INSPIRE: INvestigating Stellar Population In RElics II. First Data Release (DR1)

TL;DR

The INSPIRE project provides the first data release of 19 ultra-compact massive galaxies, identifying 10 as relics, and significantly expanding the sample of known relics to study early galaxy formation.

Contribution

This paper presents the first data release of the INSPIRE project, increasing the sample of confirmed relic galaxies and refining methods for stellar population analysis.

Findings

10 out of 19 galaxies are relics formed within 3 Gyr of the Big Bang.

The catalog now includes 10 confirmed relics, tripling the known sample.

Relics show old ages, super-solar metallicities, and high [Mg/Fe] ratios.

Abstract

The INvestigating Stellar Population In RElics is an on-going project targeting 52 ultra-compact massive galaxies at 0.1<z<0.5 with the X-Shooter@VLT spectrograph (XSH). These objects are the perfect candidates to be 'relics', massive red-nuggets formed at high-z (z>2) through a short and intense star formation burst, that evolved passively and undisturbed until the present-day. Relics provide a unique opportunity to study the mechanisms of star formation at high-z. In this paper, we present the first INSPIRE Data Release, comprising 19 systems with observations completed in 2020. We use the methods already presented in the INSPIRE Pilot, but revisiting the 1D spectral extraction. For these 19 systems, we obtain an estimate of the stellar velocity dispersion, fitting separately the two UVB and VIS XSH arms at their original resolution. We estimate [Mg/Fe] abundances via line-index strength and mass-weighted integrated stellar ages and metallicities with full spectral fitting on the combined spectrum. Ages are generally old, in agreement with the photometric ones, and metallicities are almost always super-solar, confirming the mass-metallicity relation. The [Mg/Fe] ratio is also larger than solar for the great majority of the galaxies, as expected. We find that 10 objects have formed more than 75% of their stellar mass (M*) within 3 Gyr from the Big Bang and classify them as relics. Among these, we identify 4 galaxies which had already fully assembled their M* by that time. They are therefore `extreme relics' of the ancient Universe. The INSPIRE DR1 catalogue of 10 known relics to-date augment by a factor of 3.3 the total number of confirmed relics, also enlarging the redshift window. It is therefore the largest publicly available collection. Thanks to the larger number of systems, we can also better quantify the existence of a 'degree of relicness', already hinted at the Pilot Paper.