Dark Matter Realism

According to the standard model of cosmology, $$\Lambda $$ Λ CDM, the mass-energy budget of the current stage of the universe is not dominated by the luminous matter that we are familiar with, but instead by some form of dark matter (and dark energy). It is thus tempting to adopt scientific realism about dark matter. However, there are barely any constraints on the myriad of possible properties of this entity—it is not even certain that it is a form of matter. In light of this underdetermination I advocate caution: we should not (yet) be dark matter realists. The “not(-yet)-realism” that I have in mind is different from Hacking’s (Philos Sci 56 (4), 555–581, 1989) anti-realism, in that it is semantic rather than epistemological. It also differs from the semantic anti-realism of logical empiricism, in that it is naturalistic, such that it may only be temporary and does not automatically apply to all other unobservables (or even just to all other astronomical unobservables, as with Hacking’s anti-realism). The argument is illustrated with the analogy of the much longer history of the concept of a gene, as the current state of the concept of dark matter resembles in some relevant ways that of the early concept of genes.

Fundamental Properties of the Dark and the Luminous Matter from the Low Surface Brightness Discs

Dark matter (DM) is one of the biggest mystery in the Universe. In this review, we start reporting the evidences for this elusive component and discussing about the proposed particle candidates and scenarios for such phenomenon. Then, we focus on recent results obtained for rotating disc galaxies, in particular for low surface brightness (LSB) galaxies. The main observational properties related to the baryonic matter in LSBs, investigated over the last decades, are briefly recalled. Next, these galaxies are analyzed by means of the mass modelling of their rotation curves both individual and stacked. The latter analysis, via the universal rotation curve (URC) method, results really powerful in giving a global or universal description of the properties of these objects. We report the presence in LSBs of scaling relations among their structural properties that result comparable with those found in galaxies of different morphologies. All this confirms, in disc systems, the existence of a strong entanglement between the luminous matter (LM) and the dark matter (DM). Moreover, we report how in LSBs the tight relationship between their radial gravitational accelerations g and their baryonic components gb results to depend also on the stellar disk length scale and the radius at which the two accelerations have been measured. LSB galaxies strongly challenge the ΛCDM scenario with the relative collisionless dark particle and, alongside with the non-detection of the latter, contribute to guide us towards a new scenario for the DM phenomenon.

An excess of small-scale gravitational lenses observed in galaxy clusters

Cold dark matter (CDM) constitutes most of the matter in the Universe. The interplay between dark and luminous matter in dense cosmic environments, such as galaxy clusters, is studied theoretically using cosmological simulations. Observations of gravitational lensing are used to characterize the properties of substructures—the small-scale distribution of dark matter—in clusters. We derive a metric, the probability of strong lensing events produced by dark-matter substructure, and compute it for 11 galaxy clusters. The observed cluster substructures are more efficient lenses than predicted by CDM simulations, by more than an order of magnitude. We suggest that systematic issues with simulations or incorrect assumptions about the properties of dark matter could explain our results.

Statistical deprojection of intervelocities, interdistances, and masses in the Isolated Galaxy Pair Catalog

Aims. In order to study the internal dynamics of actual galaxy pairs, we need to derive the probability distribution function (PDF) of true 3D, orbital intervelocities and interdistances between pair members from their observed projected values along with the pair masses from Kepler’s third law. For this research, we used 13 114 pairs from the Isolated Galaxy Pair Catalog (IGPC). Methods. The algorithms of statistical deprojection previously elaborated were applied to these observational data. We derived the orbital velocity PDFs for the whole catalog and for several selected subsamples. The interdistance PDF is deprojected and compared to the analytical profiles expected from semi-theoretical arguments. Results. The PDF of deprojected pair orbital velocities is characterized by the existence of a main probability peak around ≈150 km s−1 for all subsamples of the IGPC as well as for the Uppsala Galaxy Pair Catalog. The interdistance PDFs of both the projected and deprojected data are described at large distances by the same power law with exponent ≈ − 2. The whole distributions, including their cores, are fairly fitted by King profiles. The mass deprojection yields a mass/luminosity ratio for the pairs of M/L = (30 ± 5) in solar units. Conclusions. The orbital velocity probability peak is observed at the same value, ≈150 km s−1, as the main exoplanet velocity peak, which points toward a possible universality of Keplerian structures, whatever the scale. The pair M/L ratio is just seven times the standard ratio for luminous matter, which does not require the existence of nonbaryonic dark matter in these systems.

Paradigms and Scenarios for the Dark Matter Phenomenon

Well known scaling laws among the structural properties of the dark and the luminous matter in disc systems are too complex to be arisen by two inert components that just share the same gravitational field. This brings us to critically focus on the 30-year-old paradigm, that, resting on a priori knowledge of the nature of Dark Matter (DM), has led us to a restricted number of scenarios, especially favouring the collisionless Λ Cold Dark Matter one. Motivated by such observational evidence, we propose to resolve the dark matter mystery by following a new Paradigm: the nature of DM must be guessed/derived by deeply analyzing the properties of the dark and luminous mass distribution at galactic scales. The immediate application of this paradigm leads us to propose the existence of a direct interaction between Dark and Standard Model particles, which has finely shaped the inner regions of galaxies.

