Morphological Classification of Galaxies by Shapelet Decomposition in the Sloan Digital Sky Survey. II. Multiwavelength Classification

We describe the application of the `shapelet' linear decomposition of galaxyimages to multi-wavelength morphological classification using the $u,g,r,i,$and $z$-band images of 1519 galaxies from the Sloan Digital Sky Survey. Weutilize elliptical shapelets to remove to first-order the effect of inclinationon morphology. After decomposing the galaxies we perform a principal componentanalysis on the shapelet coefficients to reduce the dimensionality of thespectral morphological parameter space. We give a description of each of thefirst ten principal component's contribution to a galaxy's spectral morphology.We find that galaxies of different broad Hubble type separate cleanly in theprincipal component space. We apply a mixture of Gaussians model to the2-dimensional space spanned by the first two principal components and use theresults as a basis for classification. Using the mixture model, we separategalaxies into three classes and give a description of each class's physical andmorphological properties. We find that the two dominant mixture model classescorrespond to early and late type galaxies, respectively. The third class has,on average, a blue, extended core surrounded by a faint red halo, and typicallyexhibits some asymmetry. We compare our method to a simple cut on $u-r$ colorand find the shapelet method to be superior in separating galaxies.Furthermore, we find evidence that the $u-r=2.22$ decision boundary may not beoptimal for separation between early and late type galaxies, and suggest thatthe optimal cut may be $u-r \sim 2.4$.