INSPIRE: INvestigating Stellar Population In RElics VI -- The low-mass end slope of the stellar Initial Mass Function and chemical composition

TL;DR

This study investigates the relationship between relic stellar populations, the low-mass end slope of the IMF, and chemical composition in ultra-compact massive galaxies, revealing correlations with formation epoch and stellar mass.

Contribution

It introduces measurements of the low-mass end IMF slope and chemical ratios in relic galaxies, linking them to their formation history and degree of relicness.

Findings

Higher relicness correlates with a dwarf-rich IMF.

IMF slope correlates with the fraction of stars formed at high redshift.

Metallicity increases with stellar mass in UCMGs.

Abstract

The INSPIRE project has built the largest sample of ultra-compact massive galaxies (UCMGs) at 0.1<z<0.4 and obtained their star formation histories (SFHs). Due to their preserved very old stellar populations, relics are the perfect systems to constrain the earliest epochs of mass assembly in the Universe and the formation of massive early-type galaxies. The goal of this work is to investigate whether a correlation exists between the degree of relicness (DoR), quantifying the fraction of stellar mass formed at z>2, and the other stellar population parameters.We use the Full-Index-Fitting method to fit the INSPIRE spectra to single stellar population (SSP) models. This allows us to measure, for the first time, the low-mass end slope of the IMF, as well as stellar metallicity [M/H], [Mg/Fe], [Ti/Fe] and [Na/Fe] ratios, and study correlations between them and the DoR. Similarly to normal-sized galaxies, UCMGs with larger stellar masses have overall higher metallicities. We found a correlation between the low-mass end of the IMF slope and the DoR, that, however, breaks down for systems with a more extended SFH. An even stronger dependency is found between the IMF and the fraction of mass formed at high-z. At equal velocity dispersion and metallicity, galaxies with a higher DoR have a dwarf-richer IMF than that of low-DoR counterparts. This might indicate that the cosmic epoch and formation mechanisms influence the fragmentation of the star formation cloud and hence might be the explanation for IMF variations detected in massive ETGs.