Merger History of Galaxies and Disk+Bulge Formation

TL;DR

This paper explores galaxy morphological evolution within the CDM framework, emphasizing the roles of major and minor mergers in forming disk and bulge structures, and highlights the challenges in producing low B/T ratio massive disk galaxies.

Contribution

It demonstrates that minor mergers are crucial for forming massive low B/T disk galaxies and discusses the limitations of major mergers in creating such structures.

Findings

Major mergers disrupt disks, making low B/T galaxies hard to form after z ≥ 2.

Minor mergers significantly contribute to bulge formation in massive disks.

Re-growth of disks after major mergers is limited at late times.

Abstract

We discuss the transitions of galaxy morphologies within the CDM paradigm under the assumption of bulge formation in mergers and disk growth via cooling of gas and subsequent star formation. Based on the relative importance of these two competing processes it is possible to make predictions on the expected morphological mix of galaxies. In particular we here discuss the generation of massive disk galaxies with low bulge-to-total mass ratios. Our results indicate that it is difficult to generate enough massive disk galaxies with B/T $< 0.2$ via major mergers and subsequent disk re-growth, if during the major merger progenitor disks get disrupted completely. On average low B/T galaxies must have had there last major merger at $z \ge 2$. The main limiting factor is the ability to re-grow massive disks at late times after the last major merger of a galaxy. Taking into account the contribution from minor mergers ($4 \ge M_1/M_2$, $M_1 \ge M_2$) to the formation of bulges, we recover the right fraction of massive low B/T disk galaxies, indicating that minor mergers play an important role in the formation of massive low B/T disk galaxies.