INSPIRE: INvestigating Stellar Population In RElics

Abstract

Context. Massive elliptical galaxies are thought to form through a two-phase process. At early times (z > 2), an intense and fast starburst forms blue and disk-dominated galaxies. After quenching, the remaining structures become red, compact, and massive (i.e. red nuggets). Then, a time-extended second phase, which is dominated by mergers, causes structural evolution and size growth. Given the stochastic nature of mergers, a small fraction of red nuggets survive, without any interaction, massive and compact until today: these are relic galaxies. Since this fraction depends on the processes dominating the size growth, counting relics at low-z is a valuable way of disentangling between different galaxy evolution models. Aims. In this paper, we introduce the INvestigating Stellar Population In RElics (INSPIRE) Project, which aims to spectroscopically confirm and fully characterise a large number of relics at 0.1 < z < 0.5. We focus here on the first results based on a pilot study targeting three systems, representative of the whole sample. Methods. For these three candidates, we extracted 1D optical spectra over an aperture of r = 0.40″, which comprises ∼30% of the galaxies’ light, and we obtained the line-of-sight integrated stellar velocity and velocity dispersion. We also inferred the stellar [α/Fe] abundance from line-index measurements and mass-weighted age and metallicity from full-spectral fitting with single stellar population models. Results. Two galaxies have large integrated stellar velocity dispersion values (σ⋆ ∼ 250 km s−1), confirming their massive nature. They are populated by stars with super-solar metallicity and [α/Fe]. Both objects have formed ≥80% of their stellar mass within a short (∼0.5−1.0 Gyr) initial star formation episode occurred only ∼1 Gyr after the Big Bang. The third galaxy has a more extended star formation history and a lower velocity dispersion. Thus we confirm two out of three candidates as relics. Conclusions. This paper is the first step towards assembling the final INSPIRE catalogue that will set stringent lower limits on the number density of relics at z < 0.5, thus constituting a benchmark for cosmological simulations, and their predictions on number densities, sizes, masses, and dynamical characteristics of these objects.