Lambda CDM Predicts That There Are Lots Of Ultra-Diffuse Galaxies With Little Dark Matter

A new ΛCDM prediction is unlikely to be well supported by observational evidence.

The toy model theory MOND has predicted since 1983 that ultra-diffuse galaxies have high inferred dark matter proportions except when the external field effect applies.

A new paper proposes that there should be lots of ultra-diffuse galaxies with little dark matter relative to luminous matter without regard to external field effects.

Surveying dark matter deficient galaxies (those with dark matter mass to stellar mass ratio Mdm/Mstar<1) in the Illustris simulation of structure formation in the flat-ΛCDM cosmogony, we find $M_rm star approximately 2 times 10^8, M_sun galaxies that have properties similar to those ascribed by (vanDokkumetal 2018a) to the ultra-diffuse galaxy NGC1052-DF2. The Illustris simulation also contains more luminous dark matter deficient galaxies. Illustris galaxy subhalo 476171 is a particularly interesting outlier, a massive and very compact galaxy with M_rm star approximately 9 times 10^10 M_sun and Mdm/Mstar ≈ 0.1 and a half-stellar-mass radius of ≈ 2 kpc. If the Illustris simulation and the ΛCDM model are accurate, there are a significant number of dark matter deficient galaxies, including massive luminous compact ones. It will be interesting to observationally discover these galaxies, and to also more clearly understand how they formed, as they are likely to provide new insight into and constraints on models of structure formation and the nature of dark matter.

Hai Yu, Bharat Ratra, Fa-Yin Wang, "Dark matter deficient galaxies in the Illustris flat-ΛCDM model structure formation simulation" (September 16, 2018).

The link regarding the galaxy in question in the abstract of the article quoted above is to a prior post at this blog on the controversy, with several updates and updating comments, related to that galaxy.

So far, the data tends to favor MOND as the overwhelming majority of ultra-diffuse galaxies show a high inferred dark matter to stellar mass ratio. The exceptions are galaxies to which an external field effect applies as predicted by MOND.

ΛCDM, even to the extent that it predicts that there will be dark matter deficient galaxies, is very vague about where those galaxies will be located, something for which MOND provides a prediction.

More Successful MOND Predictions

MOND has done a good job of predicting, a priori, the dynamics of the dwarf galaxies of Andromeda, which is something that lambda CDM doesn't even have a way to predict.

MOND also, a priori, predicted that what naively looked like a satellite galaxy of Andromeda was actually a background galaxy. MOND also has an a priori rule that allows it to determine when a result is likely to be quirky and hard to estimate because the system won't be in equilibrium because the external field effect varies over time due to a highly eccentric orbit around a host galaxy (bringing the galaxy in and out of the external field effect regime leading to tidal disruption).

The Navarro-Frennk-White Model Still Doesn't Fit The Data

In other observational news, another study finds that the analytically predicted NFW distribution of cold dark matter is not the observational reality, which instead, is a better fit to the largely theoretically unjustified pseudo-isothermal halo mass structure in a dark matter particle theory. There is really no good explanation for why the analytically determined NFW profile, that should arise as a simple matter of mathematics with collisionless cold dark matter, usually isn't observed, or for why a pseudo-isothermal halo mass structure is inferred instead.

The article below also contends with the problem of reconciling the similar behavior of spiral galaxies with and without bulges also remains a problem for CDM theories (as does the higher than expected frequency of galaxies without bulges in its galactic matter assembly model). 

Mass models of 100 nearby spiral and irregular galaxies, covering morphological types from Sa to Irr, are computed using Hα rotation curves and Rc-band surface brightness profiles. The kinematics was obtained using a scanning Fabry-Perot interferometer. One of the aims is to compare our results with those from Korsaga et al. (2018), which used mid-infrared (MIR) WISE W1 (3.4 μm) photometric data. For the analysis, the same tools were used for both bands. 

