Dynamical Stellar Mass-to-light Ratio Gradients: Evidence for Very Centrally Concentrated IMF Variations in ETGs?

Abstract

Evidence from different probes of the stellar initial mass function (IMF) of massive early-type galaxies (ETGs) has repeatedly converged on IMFs more bottom heavy than in the Milky Way (MW). This consensus has come under scrutiny due to often contradictory results from different methods on the level of individual galaxies. In particular, a number of strong lensing probes are ostensibly incompatible with a non-MW IMF. Radial gradients of the IMF—related to gradients of the stellar mass-to-light ratio ϒ—can potentially resolve this issue. We construct Schwarzschild models allowing for ϒ-gradients in seven massive ETGs with MUSE and SINFONI observations. We find dynamical evidence that ϒ increases toward the center for all ETGs. The gradients are confined to subkiloparsec scales. Our results suggest that constant-ϒ models may overestimate the stellar mass of galaxies by up to a factor of 1.5. For all except one galaxy, we find a radius where the total dynamical mass has a minimum. This minimum places the strongest constraints on the IMF outside the center and appears at roughly 1 kpc. We consider the IMF at this radius characteristic for the main body of each ETG. In terms of the IMF mass-normalization α relative to a Kroupa IMF, we find on average an MW-like IMF 〈α main〉 = 1.03 ± 0.19. In the centers, we find concentrated regions with increased mass normalizations that are less extreme than previous studies suggested, but still point to a Salpeter-like IMF, 〈α cen〉 = 1.54 ± 0.15.

Gas-dynamical Mass Measurements of the Supermassive Black Holes in the Early-type Galaxies NGC 4786 and NGC 5193 from ALMA and HST Observations*

“…This is also consistent with Mehrgan et al (2024), who find that the centers of massive ETGs typically have Salpeter-like or heavier mass-to-light ratios. Mehrgan et al (2024) also find ϒ gradients in the majority of massive ETGs, which can affect the measured M BH . However, the ϒ gradients are measured at radii of ∼0.2-1.0 kpc, beyond the physical scale of the dust disks of the galaxies in this study.…”

“…This is expected, given that galaxies with large σ å tend to follow Salpeter-like or even heavier IMFs (e.g., Cappellari et al 2013;Smith 2020). This is also consistent with Mehrgan et al (2024), who find that the centers of massive ETGs typically have Salpeter-like or heavier mass-to-light ratios. Mehrgan et al (2024) also find ϒ gradients in the majority of massive ETGs, which can affect the measured M BH .…”