An isolated, compact early-type galaxy with a diffuse stellar component: merger origin?

TL;DR

This paper reports the discovery of an isolated compact early-type galaxy with a diffuse stellar component, providing insights into its possible merger origin and challenging existing formation models.

Contribution

It presents the first detailed analysis of an isolated cE galaxy, highlighting its structural and stellar population properties and proposing a merger origin scenario.

Findings

The galaxy has a half-light radius of 559 pc and a faint outer stellar component extending up to 10 kpc.

Stellar population analysis shows an intermediate age of ~7.15 Gyr and a metallicity of -0.18 dex.

The metallicity declines outward, supporting a merger-based formation scenario.

Abstract

The relation between the size and luminosity for both bright and faint early-type galaxies has been repeatedly discussed as a crucial proxy for understanding evolutionary scenarios, as galaxies grow and lose their stellar mass in different physical processes. The class of compact early-type galaxies (cEs) are, however, distinct outliers from this relation and mainly found around massive galaxies in the centres of groups and clusters. The recent discovery of a cE in isolation provided a new opportunity to understand their formation scenario in a different environment. Here, we report the discovery of an isolated cE, CGCG 036-042, using imagery from the Sloan Digital Sky Survey (SDSS). The CGCG 036-042 has an $r$-band absolute magnitude (M$_{r}$) of $-$18.21 mag and a half-light radius (R$_{e}$) of 559 pc. Interestingly, it possesses a faint outer stellar component, which extends up to 10 kpc from its centre and has a nearly constant $r$-band surface brightness of $\sim$26 mag/arcsec$^{2}$. ESO-VLT long-slit spectroscopic data reveal that the simple stellar population (SSP) properties are fairly similar to those of previously identified cEs, with an intermediate-age of 7.15$\pm$1.17 Gyr, a metallicity of --0.18$\pm$0.07 dex and a supersolar alpha-element abundance of 0.2$\pm$0.05 dex. The SSP metallicity also shows a significant decline outward from the center, by 0.5 dex over one half light radius. We discuss the possible origin of this galaxy and suggest that it might have originated from a merger of even smaller objects -- a well established scenario for the formation of massive early-type galaxies.