INSPIRE: INvestigating Stellar Population In RElics. I. Survey presentation and pilot study

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.