The structure of spiral galaxies — II. Near‐infrared properties of spiral arms

ABSTRACT We have imaged a sample of 45 face-on spiral galaxies in the K-band, to determinethe morphology of the old stellar population, which dominates the mass in the disk.The K-band images of the spiral galaxies have been used to calculate different char-acteristics of the underlying density perturbation such as arm strengths, profiles andcross-sections, and spiral pitch angles. Contrary to expectations, no correlation wasfound between arm pitch angle and Hubble type, and combined with previous resultsthis leads us to conclude that the morphology of the old stellar population bears littleresemblance to the optical morphology used to classify galaxies.The arm properties of our galaxies seem inconsistent with predictions from thesimplest density wavetheories, and some observations, such as variationsin pitch anglewithin galaxies, seem hard to reconcile even with more complex modal theories. Barshave no detectable effect on arm strengths for the present sample.We have also obtained B-band images of three of the galaxies. For these galaxieswe have measured arm cross-sections and strengths, to investigate the effects of diskdensity perturbations on star formation in spiral disks. We find that B-band arms leadK-band arms and are narrower than K-band arms, apparently supporting predictionsmade by the large scale shock scenario, although the effects of dust on B-band imagesmay contribute towards these results.Key words: galaxies: spiral - galaxies: structure - galaxies: fundamental parameters