A new method to estimate local pitch angles in spiral galaxies: Application to spiral arms and feathers in M81 and M51

Abstract

We examine $8\mu$m IRAC images of the grand design two-arm spiral galaxies M81 and M51 using a new method whereby pitch angles are locally determined as a function of scale and position, in contrast to traditional Fourier transform spectral analyses which fit to average pitch angles for whole galaxies. The new analysis is based on a correlation between pieces of a galaxy in circular windows of $(\ln R,\theta )$ space and logarithmic spirals with various pitch angles. The diameter of the windows is varied to study different scales. The result is a best-fit pitch angle to the spiral structure as a function of position and scale, or a distribution function of pitch angles as a function of scale for a given galactic region or area. We apply the method to determine the distribution of pitch angles in the arm and interarm regions of these two galaxies. In the arms, the method reproduces the known pitch angles for the main spirals on a large scale, but also shows higher pitch angles on smaller scales resulting from dust feathers. For the interarms, there is a broad distribution of pitch angles representing the continuation and evolution of the spiral arm feathers as the flow moves into the interarm regions. Our method shows a multiplicity of spiral structures on different scales, as expected from gas flow processes in a gravitating, turbulent and shearing interstellar medium. We also present results for M81 using classical 1D and 2D Fourier transforms, together with a new correlation method, which shows good agreement with conventional 2D Fourier transforms.

25 pages, 12 figures, Accepted to The Astronomical Journal

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Cited by in corpus (9)

• Local Simulations of Spiral Galaxies with the TIGRESS Framework: I. Star Formation and Arm Spurs/Feathers
• A multi-wavelength study of spiral structure in galaxies. I. General characteristics in the optical
• Spiral arm formation mechanisms: Spiral Structure in Barred galaxies. Observational constraints to spiral arm formation mechanisms
• HAWC+ Far-Infrared Observations of the Magnetic Field Geometry in M51 and NGC 891
• Optical Spectroscopy of Supernova Remnants in M81 and M82
• Galaxies decomposition with spiral arms -- II: A multiwavelength case study of M 51
• Colour jumps across the spiral arms of Hubble Ultra Deep Field galaxies
• Morphology and kinematics of the gas in M51: How interaction with NGC5195 has moulded the structure of its arms
• Formation of filaments and feathers in disc galaxies: Is self-gravity enough?