E-INSPIRE -- I. Bridging the gap with the local Universe: Stellar population of a statistical sample of ultra-compact massive galaxies at $z<0.3$

TL;DR

This study presents a comprehensive catalogue of 430 ultra-compact massive galaxies at low redshift, analyzing their stellar populations, star formation histories, and relic status to understand their evolution and identify relics.

Contribution

It provides the first extensive spectroscopic analysis of a large sample of UCMGs at low redshift, bridging the gap with the local Universe and classifying their relicness based on detailed stellar population properties.

Findings

Identification of three groups of UCMGs based on Degree of Relicness.

Extreme relics formed all their stars by z~2 with super-solar metallicities.

Most efficient relic identification involves selecting high velocity dispersion, super-solar metallicity, and high [Mg/Fe] UCMGs.

Abstract

This paper presents the first effort to Extend the Investigation of Stellar Populations In RElics (E-INSPIRE). We present a catalogue of 430 spectroscopically-confirmed ultra-compact massive galaxies (UCMGs) from the Sloan Digital Sky Survey at redshifts $0.01<z<0.3$. This increases the original INSPIRE sample eightfold, bridging the gap with the local Universe. For each object, we compute integrated stellar velocity dispersion, age, metallicity, and [Mg/Fe] through spectroscopic stellar population analysis. We infer star formation histories (SFHs), metallicity evolution histories (MEHs) and compute the Degree of Relicness (DoR) of each object. The UCMGs, covering a wide range of DoR from 0.05 to 0.88, can be divided into three groups, according to how extreme their SFH was. The first group consists of 81 extreme relics ($\text{DoR}\gtrsim0.6$) that have formed the totality of their stellar mass by $z\sim2$ and have super-solar metallicities at all cosmic epochs. The second group ($0.3\lesssim\text{DoR}\lesssim0.6$) contains 293 objects also characterised by peaked SFHs but with a small percentage of later-formed stars and with a variety of MEHs. The third group ($\text{DoR}\lesssim0.3$), has 56 objects that cannot be considered relics since they have extended SFHs and formed a non-negligible fraction ($>25\%$) of their stellar mass at $z<2$. We confirm that an efficient method of finding relics is to select UCMGs with large velocity dispersion values but we believe that the most efficient way is to select high velocity dispersion objects that also have super-solar metallicities and high [Mg/Fe].