Are the total mass density and the low-mass end slope of the IMF anti-correlated?

TL;DR

This study analyzes nine massive early-type galaxies using lensing, dynamics, and stellar population data to investigate the relationship between total mass density and the low-mass slope of the IMF, finding an anti-correlation.

Contribution

It provides new measurements of the low-mass IMF slope and cut-off mass in massive galaxies and explores their correlation with mass density, using combined lensing and spectroscopic data.

Findings

No significant correlation between IMF slope and galaxy properties except an anti-correlation with mass density.

IMF slope is steeper than that of the Milky Way in all studied galaxies.

Anti-correlation suggests denser galaxies tend to have shallower low-mass IMFs.

Abstract

We conduct a detailed lensing, dynamics and stellar population analysis of nine massive lens early-type galaxies (ETGs) from the X-Shooter Lens Survey (XLENS). Combining gravitational lensing constraints from HST imaging with spatially-resolved kinematics and line-indices constraints from VLT X-Shooter (XSH) spectra, we infer the low-mass slope and the low cut-off mass of the stellar Initial Mass Function (IMF): $x_{250}=2.37^{+0.12}_{-0.12}$ and $M_{{\rm low}, 250}= 0.131^{+0.023}_{-0.026}\, M_{\odot}$, respectively, for a reference point with $\sigma \equiv 250\, {{\rm kms}}^{-1}$ and R$_{{\rm eff}} \equiv 10$ kpc. All the XLENS systems are consistent with an IMF slope steeper than Milky Way-like. We find no significant correlations between IMF slope and any other quantity, except for an anti-correlation between total dynamical mass density and low-mass IMF slope at the 87% CL [$dx/d\log(\rho)$ = $ -0.19^{+0.15}_{-0.15}$]. This anti-correlation is consistent with the low redshift lenses found by Smith et al. (2015) that have high velocity dispersions and high stellar mass densities but surprisingly shallow IMF slopes.