A Connection between Bulge Properties and the Bimodality of Galaxies

Abstract

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 Sérsic 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.