We describe a Fourier-based method of separating bars from spirals innear-infrared images. The method takes advantage of the fact that a bar istypically a feature with a relatively fixed position angle, and uses the simpleassumption that the relative Fourier amplitudes due to the bar decline withradius past a maximum in the same or a similar manner as they rose to thatmaximum. With such an assumption, the bar can be extrapolated into the spiralregion and removed from an image, leaving just the spiral and the axisymmetricbackground disk light. The procedure allows us to estimate the maximumgravitational torque per unit mass per unit square of the circular speed forthe bar and spiral forcing separately, parameters which quantitatively definethe bar strength Q_b and the spiral strength Q_s following the recent study ofButa & Block. We outline the complete procedure here using a 2.1 micron imageof NGC 6951, a prototypical SAB(rs)bc spiral. We justify our assumption on howto make the bar extrapolation using an analysis of NGC 4394, a barred spiralwith only weak near-infrared spiral structure, and we justify the number ofneeded Fourier terms using NGC 1530, one of the most strongly-barred galaxies(bar class 7) known. We also evaluate the main uncertainties in the technique.Allowing for uncertainties in vertical scaleheight, bar extrapolation, skysubtraction, orientation parameters, and the asymmetry in the spiral armsthemselves, we estimate Q_b=0.28\pm0.04 and Q_s=0.21\pm0.06 for NGC 6951.(abridged)
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