compared with observations. The linear stability analysis of the disk of NGC 1566, made under the assumption that the ratio of the vertical and the radial velocity dispersions is equal to 0.6, shows that the disk is stable with respect to spiral perturbations. We confirm this conclusion with 2D hydrodynamical simulations. National Astronomical Observatory, Mitaka, Tokyo 181–8588, Japan