Evidence of a distinct stellar population in the counter-rotating core of NGC 1700

TL;DR

This study identifies a younger, distinct stellar population in the counter-rotating core of NGC 1700, suggesting a formation history involving accretion of a small stellar companion, which impacts the galaxy's core properties.

Contribution

It provides evidence of a unique stellar population in the galaxy's core and links it to a specific formation scenario involving accretion of a small companion.

Findings

Younger age of the core compared to the main galaxy body.

Significant change in absorption line index gradients at the core edge.

Lower metallicity and Mg/Fe ratio in the core.

Abstract

We find a distinct stellar population in the counter-rotating and kinematically decoupled core of the isolated massive elliptical galaxy NGC 1700. Coinciding with the edge of this core we find a significant change in the slope of the gradient of various representative absorption line indices. Our age estimate for this core is markedly younger than the main body of the galaxy. We find lower values for the age, metallicity and Mg/Fe abundance ratio in the center of this galaxy when we compare them with other isolated elliptical galaxies with similar velocity dispersion. We discuss the different possible scenarios that might have lead to the formation of this younger kinematically decoupled structure and conclude that, in light of our findings, the ingestion of a small stellar companion on a retrograde orbit is the most favoured.