Photometric studies of spiral structure. I - The disks and arms of six SB I and SC I galaxies

Abstract

This paper presents detailed surface photometry of the galaxies NGC 3031, 4254, 4321, 5194, 5364, and 5457. Profiles showing the variation of surface brightness with azimuthal angle in each galaxy are given and used to separate the light into disk and arm components. The disk level at each radius is defined by the two main minima of the azimuthal fight distribution, and the fight above this level is assigned to the arms. The disks are exponential with an extrapolated central surface brightness of (B¿) D = 21.67 ± 0.35 mag arcsec" 2 , in excellent agreement with Freeman. The disk colors are very uniform, with radial gradients mostly below the detection limit (±0.010 mag kpc-1 in R-V), and lie in the narrow range of colors between old galactic clusters (B-V x +0.7) and giant elliptical galaxies (B-Vz +0.9). The arms are significantly bluer than the disks and have a highly composite spectrum, as judged from the color indices. The ratio of arm intensity to disk intensity increases with radius, which explains the well-known outward bluing of spiral galaxies. There are indications that the ratio of total arm intensity to total disk intensity, which is a coarse measure of the strength of star formation , correlates with van der Kruifs compression strength of the gas. The most interesting result is the detection of broad spiral patterns in the underlying disks of the galaxies. We present evidence that these patterns represent variations in the surface mass density of the old disk population. The fractional amplitudes of the patterns increase with radius, reaching about ±20 percent to ±30 percent of the mean surface mass density near the outermost H n regions. The intriguing "red arms" discovered by Zwicky (1955) on composite photographs of NGC 5194 are a good example of such a disk pattern. By reanalyzing the photographic composition process, we show that these arms are regions of enhanced surface brightness in the old disk, rather than regions of enhanced color; this agrees with their interpretation as "massive" arms.