Inclination-Independent Galaxy Classification

We present a new method to classify galaxies from large surveys such as the Sloan Digital Sky Survey using inclination-corrected concentration, inclination-corrected location on the color-magnitude diagram, and apparent axis ratio. Explicitly accounting for inclination tightens the distribution of each of these parameters and enables simple boundaries to be drawn that delineate three different galaxy populations: early-type galaxies, which are red, highly concentrated, and round; late-type galaxies, which are blue, have low concentrations, and are disk dominated; and intermediate-type galaxies, which are red, have intermediate concentrations, and have disks. We have validated our method by comparing to visual classifications of high-quality imaging data from the Millennium Galaxy Catalogue. The inclination correction is crucial to unveiling the previously unrecognized intermediate class. Intermediate-type galaxies, roughly corresponding to lenticulars and early spirals, lie on the red sequence. The red sequence is therefore composed of two distinct morphological types, suggesting that there are two distinct mechanisms for transiting to the red sequence. We propose that intermediate-type galaxies are those that have lost their cold gas via strangulation, while early-type galaxies are those that have experienced a major merger either that consumed their cold gas, or whose merger progenitors were already devoid of cold gas (the “dry merger” scenario).