Surface photometry of barred spiral galaxies

Abstract

Detailed surface photometry has been made for six early-type barred spiral galaxies with use of high-resolution photographic plates. The main results obtained by detailed examination of several kinds of profiles are the following: (1) On the azimuthal luminosity profiles along the concentric ellipses, which should appear as circles centered on the nuclei in the planes of the galaxies, the bars appear as narrow humps superposed on the nearly flat axisymmetric components. The luminosity contrasts between the bars and the underlying axisymmetric components are quite large and have values ranging from 2.5-5.5. Fourier decompositions of these azimuthal profiles reveal that the largest contribution comes from the m = 2 component, but higher even-order components (m = 4, 6) are not negligible; a density distribution approximated by the m = 2 component alone is too simple to represent the observed bars. (2) The luminosity profiles along the bar major axes protrude prominently in the bar regions on the extrapolated straight lines of the outer exponential disks and have sharp cutoffs at the bar ends. The luminosity profiles along the bar minor axes have Freeman's type II shape. However, the azimuthally averaged radial luminosity profiles are Freeman's type I profile and are indistinguishable from those of ordinary spirals. (3) The bar components are extracted as nonaxisymmetric components with rectangular shapes by subtracting the underlying axisymmetric components from the original images. The luminosity distributions along the major axes of the extracted bars are expressed by exponential functions with shallow luminosity gradients and sharp cutoffs. The distributions perpendicular to the bars are described by Gaussian functions. (4) These observed properties of barred spirals are compared with the model bars formed in AT-body simulations of a stellar disk. Significant differences in the luminosity distribution of the model bar have been found: (i) The gradient of the bar major axis profile is much steeper in the bar region than in the outer disk region, and there is no sharp cutoff at the bar ends, (ii) The azimuthal profiles in the bar region is well approximated only by the m = 2 and m = 4 components.