The detection of spiral arm modulation in the stellar disk of an opticallyflocculent and an optically grand design galaxy

Two dimensional Fourier spectra of near-infrared images of galaxies provide a powerful diagnostic tool for the detection of spiral arm modulation in s tellar disks. Spiral arm modulation may be understood in terms of interfere nce patterns of outgoing and incoming density wave packets or modes. The br ightness along a spiral arm will be increased where two wave crests meet an d constructively interfere, but will be decreased where a wave crest and a wave trough destructively interfere. Spiral arm modulation has hitherto onl y been detected in grand design spirals (such as Messier 81), Spiral arm am plitude variations have the potential to become a powerful constraint for t he study of galactic dynamics. We illustrate our method in two galaxies: NG C 4062 and NGC 5248. In both cases, we have detected trailing and leading m =2 waves with similar pitch angles. This suggests that the amplification me chanism is the WASER type II. In this mechanism, the bulge region reflects (rather than refracts) incoming waves with no change of pitch angle, but on ly a change of their sense of winding. The ratio between the amplitudes of the leading and the trailing waves is about 0.5 in both cases, wherein the higher amplitude is consistently assigned to the trailing (as opposed to le ading) mode. The results are particularly significant because NGC 5248 is a n optically grand design galaxy, whereas NGC 4062 is optically flocculent. NGC 4062 represents the very first detection of spiral arm modulation in th e stellar disk of an optically flocculent galaxy.