Wednesday, May 6, 2020

Dark Matter Simulations Systemically Predict Too Few Stars Per Halo

One of the ways to try to understand the discrepancy between the apparent mass of systems of galaxy size or larger in astronomy based upon general relativity (usually in a weak field Newtonian gravity approximation), and the mass that can be observed from the stars in that galaxy, is to infer the presence of dark matter with a sufficiently small mean velocity that interacts only weakly via means other than gravity with ordinary matter or other dark matter.

Often, investigators attempt to determine what a particular kind of dark matter hypothesis implies in terms of predictions with simulations which they compare to what astronomers observe.

The paper below points out that the leading simulations get some key points wrong. Specifically, they systemically underestimate the amount of luminous matter in the form of stars (which where the lion's share of ordinary matter in the universe is found) relative to the inferred or simulated dark matter halos in which they are found, in a consistent and unbiased manner that can be quantified in several different ways.

But, because the simulations are wrong in some very predictable mays and magnitudes which the paper describes, it is also one of the more encouraging ones for the dark matter particle hypothesis in a long time. This is because the results suggest that only a small number of problems which a very specific character are wrong in the simulations, with shines a flashlight on how to look for a fix to the simulations and the dark matter and galaxy formation models that the simulations virtually test. It seems that the main problem with the simulations is probably related to stellar or active galactic nucleus feedback with the dark matter halos in the models. 

In contrast, many simulations have produced results that are wrong when compared to astronomy observations, while leaving astrophysicists to look under the metaphorical street light to look for possible solutions to explain why that might be the case.

Massive disc galaxies in cosmological hydrodynamical simulations are too dark matter-dominated

We investigate the disc-halo connection in massive (Mstar/Msun>5e10) disc galaxies from the cosmological hydrodynamical simulations EAGLE and IllustrisTNG, and compare it with that inferred from the study of HI rotation curves in nearby massive spirals from the Spitzer Photometry and Accurate Rotation Curves (SPARC) dataset. We find that discrepancies between the the simulated and observed discs arise both on global and on local scales. Globally, the simulated discs inhabit halos that are a factor ~4 (in EAGLE) and ~2 (in IllustrisTNG) more massive than those derived from the rotation curve analysis of the observed dataset. We also use synthetic rotation curves of the simulated discs to demonstrate that the recovery of the halo masses from rotation curves are not systematically biased. We find that the simulations predict dark-matter dominated systems with stellar-to-total enclosed mass ratios that are a factor of 1.5-2 smaller than real galaxies at all radii. 
This is an alternative manifestation of the `failed feedback problem', since it indicates that simulated halos hosting massive discs have been too inefficient at converting their baryons into stars, possibly due to an overly efficient stellar and/or AGN feedback implementation.
Comments:6 pages, 3 figures, submitted to A&A. Comments are welcome
Subjects:Astrophysics of Galaxies (astro-ph.GA)
Cite as:arXiv:2005.01724 [astro-ph.GA]
(or arXiv:2005.01724v1 [astro-ph.GA] for this version)


neo said...

A fundamental test for MOND
Valerio Marra, Davi C. Rodrigues, Álefe O. F. de Almeida

The Radial Acceleration Relation (RAR) shows a strong correlation between two accelerations associated to galaxy rotation curves. The relation between these accelerations is given by a nonlinear function which depends on an acceleration scale a†. Some have interpreted this as an evidence for a gravity model, such as Modified Newtonian Dynamics (MOND), which posits a fundamental acceleration scale a0 common to all the galaxies. However, it was later shown, using Bayesian inference, that this seems not to be the case: the a0 credible intervals for individual galaxies were not found to be compatible among themselves. This type of test is a fundamental test for MOND as a theory for gravity, since it directly evaluates its basic assumption and this using the data that most favor MOND: galaxy rotation curves. Here we improve upon the previous analyses by introducing a more robust method to assess the compatibility between the credible intervals, in particular without Gaussian approximations. We directly estimate, using a Monte Carlo simulation, that the existence of a fundamental acceleration is incompatible with the data at more than 5σ. We also consider quality cuts in order to show that our results are robust against outliers. In conclusion, the new analysis further supports the claim that the acceleration scale found in the RAR is an emergent quantity.

Comments: 12 pages, 4 figures; version accepted for publication in MNRAS
Subjects: Astrophysics of Galaxies (astro-ph.GA); Cosmology and Nongalactic Astrophysics (astro-ph.CO); General Relativity and Quantum Cosmology (gr-qc)
Journal reference: MNRAS 2020
DOI: 10.1093/mnras/staa890
Cite as: arXiv:2002.03946 [astro-ph.GA]

neo said...

Galaxies with Declining Rotation Curves
D. I. Zobnina, A. V. Zasov

A sample of 22 spiral galaxies compiled from published data is studied. The galaxy rotation curves pass through a maximum distance of more than ∼1 kpc from the center with a subsequent decrease in the rotation velocity. The galaxy position in the Tully-Fisher (TF) and baryonic Tully-Fisher (BTF) diagrams show that the values of maximum rotation velocities are located on the same sequence with other galaxies, while the velocities at the disk periphery for some galaxies are significantly lower than the expected values for a given mass or luminosity. Thus, the decrease in the rotation curve can be associated with a reduced contribution of the dark halo to the rotation velocity. For seven galaxies with the longest rotation curves, the disk mass was estimated to be with the dark halo (Newtonian model) and without the halo (modified Newtonian dynamics (MOND) model). In four of the galaxies, the MOND model encounters difficulties in interpreting the rotation curve: in order to be consistent with the observations, the MOND parameter a0 should differ significantly from the expected value a0∼10−8 cm/s2, while the disk mass exceeds the value based on IR photometry and maximum disk model. The conflict with MOND is the most significant for NGC 157.

Comments: 15 pages, 11 figures
Subjects: Astrophysics of Galaxies (astro-ph.GA); Cosmology and Nongalactic Astrophysics (astro-ph.CO)
Journal reference: Astronomy Reports, 2020, Vol. 64, No. 4, pp. 295-309
DOI: 10.1134/S1063772920050054
Cite as: arXiv:2003.08845 [astro-ph.GA]

andrew said...

I'll take a look. Thanks for pointing out the papers.