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Abstract: We present a new method to classify galaxies from large surveys like theSloan Digital Sky Survey using inclination-corrected concentration,inclination-corrected location on the color-magnitude diagram, and apparentaxis ratio. Explicitly accounting for inclination tightens the distribution ofeach of these parameters and enables simple boundaries to be drawn thatdelineate three different galaxy populations: Early-type galaxies, which arered, highly concentrated, and round; Late-type galaxies, which are blue, havelow concentrations, and are disk dominated; and Intermediate-type galaxies,which are red, have intermediate concentrations, and have disks. We havevalidated our method by comparing to visual classifications of high-qualityimaging data from the Millennium Galaxy Catalogue. The inclination correctionis crucial to unveiling the previously unrecognized Intermediate class.Intermediate-type galaxies, roughly corresponding to lenticulars and earlyspirals, lie on the red sequence. The red sequence is therefore composed of twodistinct morphological types, suggesting that there are two distinct mechanismsfor transiting to the red sequence. We propose that Intermediate-type galaxiesare those that have lost their cold gas via strangulation, while Early-typegalaxies are those that have experienced a major merger that either consumedtheir cold gas, or whose merger progenitors were already devoid of cold gasthe ``dry merger- scenario.

Autor: Jeremy Bailin, William E. Harris McMaster University


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