Sigma-drop in galaxies and the sigma-metallicity degeneracy

TL;DR

This paper discusses the sigma-drop phenomenon in galaxies, highlighting how observational artifacts caused by sigma-metallicity degeneracy can lead to misinterpretation of central velocity dispersion depressions, with implications for galaxy dynamics studies.

Contribution

It emphasizes the significance of observational artifacts due to sigma-metallicity degeneracy in interpreting sigma-drop features in galaxy spectra, supported by analysis of VLT/FORS data.

Findings

Sigma-drop can be an artifact caused by metallicity mismatches.

Some reported sigma-drop galaxies may be analysis artifacts.

Spectral analysis must account for sigma-metallicity degeneracy.

Abstract

In some galaxies, the central velocity dispersion, sigma, is depressed with respect to the surroundings. This sigma-drop phenomenon may have different physical origins, bearing information about the internal dynamics of the host galaxy. In this article, we stress the importance also of observational artifacts due to the sigma-metallicity degeneracy: when a spectrum of a population is compared with a template of miss-matched metallicity, the velocity dispersion may be wrongly estimated. A sigma-drop may appear in place of a metallicity peak. The discussion is illustrated using VLT/FORS spectra of diffuse elliptical galaxies. Some of the sigma-drop galaxies reported in the literature may be analysis artifacts.