Structural and photometric classification of galaxies. I. Calibration based on a nearby galaxy sample

In this paper we define an observationally robust, multiparameter space for the classification of nearby and distant galaxies. The parameters include luminosity, color, and the image-structure parameters: size, image concentr ation, asymmetry, and surface brightness. Based on an initial calibration o f this parameter space using the "normal" Hubble types surveyed in 1996 by Frei et al., we find that only a subset of the parameters provide useful cl assification boundaries for this sample. Interestingly, this subset does no t include distance-dependent scale parameters such as size or luminosity. T he essential ingredient is the combination of a spectral index (e.g., color ) with parameters of image structure and scale: concentration, asymmetry, a nd surface brightness. We refer to the image structure parameters (concentr ation and asymmetry) as indices of "form." We define a preliminary classifi cation based on spectral index, form, and surface brightness (a scale) that successfully separates normal galaxies into three classes. We intentionall y identify these classes with the familiar labels of early, intermediate, a nd late. This classification, or others based on the above four parameters, can be used reliably to define comparable samples over a broad range in re dshift. The size and luminosity distribution of such samples will not be bi ased by this selection process except through astrophysical correlations be tween spectral index, form, and surface brightness.