The Role of Dissipation in the Scaling Relations of Cosmological Merger Remnants

TL;DR

This study demonstrates that incorporating gas dissipation and star formation into merger models helps reproduce the observed scaling relations of elliptical galaxies, including their tilt and scatter, across redshifts 0 to 3.

Contribution

The paper introduces a simple merger model with gas dissipation that, combined with semi-analytic models, successfully reproduces observed elliptical galaxy scaling relations without parameter tuning.

Findings

The model matches observed slope and scatter of size-mass relations.

Gas fraction decrease with mass explains the fundamental plane tilt.

Scatter reduction is due to correlation between progenitor size and gas fraction.

Abstract

There are strong correlations between the three structural properties of elliptical galaxies -- stellar mass, velocity dispersion and size -- in the form of a tight "fundamental plane" and a "scaling relation" between each pair. Major mergers of disk galaxies are assumed to be a mechanism for producing ellipticals, but semi-analytic galaxy formation models (SAM) have encountered apparent difficulties in reproducing the observed slope and scatter of the size-mass relation. We study the scaling relations of merger remnants using progenitor properties from two SAMs. We apply a simple merger model that includes gas dissipation and star formation based on theoretical considerations and simulations. Combining the SAMs and the merger model allows calculation of the structural properties of the remnants of major mergers that enter the population of elliptical galaxies at a given redshift. Without tuning the merger model parameters for each SAM, the results roughly match the slope and scatter in the observed scaling relations and their evolution in the redshift range $z=0-3$. Within this model, the observed scaling relations, including the tilt of the fundamental plane relative to the virial plane, result primarily from the decrease of gas fraction with increasing progenitor mass. The scatter in the size-mass relation of the remnants is reduced from that of the progenitors because of a correlation between progenitor size and gas fraction at a given mass.