INSPIRE: INvestigating Stellar Population In RElics V. A catalogue of ultra-compact massive galaxies outside the local Universe and their degree of relicness

TL;DR

This study presents a catalog of 52 ultra-compact massive galaxies outside the local universe, analyzing their stellar populations and degree of relicness to understand galaxy formation and evolution.

Contribution

It provides the first large sample of relic galaxies beyond the local universe, with detailed stellar population measurements and relicness classification.

Findings

High degree of relicness correlates with higher [Mg/Fe] and metallicity.

38 out of 52 galaxies formed over 75% of their mass by z=2.

Nine galaxies are extreme relics formed entirely by z=2.

Abstract

This paper presents the third data release of the INvestigating Stellar Population In RElics (INSPIRE) project, comprising 52 ultra-compact massive galaxies (UCMGs) observed with the X-Shooter spectrograph. We measure integrated stellar velocity dispersion, [Mg/Fe] abundances, ages, and metallicities for all the INSPIRE objects. We thus infer star formation histories and confirm the existence of a degree of relicness (DoR), defined in terms of the fraction of stellar mass formed by $z=2$, the time at which a galaxy has assembled 75\% of its mass, and the final assembly time. Objects with a high DoR assembled their stellar mass at early epochs, while low-DoR objects show a non-negligible fraction of later-formed populations and hence a spread in ages and metallicities. A higher DoR correlates with larger [Mg/Fe], super-solar metallicity, and larger velocity dispersion values. The 52 UMCGs span a large range of DoR from 0.83 to 0.06, with 38 of them having formed more than 75\% of their mass by $z=2$. Of these, nine are extreme relics (DoR$>0.7$), since they formed the totality ($>99\%$) of their stellar mass by redshift $z=2$. The remaining 14 UCMGs cannot be considered relics, as they are characterised by more extended star formation histories. With INSPIRE, we built the first sizeable sample of relics outside the local Universe, up to $z\sim0.4$, increasing the number of confirmed relics by a factor of $>10$, and opening up an important window to explain the mass assembly of massive galaxies in the high-z Universe.