Multiple minor mergers: formation of elliptical galaxies and constraints for the growth of spiral disks

TL;DR

This study uses numerical simulations to show that multiple minor mergers can produce elliptical galaxies with properties similar to observed ellipticals, challenging the idea that major mergers are the sole formation mechanism.

Contribution

It demonstrates that repeated minor mergers can form elliptical galaxies, providing an alternative to major mergers and constraining spiral galaxy growth.

Findings

Multiple minor mergers can produce elliptical-like galaxies.

Repeated minor mergers can explain properties of observed ellipticals.

Minor mergers destroy disks, limiting spiral galaxy growth by this process.

Abstract

Multiple, sequential mergers are unavoidable in the hierarchical build-up picture of galaxies, in particular for the minor mergers that are frequent and highly likely to have occured several times for most present-day galaxies. However, the effect of repeated minor mergers on galactic structure and evolution has not been studied systematically so far. We present a numerical study of multiple, subsequent, minor galaxy mergers, with various mass ratios ranging from 4:1 to 50:1. The N-body simulations include gas dynamics and star formation. We study the morphological and kinematical properties of the remnants, and show that several so-called "minor" mergers can lead to the formation of elliptical-like galaxies that have global morphological and kinematical properties similar to that observed in real elliptical galaxies. The properties of these systems are compared with those of elliptical galaxies produced by the standard scenario of one single major merger. We thus show that repeated minor mergers can theoretically form elliptical galaxies without major mergers, and can be more frequent than major mergers, in particular at moderate redshift. This process must then have formed some elliptical galaxies seen today, and could in particular explain the high boxiness of massive ellipticals, and some fundamental relations observed in ellipticals. In addition, because repeated minor mergers, even at high mass ratios, destroy disks into spheroids, these results indicate that spiral galaxies cannot have grown only by a succession of minor mergers.