Alfie Russell | Searcharxiv

arXiv2026

The Dynamical Mass Profiles of Galaxy Groups - Application to IllustrisTNG-300

We present a new rescaling and stacking method to test cold dark matter predictions through the dynamical modelling of galaxy groups using the Jeans equation. We test the viability of this method on idealised, line-of-sight group data at $z=0$ in the IllustrisTNG-300 simulation, using subhalos and satellite galaxies as tracers. Dynamical mass profiles are measured using the code `Gravsphere' for seven distinct quality cuts, three on the dark matter substructure and four on the `visible' satellite galaxies. When using the dark matter substructure, we find that selecting subhalos by their infall time onto the group provides the best recovery of the true mass profile. When using satellite galaxies we instead find that selecting quenched satellites provides the best recovery. In both cases the mass profile is recovered within $<1σ$ of the true value up to $\sim 1.5$ times the median host virial radius. However, we also find the recovered velocity anisotropy to be biased towards negative values for all seven of our subhalo samples beyond the median host virial radius. This suggests dynamical systematics may still be present in the data, which will need to be quantified in future work before application to surveys.

astro-ph.CO↗

arXiv2026

How does a MOND cosmology fare on Gpc scales? - Collisionless $N$-body simulations of $ν$HDM

We present the largest collisionless $N$-body cosmological simulations in a MOdified Newtonian Dynamics (MOND) cosmology to date. Our 4 simulations cover $Λ$CDM as a baseline, a MOND with hot dark matter model known as $ν$HDM, and 2 unphysical models we call $Λ$HDM and $ν$CDM to test the individual contributions of hot dark matter and MOND gravity, respectively. $ν$HDM reproduces the CMB power spectrum whilst also theoretically matching cluster dynamics and preserving MOND predictions for galactic rotation curves. We test its viability on cosmological scales using simulations with $256^{3}$ particles in a box of size $800/h$ comoving Mpc. We find generically that the MOND models massively overproduce large-scale structures by $z=0$, with a most massive cluster in $ν$HDM of $\approx 5 \times 10^{17} M_{\odot}/h$ and typical peculiar velocities of several thousand km/s. We also explore a local void solution to the Hubble tension in these models. Analogues to the observed "Local Hole'' do form in the MOND models, but values for the deceleration parameter $<-1.5$ in these regions prevent a satisfactory resolution to the Hubble tension. Whilst $Λ$CDM significantly underpredicts the observed bulk flow in Cosmicflows-4, the high peculiar velocities that arise in the MOND models create the opposite problem, ruling out $ν$HDM at $>5σ$ confidence. Observations clearly require a much milder enhancement to the rate of structure growth in $Λ$CDM than is provided by the $ν$HDM paradigm. Our results also suggest that replacing cold dark matter with hot dark matter is unlikely to provide a viable cosmological model, regardless of the gravity law.

astro-ph.CO↗