The Carnegie-Irvine Galaxy Survey. IX. Classification of Bulge Types and Statistical Properties of Pseudo Bulges

TL;DR

This study analyzes the structural and physical properties of 320 galaxy bulges, classifying them into classical and pseudo bulges, revealing their distinct characteristics and challenging some previous findings due to improved methods.

Contribution

The paper introduces a robust classification of bulge types using the Kormendy relation and provides new statistical insights into bulge properties with improved image analysis techniques.

Findings

Pseudo bulges are more flattened than classical bulges.

Pseudo bulges are most common in late-type spiral galaxies.

Differences with previous studies are due to improved image quality and analysis methods.

Abstract

We study the statistical properties of 320 bulges of disk galaxies in the Carnegie-Irvine Galaxy Survey, using robust structural parameters of galaxies derived from image fitting. We apply the Kormendy relation to classify classical and pseudo bulges and characterize bulge dichotomy with respect to bulge structural properties and physical properties of host galaxies. We confirm previous findings that pseudo bulges on average have smaller S\'{e}rsic indices, smaller bulge-to-total ratios, and fainter surface brightnesses when compared with classical bulges. Our sizable sample statistically shows that pseudo bulges are more intrinsically flattened than classical bulges. Pseudo bulges are most frequent (incidence $\gtrsim 80\%$) in late-type spirals (later than Sc). Our measurements support the picture in which pseudo bulges arose from star formation induced by inflowing gas, while classical bulges were born out of violent processes such as mergers and coalescence of clumps. We reveal differences with the literature that warrant attention: (1) the bimodal distribution of S\'{e}rsic indices presented by previous studies is not reproduced in our study; (2) classical and pseudo bulges have similar relative bulge sizes; and (3) the pseudo bulge fraction is considerably smaller in early-type disks compared with previous studies based on one-dimensional surface brightness profile fitting. We attribute the above differences to our improved image quality, more robust bulge-to-disk decomposition technique, and different classification criteria applied. Moreover, we find that barred galaxies do not host more pseudo bulges or more prominent pseudo bulges than unbarred galaxies. Various implications of these findings are discussed.