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

摘要

Two dimensional Fourier spectra of near-infrared images of galaxies provide apowerful diagnostic tool for the detection of spiral arm modulation in stellardisks. Spiral arm modulation may be understood in terms of interferencepatterns of outgoing and incoming density wave packets or modes. The brightnessalong a spiral arm will be increased where two wave crests meet andconstructively interfere, but will be decreased where a wave crest and a wavetrough destructively interfere. Spiral arm modulation has hitherto only beendetected in grand design spirals (such as Messier 81). Spiral arm amplitudevariations have the potential to become a powerful constraint for the study ofgalactic dynamics. We illustrate our method in two galaxies: NGC 4062 and NGC5248. In both cases, we have detected trailing and leading m=2 waves withsimilar pitch angles. This suggests that the amplification mechanism is theWASER type II. In this mechanism, the bulge region reflects (rather thanrefracts) incoming waves with no change of pitch angle, but only a change oftheir sense of winding. The ratio between the amplitudes of the leading and thetrailing waves is about 0.5 in both cases, wherein the higher amplitude isconsistently assigned to the trailing (as opposed to leading) mode. The resultsare particularly significant because NGC 5248 is an optically grand designgalaxy, whereas NGC 4062 is optically flocculent. NGC 4062 represents the veryfirst detection of spiral arm modulation in the stellar disk of an opticallyflocculent galaxy.