NARROW-BAND PHOTOMETRY AND THE EVOLUTION OF GALAXIES

A new approach to the use of narrow-band filters in extragalactic research is described. The photometric system vz, bz, yz essentially imitates, at the redshift of the cluster of galaxies in question, the Stromgren v, b, y passbands. This system has some obvious advantages: the uncertainties associated with k-corrections are bypassed, and the passbands avoid the stronger emission lines appearing in the spectra of some normal and most active galaxies. It is necessary to acquire filters for each redshift value and to have accurate spectrophotometry for at least one galaxy at the redshift in order to calibrate the photometry. It is shown that the vz, bz, yz filter system is especially suitable for testing the evolutionary synthesis models, such as those published by Bruzual (1983a,b). For that purpose 48 CCD frames of three clusters of galaxies (Abell 665, the small cluster around 3C 234, and C1 1021.1+0427) were obtained with the University of Hawaii 2.2-m telescope at Mauna Kea Observatory. The complete photometric reduction was carried out with the ESO image-processing system MIDAS (Munich Image Data Analysis System). To discriminate between stars, image defects, and galaxies, the profile gradient relative to a point-spread function was calculated for every object found. For galaxies, metric magnitudes were derived corresponding to a linear physical standard diameter of 32 kpc (H₀ = 60 km s⁻¹ Mpc⁻¹, q₀ = 0.5). Galaxies showing a strong color gradient along their images (disk galaxies) are distinguished from galaxies with no, or very small, color gradient along their images (ellipticals and lenticulars). The colors of galaxies derived from the CCD vz, bz, yz photometry and synthetic colors derived from published spectrophotometric data of galaxies are compared with the evolutionary synthesis models of Bruzual (1983a,b). The color index (vz–yz) gives the best discrimination between various models. All data fit much better to the models calculated with H₀ = 50 km s⁻¹ Mpc and q₀ = 0 than for high values of H₀. The colors derived from CCD observations are in agreement with the mildly or weakly evolving models.