INSPIRE: INvestigating Stellar Population In RElics IV. The Initial Mass Function slope in relics

TL;DR

This study compares the stellar initial mass function in relic galaxies versus non-relics, providing evidence that relics have a systematically bottom-heavier IMF, supporting the idea of IMF non-universality and its link to early star formation conditions.

Contribution

It presents the first detailed spectral analysis of relic galaxies, showing they have a bottom-heavier IMF than similar non-relic galaxies, supporting theories of IMF variation.

Findings

Relics exhibit a systematically bottom-heavier IMF slope.

Non-relics show a more extended star formation history.

Supports the non-universality of the IMF across different galaxy types.

Abstract

In the last decade, growing evidence has emerged supporting a non-universal stellar Initial Mass Function (IMF) in massive galaxies, with a larger number of dwarf stars with respect to the Milky-Way (bottom-heavy IMF). However, a consensus about the mechanisms that cause IMF variations is yet to be reached. Recently, it has been suggested that stars formed early-on in cosmic time, via a star formation burst, could be characterised by a bottom-heavy IMF. A promising way to confirm this is to use relics, ultra-compact massive galaxies, almost entirely composed by these "pristine" stars. The INSPIRE Project aims at assembling a large sample of confirmed relics, that can serve as laboratory to investigate on the conditions of star formation in the first 1-3 Gyr of the Universe. In this third INSPIRE paper, we build a high signal-to-noise spectrum from five relics and one from five galaxies with similar sizes, masses, and kinematical properties, but characterised by a more extended star formation history (non-relics). Our detailed stellar population analysis suggests a systematically bottom-heavier IMF slope for relics than for non-relics, adding new observational evidence for the non-universality of the IMF at various redshifts and further supporting the above proposed physical scenario.