Insights into spiral structure in galaxies from SINGS infrared imaging

Spiral structure in galaxies can be traced in the optical (reflecting recent star formation) in the near infrared (tracing the dominant stellar mass component) and in tracers of the disc gas (such as thermal emission at 8 μm). We first review the predictions (based on both analytic and numerical studies) for the spatial relationship between these features according to various models for spiral structure formation and then present the result of our recent infrared imaging study of nearby galaxies with the Spitzer Space Telescope (SINGS). We find that in the case of M81 the shock in the disc gas is azimuthally displaced from the potential minimum and that this offset increases in the upstream direction with increasing radius. This result is consistent with the predictions of a model in which the gas responds to a long lived rigidly rotating spiral pattern whose pattern speed is consistent with previous estimates in M81. On the other hand we find that the potential arms are only sufficiently regular to attempt such an analysis in a further 7 galaxies and that in these cases—although there is a general tendency towards upstream offsets at large radius—there is no consistent radial trend that would allow one to identify a pattern speed in these cases. We draw attention to the fact that the spread in pitch angles that we measure in the infrared is much smaller than in the optical and that there is no correlation between the pitch angle of potential arms and a galaxy’s Hubble type.