On the formation mechanisms of compact elliptical galaxies

TL;DR

This study investigates the origins of compact elliptical galaxies by analyzing their properties and environments, finding evidence for both stripping and intrinsic formation mechanisms depending on their surroundings.

Contribution

It provides a comprehensive analysis combining multiple diagnostics to distinguish between intrinsic and stripped origins of cEs, clarifying their formation pathways.

Findings

Stripping likely causes cEs near hosts in groups and clusters.

Intrinsic formation favored for isolated cEs in loose environments.

Combining diagnostics effectively determines galaxy origins.

Abstract

In order to investigate the formation mechanisms of the rare compact elliptical galaxies (cE) we have compiled a sample of 25 cEs with good SDSS spectra, covering a range of stellar masses, sizes and environments. They have been visually classified according to the interaction with their host, representing different evolutionary stages. We have included clearly disrupted galaxies, galaxies that despite not showing signs of interaction are located close to a massive neighbor (thus are good candidates for a stripping process), and cEs with no host nearby. For the latter, tidal stripping is less likely to have happened and instead they could simply represent the very low-mass, faint end of the ellipticals. We study a set of properties (structural parameters, stellar populations, star formation histories and mass ratios) that can be used to discriminate between an intrinsic or stripped origin. We find that one diagnostic tool alone is inconclusive for the majority of objects. However, if we combine all the tools a clear picture emerges. The most plausible origin, as well as the evolutionary stage and progenitor type, can be then determined. Our results favor the stripping mechanism for those galaxies in groups and clusters that have a plausible host nearby, but favors an intrinsic origin for those rare cEs without a plausible host and that are located in looser environments.