About Dark Matter and Gravitation

A close inspection of Zwicky's seminal papers on the dynamics of galaxy clusters reveals that the discrepancy discovered between the dynamical mass and the luminous mass of clusters has been widely overestimated in 1933 as a consequence of several factors, among which the excessive value of the Hubble constant $H_0$, then believed to be about seven times higher than today's average estimate. Taking account, in addition, of our present knowledge of classical dark matter inside galaxies, the contradiction can be reduced by a large factor. To explain the rather small remaining discrepancy of the order of 5, instead of appealing to a hypothetic exotic dark matter, the possibility of a inhomogeneous gravity is suggested. This is consistent with the ``cosmic tapestry" found in the eighties by De Lapparent and her co-authors, showing that the cosmos is highly inhomogeneous at large scale. A possible foundation for inhomogeneous gravitation is the universally discredited ancient theory of Fatio de Duillier and Lesage on pushing gravity, possibly revised to avoid the main criticisms which led to its oblivion. This model incidentally opens the window towards a completely non-standard representation of cosmos, and more basically calls to develop fundamental investigation to find the origin of the large scale inhomogeneity in the distribution of luminous matter

A discrete chemo-dynamical model of M87’s globular clusters: Kinematics extending to ∼400 kpc

ABSTRACT We study the mass distribution and kinematics of the giant elliptical galaxy M87 (NGC 4486) using discrete chemo-dynamical, axisymmetric Jeans equation modelling. Our catalogue comprises 894 globular clusters (GCs) extending to a projected radius of ∼430 kpc with line-of-sight velocities and colours, and Multi Unit Spectroscopic Explorer integral field unit data within the central 2.4 kpc of the main galaxy. The gravitational potential for our models is a combination of a luminous matter potential with a varying mass-to-light ratio for the main galaxy, a supermassive black hole and a dark matter (DM) potential with a cusped or cored DM halo. The best-fitting models with either a cusped or a cored DM halo show no significant differences and both are acceptable. We obtain a total mass of $(2.16 \pm 0.38) \times 10^{13} \, \mathrm{M}_{\odot }$ within ∼400 kpc. By including the stellar mass-to-light ratio gradient, the DM fraction increases from ∼26 per cent (with no gradient) to ∼73 per cent within $1\, R_e^{\rm maj}$ (major axis of half-light isophote, 14.2 kpc), and from ∼84 per cent to ∼94 per cent within $5\, R_e^{\rm maj}$ (71.2 kpc). Red GCs have moderate rotation with Vmax/σ ∼ 0.4, and blue GCs have weak rotation with Vmax/σ ∼ 0.1. Red GCs have tangential velocity dispersion anisotropy, while blue GCs are consistent with being nearly isotropic. Our results suggest that red GCs are more likely to be born in situ, while blue GCs are more likely to be accreted.

Inner dark matter distribution of the Cosmic Horseshoe (J1148+1930) with gravitational lensing and dynamics

We present a detailed analysis of the inner mass structure of the Cosmic Horseshoe (J1148+1930) strong gravitational lens system observed with the Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3). In addition to the spectacular Einstein ring, this systems shows a radial arc. We obtained the redshift of the radial arc counterimage zs, r = 1.961 ± 0.001 from Gemini observations. To disentangle the dark and luminous matter, we considered three different profiles for the dark matter (DM) distribution: a power law profile, the Navarro, Frenk, and White (NFW) profile, and a generalized version of the NFW profile. For the luminous matter distribution, we based the model on the observed light distribution that is fitted with three components: a point mass for the central light component resembling an active galactic nucleus, and the remaining two extended light components scaled by a constant mass-to-light ratio (M/L). To constrain the model further, we included published velocity dispersion measurements of the lens galaxy and performed a self-consistent lensing and axisymmetric Jeans dynamical modeling. Our model fits well to the observations including the radial arc, independent of the DM profile. Depending on the DM profile, we get a DM fraction between 60% and 70%. With our composite mass model we find that the radial arc helps to constrain the inner DM distribution of the Cosmic Horseshoe independently of the DM profile.

To see the invisible: Image of the event horizon within the black hole shadow

How the supermassive black hole SgrA* in the Milky Way Center looks like for a distant observer? It depends on the black hole highlighting by the surrounding hot matter. The black hole shadow (the photon capture cross-section) would be viewed if there is a stationary luminous background. The black hole event horizon is invisible directly (per se). Nevertheless, a more compact (with respect to black hole shadow) projection of the black hole event horizon on the celestial sphere may be reconstructed by detecting the highly redshifted photons emitted by the nonstationary luminous matter plunging into the black hole and approaching the event horizon. It is appropriate to call this reconstructed projection of the event horizon on the celestial sphere for a distant observer as the “lensed event horizon image”, or simply the “event horizon image”. This event horizon image is placed on the celestial sphere within the position of black hole shadow. Amazingly, the event horizon image is a gravitationally lensed projection on the celestial sphere of the whole surface of the event horizon globe. As a result, the black holes may be viewed at once from both the front and back sides. The lensed event horizon image may be considered as a genuine silhouette of the black hole. For example, a dark northern hemisphere of the event horizon image is the simplest model for a black hole silhouette in the presence of a thin accretion disk.