Low-mass compact elliptical galaxies: spatially-resolved stellar populations and kinematics with the Keck Cosmic Web Imager

TL;DR

This study uses spatially-resolved spectroscopy to analyze six compact elliptical galaxies, revealing their stellar populations, kinematics, and formation histories, and establishing links between different formation channels of compact galaxies.

Contribution

First to provide spatially-resolved stellar population and kinematic maps for cEs, linking their properties to formation scenarios and evolutionary pathways.

Findings

Most cEs are old and metal-rich, with decreasing age and metallicity outward.

All cEs are fast rotators with high rotation relative to their ellipticities.

Five out of six cEs formed through stripping of larger galaxies, one formed intrinsically.

Abstract

We present spatially-resolved two-dimensional maps and radial trends of the stellar populations and kinematics for a sample of six compact elliptical galaxies (cE) using spectroscopy from the Keck Cosmic Web Imager (KCWI). We recover their star formation histories, finding that all except one of our cEs are old and metal rich, with both age and metallicity decreasing toward their outer radii. We also use the integrated values within one effective radius to study different scaling relations. Comparing our cEs with others from the literature and from simulations we reveal the formation channel that these galaxies might have followed. All our cEs are fast rotators, with relatively high rotation values given their low ellipticites. In general, the properties of our cEs are very similar to those seen in the cores of more massive galaxies, and in particular, to massive compact galaxies. Five out of our six cEs are the result of stripping a more massive (compact or extended) galaxy, and only one cE is compatible with having been formed intrinsically as the low-mass, compact object that we see today. These results further confirm that cEs are a mixed-bag of galaxies that can be formed following different formation channels, reporting for the first time an evolutionary link within the realm of compact galaxies (at all stellar masses).