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

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. 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 ~26 mag arcsec^-2. European Southern Observatory- Very Large Telescope 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 ± 1.17 Gyr, a metallicity of -0.18 ± 0.07 dex and a supersolar alpha-element abundance of 0.2 ± 0.05 dex. The SSP metallicity also shows a significant decline outwards from the centre, 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.