A connection between bulge properties and the bimodality of galaxies

The global colors and structure of galaxies have recently been shown to follow a bimodal distribution: a red sequence populated prototypically by early-type galaxies, and a blue cloud whose typical objects are late-type disk galaxies. Intermediate-type (Sa-Sbc) galaxies populate both regions. It has been suggested that this bimodality reflects the two-component nature of disk-bulge galaxies. However, it has now been established that there are two types of bulges: classical bulges that are dynamically hot systems resembling (little) ellipticals, and pseudobulges, dynamically cold, flattened, disklike structures that could not have formed via violent relaxation. Given the different formation mechanisms of these bulges, the question is whether at types Sa-Sbc, where both bulge types are found, the red-blue dichotomy separates galaxies at some value of the disk-to-bulge ratio, B/T, or whether it separates galaxies of different bulge type, irrespective of B/T. Here we identify classical bulges and pseudobulges morphologically with HST images in a sample of nearby galaxies. Surface photometry reveals that (1) the red-blue dichotomy is a function of bulge type: at the same B/T, pseudobulges are in globally blue galaxies and classical bulges are in globally red galaxies; (2) bulge type also predicts where the galaxy lies in other (bimodal) global structural parameters: global Sersic index and central surface brightness. Hence, the red-blue dichotomy is not due to decreasing bulge prominence alone, and the bulge type of a galaxy carries significance for the galaxy's evolutionary history. Classical bulges are thought to indicate that a galaxy has undergone violent relaxation, e. g., during a major merger in its past, while pseudobulges are disk components and therefore indicate a disk-only galaxy that has not suffered a major merger. The circumstances that lead to these differing histories also cause the color-structural bimodality.