A Connection between Bulge Properties and the Bimodality of Galaxies

The global colors and structural properties of galaxies have recently been shown to follow bimodal distributions. Galaxies separate into 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, disk-like structures that could not have formed via violent relaxation. Alas, 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 disk-to-bulge ratio, B/T , or, whether it separates galaxies of different bulge type, irrespective of their B/T . In this paper, we identify classical bulges and pseudobulges morphologically with HST images in a sample of nearby galaxies. Detailed 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. (3) 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. We interpret this result as showing that the type of bulge a galaxy has is a signpost of the evolutionary history of the whole galaxy. Classical bulges are thought to indicate that a galaxy has undergone violent relaxation, e.g. during a major merger (of smaller fragments) in its past. This is more likely to have happened earlier when merging was frequent, in higher-density environments, and when there was still enough gas to subsequently form the disk. Therefore, these galaxies are likely to be red today. Pseudobulges are disk components and therefore indicate a disk-only galaxy. Such a galaxy has not suffered a major merger since the formation of its disk. This is more likely at later epochs, when the merger rate is lower and in low-density environments. Therefore, these are likely to be younger, blue galaxies. In addition, there is evidence that pseudobulge galaxies harbor supermassive black holes that follow the MBH–σ relation. We discuss the effect of black hole feedback in the host galaxy. If feedback during black hole growth in classical bulges is what quenches star formation in their host galaxies, why does this not happen in galaxies with pseudobulges? Subject headings: galaxies: bulges — galaxies: formation — galaxies: evolution — galaxies: structure — galaxies: fundamental parameters