Tracing spiral density waves in M81

TL;DR

This study maps the spiral density waves in galaxy M81 by combining infrared, optical, and near-infrared data to analyze the structure and phase relationship between stars and gas, revealing insights into the longevity and dynamics of spiral arms.

Contribution

It introduces a method to extract and analyze spiral density waves in M81 using multi-wavelength data and residual mass maps, providing new insights into spiral structure longevity.

Findings

Spiral density waves in M81 are long-lived.

The phase offset between stars and gas indicates the position of corotation.

The method effectively reveals non-axisymmetric structures in galaxy mass maps.

Abstract

We use SPITZER IRAC 3.6 and 4.5micron near infrared data from the Spitzer Infrared Nearby Galaxies Survey (SINGS), optical B, V and I and 2MASS Ks band data to produce mass surface density maps of M81. The IRAC 3.6 and 4.5micron data, whilst dominated by emission from old stellar populations, is corrected for small-scale contamination by young stars and PAH emission. The I band data are used to produce a mass surface density map by a B-V colour-correction, following the method of Bell and de Jong. We fit a bulge and exponential disc to each mass map, and subtract these components to reveal the non-axisymmetric mass surface density. From the residual mass maps we are able to extract the amplitude and phase of the density wave, using azimuthal profiles. The response of the gas is observed via dust emission in the 8micron IRAC band, allowing a comparison between the phase of the stellar density wave and gas shock. The relationship between this angular offset and radius suggests that the spiral structure is reasonably long lived and allows the position of corotation to be determined.