Jeans Dynamical Constraints on the IMF in Massive Quiescent Galaxies at $z\sim0.7$ from JWST/NIRSpec
We present dynamical masses for nine massive quiescent galaxies at $z\sim0.7$ from the JWST-IMFERNO program, measured from ultra-deep ($\sim32$hr), spatially resolved NIRSpec/MSA spectroscopy combined with NIRCam imaging. The most massive of these galaxies have previously been found to have a bottom-heavy initial mass function (IMF), based on measurements of dwarf-sensitive absorption features. Here, we examine whether the stellar masses inferred for these IMFs are consistent with the dynamical masses of these galaxies. We derive the dynamical masses by including Jeans Anisotropic Modelling within a forward-modelling framework that accounts for instrumental optics, source morphology, and geometric effects. We compare the resulting dynamical masses to the stellar masses for different IMF assumptions. A Milky Way IMF produces physically plausible dynamical-to-stellar mass ratios but implies central dark matter fractions ($f_{\rm DM,1R_e}=0.54\pm0.06$) that are a factor $\sim2$ higher compared to local ETGs and simulations. When uniformly applying the IMF that was measured from absorption features, the mean dark matter fraction becomes low ($f_{\rm DM,1R_e}=0.02\pm0.18$), and the stellar masses exceed the dynamical mass at $2σ$ significance for two out of seven galaxies in the sample. When we adopt the measured IMF only in the central $R_e$ and use a MW IMF in the outskirts, we find realistic dark matter fractions ($f_{\rm DM,1R_e}=0.28\pm0.12$) and consistent dynamical masses. These results support a scenario where the cores of the most massive quiescent galaxies at $z\sim0.7$ had a bottom-heavy IMF, similar to those observed in the centres of local early-type galaxies. Our results are consistent with an inside-out growth scenario, in which size and mass growth of massive galaxies proceeds after quenching through minor mergers, leaving the bottom-heavy cores mostly intact.