Are dry mergers of Ellipticals the way to reconcile model predictions with the downsizing?

TL;DR

This study investigates whether dry mergers can reconcile hierarchical galaxy formation models with observed galaxy downsizing, finding that dry mergers alone cannot fully explain the observed chemical and mass relations in massive ellipticals.

Contribution

The paper demonstrates that dry mergers alone cannot account for the observed chemical and mass relations, highlighting the need for star formation processes in galaxy evolution models.

Findings

Dry mergers cannot fit both the mass-[Mg/Fe] and mass-metallicity relations simultaneously.

Major dry mergers between galaxies obeying observed relations do not worsen the fit to observations.

Dry mergers alone cannot explain the physical reasons behind galaxy downsizing.

Abstract

To show that the bulk of the star formation and the galaxy assembly should occur simultaneously in order to reproduce at the same time the downsizing and the chemical properties of present-day massive spheroids within one effective radius.By means of chemical evolution models we create galactic building blocks of several masses and different chemical properties. We then construct a sample of possible merger histories going from a multiple minor merger scenario to a single major merger event aimed at reproducing a single massive elliptical galaxy. We compare our results against the mass-[Mg/Fe] and the mass-metallicity relations. We found that a series of multiple dry-mergers (no star formation in connection with the merger) involving building-blocks which have been created ad hoc in order to satisfy the [Mg/Fe]-mass relation cannot fit the mass-metallicity relation and viceversa. A major dry merger, instead, does not worsen the agreement with observation if it happens between galaxies which already obey to both the mass- or sigma-[Mg/Fe] and the mass-(sigma-) metallicity relations. However, this process alone cannot explain the physical reasons for these trends. Dry mergers alone cannot be the way to reconcile the need of a more efficient star formation in the most massive galaxies with the late time assembly suggested in the hierarchical paradigm in order to recover the galaxy downsizing.