Why do different early-type-galaxies have different amounts of rotational support?

TL;DR

This study investigates why early-type galaxies exhibit varying degrees of rotational support, linking their current properties to their merger histories and suggesting multiple minor wet mergers at high redshift as a key factor.

Contribution

The paper provides an observational archaeological analysis connecting local ETG properties with merger history, supporting the theory that minor wet mergers reduce rotational support.

Findings

Slowly rotating ETGs often have tidal features and kinematically distinct components.

Lower metallicities are associated with galaxies that have less rotational support.

Multiple minor wet mergers at high redshift likely cause the loss of rotational support in ETGs.

Abstract

Early-type galaxies (ETGs, i.e. elliptical and lenticular galaxies) differ in their amount of rotational support -- some are purely supported by velocity dispersion, while others show pronounced ordered rotation. Cosmological hydrodynamical simulations show that the progenitors of all ETGs were first rotating quickly, but then mergers decreased their rotational support. In the presented work, we studied this process using an observational archaeological approach. Namely, we inspected the correlations of 23 merger-sensitive characteristics of local ETGs with a parameter quantifying the rotational support. We used a volume-limited sample of local ETGs, that are not in galaxy clusters, from the MATLAS survey. We found, for example, that slowly rotating galaxies have tidal features and kinematically distinct components more often and have lower metallicities. We sought for mutual interpretation of the correlations among all 23 quantities, together with literature results on high-redshift massive galaxies. There seems to be only one interpretation possible: on average, ETGs lose their rotational support through multiple minor wet mergers happening at the redshifts above about two.