The MAGPI survey: evidence against the bulge–halo conspiracy

Abstract

ABSTRACT Studies of the internal mass structure of galaxies have observed a ‘conspiracy’ between the dark matter and stellar components, with total (stars$+$dark) density profiles showing remarkable regularity and low intrinsic scatter across various samples of galaxies at different redshifts. Such homogeneity suggests the dark and stellar components must somehow compensate for each other in order to produce such regular mass structures. We test the conspiracy using a sample of 22 galaxies from the ‘Middle Ages Galaxy Properties with Integral field spectroscopy’ Survey that targets massive galaxies at $z \sim 0.3$. We use resolved, 2D stellar kinematics with the Schwarzschild orbit-based modelling technique to recover intrinsic mass structures, shapes, and dark matter fractions. This work is the first implementation of the Schwarzschild modelling method on a sample of galaxies at a cosmologically significant redshift. We find that the variability of structure for combined mass (baryonic and dark) density profiles is greater than that of the stellar components alone. Furthermore, we find no significant correlation between enclosed dark matter fractions at the half-light radius and the stellar mass density structure. Rather, the total density profile slope, $\gamma _{\mathrm{tot}}$, strongly correlates with the dark matter fraction within the half-light radius, as $\gamma _{\mathrm{tot}} = (1.3 \pm 0.2) f_{\mathrm{DM}} - (2.44 \pm 0.04)$. Our results refute the bulge–halo conspiracy and suggest that stochastic processes dominate in the assembly of structure for massive galaxies.