The role of dry mergers in shaping the scaling relations of galaxies

TL;DR

This paper develops a simple dry merger theory based on the scalar Virial Theorem to explain galaxy scaling relations, successfully matching observational data and detailed simulations across cosmic time.

Contribution

It introduces a novel, rapid theoretical model of dry mergers that accurately reproduces galaxy scaling relations and their evolution, complementing complex numerical simulations.

Findings

Dry mergers explain observed galaxy scaling relation characteristics.

The model matches data across different cosmic epochs.

It offers a fast alternative to detailed simulations.

Abstract

In the context of the hierarchical formation of galaxies, we investigated the role played by mergers in shaping the scaling relations of galaxies, that is the projections of their Fundamental Plane onto the Ie-Re, Ie-Sigma, Ms-Re and L-Sigma planes. To this aim, based on the scalar Virial Theorem, we developed a simple theory of multiple dry mergers to read both the large scale simulations and the companion scaling relations. The aim was to compare the results of this approach with the observational data and with two of the most recent and detailed numerical cosmo-hydro-dynamical simulations, that is Illustris-TNG and EAGLE (Evolution and Assembly of GaLaxies and their Environments). We derived the above scaling relations for the galaxies of the MaNGA (Mapping Nearby Galaxies at APO) and WINGS (Wide-field Imaging of Nearby Galaxy-Clusters Survey) databases and compared them with the observational data, the numerical simulations, and the results of our simple theory of dry mergers. The multiple dry merging mechanism is able to explain all the main characteristics of the observed scaling relations of galaxies, such as slopes, scatters, curvatures and zones of exclusion. The distribution of galaxies in these planes is continuously changing across time because of the merging activity and other physical processes, such as star formation, quenching, energy feedback, and so forth. The simple merger theory presented here yields the correct distribution of galaxies in the main scaling relations at all cosmic epochs. The precision is comparable with that obtained by the modern cosmo-hydro-dynamical simulations, with the advantage of providing a rapid exploratory response on the consequences engendered by different physical effects.