Unveiling the formation route of the largest galaxies in the universe

TL;DR

This paper demonstrates through simulations that dissipationless hierarchical merging can produce the largest galaxies and their observed properties, including the Fundamental Plane, challenging the necessity of gas dissipation in galaxy formation.

Contribution

It shows that multiple collisionless mergers can form realistic massive galaxies that align with observed scaling relations, providing a new perspective on galaxy formation mechanisms.

Findings

Mock remnants define a thin Fundamental Plane fitting observed data.

High-ranked galaxies show zero-point offsets in the FP.

Hierarchical dissipationless merging is a viable formation route.

Abstract

Observational evidence indicates that the role of gas is secondary to that of gravity in the formation of the most luminous spheroids inhabiting the centres of galaxy associations, as originally conjectured in the late 80's/early 90's. However, attempts to explain the origin of the Fundamental Plane (FP) of massive early-type galaxies (ETGs) -- a tilted version of the scaling relation connecting the size, velocity dispersion and mass of virialized homologous systems -- based on sequences of pairwise mergers, have systematically concluded that dissipation cannot be ignored. We use controlled simulations of the previrialization stage of galaxy groups to show that multiple collisionless merging is capable of creating realistic first-ranked galaxies. Our mock remnants define a thin FP that perfectly fits data from all kinds of giant ETGs in the local volume, showing the existence of a unified relationship for these systems. High-ranked galaxies occupy in the FP different areas than standard objects, a segregation which is viewed essentially as zero-point offsets in the 2D correlations arising from standard projections of this plane. Our findings make a strong case for considering hierarchical dissipationless merging a viable route for the formation of the largest galaxies in the universe.