c) Preliminary Intensity Estimates
Quantitative analyses of the galaxies in this atlas will be made when all of the plates have been scanned with a microdensitometer. However, in order to provide a guideline for examining and comparing the galaxy photographs, some initial results will be described. These are not intended to be exact, but are merely illustrative.
Photoelectric photometry data in the B and I passbands previously were obtained for six of the galaxies in this atlas: NGC 3031, 4321, 5194, 5457 (Schweizer 1976), and NGC 628 and 7793 (Elmegreen 1980a). Apertures of 17".8 and 16".0, respectively, were used. Each galaxy was measured at 6 to 11 points on its E-W axis. A point-by-point comparison of these results indicates that the surface brightnesses typically are 2 mag arcsec-2 fainter in the B than in the I. The central regions average 19.3±1.0 mag arcsec-2 in the B and 17.3±1.3 mag arcsec-2 in the I, while the outer regions are 23 to 24 mag arcsec-2 in the B and 21 to 22 mag arcsec-2 in the I.
Several galaxies (NGC 2403, 4321, 5033, 5248) were examined in order to estimate the relative intensities of the features seen in the galaxy photographs. Arm and interarm densities were visually determined with a step wedge, and intensities were obtained from the characteristic curves. In all cases, the figures quoted below should be regarded as upper limits since the largest errors result for features that are close to the sky brightness. In NGC 2403, the arms are brighter than the interarm regions by about 130% in the B, and 40% in the I. In NGC 4321, the average arm intensity, excluding the brightest areas, is 170% brighter than the interarm for both the B and the I plates. In NGC 5033, the arms are about 270% brighter than the interarm in the B, and 130% brighter in the I. In NGC 5248, the arms are brighter than the interarm by a factor of 2 in the B, and an upper limit of 18 in the I.
Selected surface photometry of the galaxies NGC 628, 5194, 5457, and 7793 (Elmegreen 1980a) provides more details regarding the intensities of different features. A total of 44 dusty regions were measured relative to their surrounding neighborhood; these features are identified in the above paper. In the B band, these dust areas are 0.3 to 1.4 mag fainter than nearby comparison regions; in the I, the differential magnitudes range from 0.1 to 0.6.
It is instructive to consider the differential magnitudes that correspond to different intensity enhancements. A spiral arm with a 5% amplitude relative to its surroundings is only 0.05 mag brighter than the interarm. Such a pattern would be difficult to measure even with surface photometry. A 10% amplitude would result in an 0.1 mag differential, which can be measured. The arms in NGC 4321 are ~ 1.1 mag brighter than the interarm regions. Features with a 20% enhancement would be easy to recognize on the photographs (e.g., the dust regions mentioned previously); most of the arms in both the B and I passbands are probably considerably brighter than this, based on the previous estimates. The detection of fainter patterns will require microdensitometer measurements. Therefore, the simplistic divisions into grand design and flocculent galaxies in this atlas represent gross separations based on the presence or absence of smooth continuous arms showing a conservatively estimated 20% or more enhancement over the interarm regions.