Pseudo-Isothermal (ISO) core and Navarro-Frenk-White (NFW) cuspy models have been used. We test Best Fit Models (BFM), Maximum Disc Models (MDM) and models for which M/L is fixed using the B - V colors. Similarly to what was found in the MIR 3.4 μm band, most of the observed rotation curves are better described by a central core density profile (ISO) than a cuspy one (NFW) when using the optical Rc-band. In both bands, the dispersion in the (M/L) values is smaller for the fixed M/L fits. As for the W1 photometry, the derived DM halos' parameters depend on the morphological types. We find similar relations than those in the literature, only when we compare our results for the bulge-poor sub-sample because most of previous results were mainly based on late-type spirals. Because the dispersion in the model parameters is smaller and because stellar masses are better defined in that band, MIR photometry should be preferred, when possible, to the optical bands. It is shown that for high-z galaxies, sensible results can still be obtained without full profile decomposition.

Marie Korsaga, et al., "GHASP: an Hα kinematics survey of spiral galaxies - XII. Distribution of luminous and dark matter in spiral and irregular nearby galaxies using Rc-band photometry" (September 17, 2018).

Yet another article also alludes to these issues in the model formation context:

In this paper we develop a new semianalytical approach to quantifying the density profile of outer dark matter halos, motivated by the remarkable universality those profiles, for a wide range of dynamical parameters of the inner halos. We show that our minimalist model is robust under significant variations of its input parameters around the currently known values and we identify the turnaround radius as the most important scale of the problem. Based on that observation, we argue that the turnaround radius accurately represents the transition to the external density profile, and we provide a universal test based on geometrical characteristics of the outer profile as a proposal for measuring the turnaround radius of structures.

Kyriakos Grammatikos, Vasiliki Pavlidou, "Getting the tiger by the tail: Probing the turnaround radius of structures with outer halo density profiles" (September 17, 2018).

The introduction to this paper does a good job, often omitted in other papers, of laying out the true complexities and uncertainties involved in inferring dark matter halo mass distributions from observable luminous matter, so I will quote it at length (bold emphasis, underlining, and bullet point before questions added editorially for reading ease in blog format):

One of many widely discussed problems in cosmology is the problem of the definition of a structure. In general, due to hardships posed by the non-linearity of Einstein’s equations, the lack of a globally accurate description of the so-called ”dark matter”, but most importantly by the complexity of many-body problems in general relativity, defining a structure has been non-trivial. Many notions regarding a structure are ill-defined. Even when the structure is in equilibrium, there is no concrete way of determining the boundaries of a structure (see [9] and [10] for a detailed discussion). Consequently, measuring the total mass of a cosmological structure is also a poorly defined problem. In this context, other measurable quantities such as the density profile and the average radial velocity profile become important.

More specifically, the mass of a structure is a difficult quantity to define experimentally. In principle, it is easy to imagine that the mass of a structure corresponds to the mass of the maximum number of objects we can include in the structure that form a gravitationally bound system. As simple as this definition may seem, the lack of accurate information on the velocities and relative positions of the constituents of the structure demonstrates the difficulty of implementing the definition directly. Even theoretical calculations pose difficulties, due to the innate complexity structures exhibit as many body systems. The result of those difficulties is that the prevailing physical scales used to describe structures are not always suitable for all kinds of phenomena. The necessity of accurate description of structure formation has therefore led to various proposals of scales which depend on the velocity or density profiles, which can substitute ambiguous concepts used up to present day.

The first exact estimate for the density profile of a structure came from the pioneering work of Gunn & Gott (1972), who utilized the spherical top hat model in order to approach in a simple way a highly non-linear problem. Since then and in the following few decades the field has known significant advancements. Earlier work in these steps was primarily focused on the density profiles of the innermost regions of structures as in [11]. Progress in calculating the density profile of the outskirts of structures has been made only fairly recently. Furthermore, in the absence of observations for outer regions of halos, most effort was put in interpreting results for the innermost regions. However, these regions nowadays can be effectively probed by virtue of several methods (see [12]),  thus necessitating the theoretical interpretation of existing observations.

Numerical simulations, such as the ones presented by Diemer & Kravtsov (2014), proved that the outskirts of simulated halos exhibit strong deviation from the commonly used density profiles of inner regions (NFW, Einasto), which manifests itself through a steep drop in the power law locally describing the density profile over a narrow interval of radii. Additionally, their simulations revealed that the radial density profile tends to evolve almost self-similarly in time. Although it has been confirmed that there are several universal scalings related to external profiles, their physical significance and role are yet to be specified, since such studies along with semianalytical treatments cannot reveal whether this apparent universality is a well-established consequence of some physical aspect of the system. Characterization of halos has not been an easy task to tackle, there is no rule of thumb for picking out the best criteria for detecting density peaks, and inevitably results, especially concerning halo counting purposes, differ from author to author, depending on their particular choice of definition scale. For a discussion and comparison between several halo simulation algorithms see [13].

