Signatures of long-lived spiral patterns
  Azimuthal age/color gradients across spiral arms are a signature of
long-lived spirals. From a sample of 19 normal (or weakly barred) spirals where
we have previously found azimuthal age/color gradient candidates, 13 objects
were further selected if a two-armed grand-design pattern survived in a surface
density stellar mass map. Mass maps were obtained from optical and
near-infrared imaging, by comparing with a Monte Carlo library of stellar
population synthesis models that allowed us to obtain the mass-to-light ratio
in the J band, (M/L)_J, as a function of (g-i) versus (i-J) color. The selected
spirals were analyzed with Fourier methods in search for other signatures of
long-lived modes related to the gradients, such as the gradient divergence
toward corotation, and the behavior of the phase angle of the two-armed spiral
in different wavebands, as expected from theory. The results show additional
signatures of long-lived spirals in at least 50% of the objects.
