The MAGPI Survey: Evidence Against the Bulge-Halo Conspiracy

• Main Authors: Derkenne, C, McDermid, R. M, Santucci, G, Poci, A, Thater, S, Bellstedt, S, Mendel, J. T, Foster, C, Harborne, K. E, Lagos, C. D. P, Wisnioski, E, Croom, S, Remus, R-S, Valenzuela, L. M, van de Sande, J, Sweet, S. M, Ziegler, B
• Format: Journal Article
• Language: English
• Published: 08-08-2024
• Subjects: Physics - Astrophysics of Galaxies
• DOI: 10.48550/arxiv.2408.04834

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' (MAGPI) 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.