Another issue is that the most important and actual physical changes in the behavior of the matter density profile of structure happen at lengths which are not described well by the standard scales used for the analysis of structure formation. Analysis of the spherical top hat model gives a concrete result for the radius of virialization of an idealized structure and predicts no other significant length scale for its density profile. More detailed cosmological simulations however contradict this result, proving that structures are far more complex, both inside and outside the virial radius. Important characteristics of the structures, such as peaks and second order critical points of the density profile, tend to appear around the virial radius, but also at greater distances, see [5] and [14]. Recent results [16] have shown that a reasonably robust choice of scale that distinguishes between regions where accretion is happening is the splashback radius, and that this transition manifests itself as a steep drop in the logarithmic derivative of the density profile.

In the light of these considerations, several related questions arise: 

* Does the category of spherical collapse models suffice for the description of radial density profiles of structures at large radii, where effects of nonsphericities are minimal? 

* If that is true, could it possibly succumb to analytical methods and produce a prediction for the density profile of structures outside their virial radius that agrees with simulations? 

* What physical mechanisms dictate the special characteristics of the external matter profiles? 

* Is there any chance outer halo profiles are universal for all types of structures? 

* Finally, at a more practical level, is there an easy and intuitive way for an experimenter to determine useful physical lengths of a structure, assuming that he is able to measure the outer radial density profile of a structure?

In this paper we calculate analytically the post virialization equilibrium density profile of a spherically evolving structure, which is assumed static. This analytical treatment is aimed at being reasonably accurate a considerable distance away from the center of collapse. We intend to draw conclusions from this treatment about the universality of the external matter profiles and determine explicitly on which parameters of the collapse they depend on. Also, we show that the scale that signifies the transition to the outer halo is no other but the well known turnaround radius of the overdensity. Finally, we propose a fitting function in the range of distances we are interested in, along with the best fit parameters of our approximate approach.

Another Dark Matter Candidate Ruled Out

Strange dibaryons have never been a very strong contender as a dark matter candidate, even though a tiny number of these particles may exist. This paper puts a nail in that coffin.

The hypothetical SU(3) flavor-singlet dibaryon state S with strangeness −2 has been discussed as a dark-matter candidate capable of explaining the curious 5-to-1 ratio of the mass density of dark matter to that of baryons. We study the early-universe production of dibaryons and find that irrespective of the hadron abundances produced by the QCD quark/hadron transition, rapid particle reactions thermalized the S abundance, and it tracked equilibrium until it "froze out" at a tiny value. For the plausible range of dibaryon masses (1860 - 1890 MeV) and generous assumptions about its interaction cross sections, S's account for at most 10^−11 of the baryon number, and thus cannot be the dark matter. Although it is not the dark matter, if the S exists it might be an interesting relic.

Edward W. Kolb, Michael S. Turner, "Dibaryons cannot be the dark matter" (September 17, 2018).

Neutron star behavior also disfavors the existence of these kinds of dibaryons in Nature.

We study the effect of a dibaryon, S, in the mass range 1860 MeV < m_S < 2054 MeV, which is heavy enough not to disturb the stability of nuclei and light enough to possibly be cosmologically metastable. Such a deeply bound state can act as a baryon sink in regions of high baryon density and temperature. We find that the ambient conditions encountered inside a newly born neutron star are likely to sustain a sufficient population of hyperons to ensure that a population of S dibaryons can equilibrate in less than a few seconds. This would be catastrophic for the stability of neutron stars and the observation of neutrino emission from the proto-neutron star of Supernova 1987A over ~ O(10)s. A deeply bound dibaryon is therefore incompatible with the observed supernova explosion, unless the cross section for S production is severely suppressed.

Samuel D. McDermott, Sanjay Reddy, Srimoyee Sen, "A Deeply Bound Dibaryon is Incompatible with Neutron Stars and Supernovae" (September 18, 2018).