Tracing spiral density waves in M81

ABSTRACT We use SPITZER IRAC 3.6 and 4.5µm near infrared data from the Spitzer InfraredNearby Galaxies Survey (SINGS), optical B, V and I and 2MASS K s band data to producemass surface density maps of M81. The IRAC 3.6 and 4.5µm data, whilst dominatedby emis-sion from old stellar populations, is corrected for small-scale contamination by young starsand PAH emission. The I band data are used to produce a mass surface density map by aB-Vcolour-correction, following the method of Bell and de Jong. We fit a bulge and expo-nential disc to each mass map, and subtract these components to reveal the non-axisymmetricmass surface density. From the residual mass maps we are able to extract the amplitude andphase of the density wave, using azimuthal profiles. The resp onse of the gas is observed viadust emission in the 8µm IRAC band, allowing a comparison between the phase of the stellardensity wave and gas shock. The relationship between this angular offset and radius suggeststhat the spiral structure is reasonably long lived and allows the position of corotation to bedetermined.Key words: galaxies:individual:M81–galaxies:spiral–galaxies:st ructure–infrared:galaxies

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