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8 pages, 3821 KiB

Open AccessCommunication

Ultraviolet Background Radiation from Not-So-Dark Matter in the Galactic Halo

by Richard Conn Henry, Jayant Murthy and James Overduin

Universe 2025, 11(5), 148; https://doi.org/10.3390/universe11050148 - 3 May 2025

Viewed by 374

Murthy et al. (2025) (hereafter Paper I) have recently reported the discovery of unexpectedly bright diffuse extreme-ultraviolet radiation at high latitudes in both the Northern and Southern Galactic Hemispheres. After correction for extinction by the total interstellar dust in the direction of each [...] Read more.

Murthy et al. (2025) (hereafter Paper I) have recently reported the discovery of unexpectedly bright diffuse extreme-ultraviolet radiation at high latitudes in both the Northern and Southern Galactic Hemispheres. After correction for extinction by the total interstellar dust in the direction of each observation, the spectra are nearly identical, suggesting that the radiation has a unique source and likely originates in the halo of our galaxy. The observed spectrum extends down to 912 Å, the interstellar hydrogen absorption edge. Radiation even slightly short of that edge would, if ubiquitous, be sufficient to explain the high degree of ionization in our galaxy and throughout the universe. We hypothesize that this newly discovered radiation originates in the slow decay of dark matter. The intensity of the radiation implies that the decay cannot be via the weak interaction, suggesting the existence of a new, even weaker fundamental interaction, consistent with the exceedingly long decay lifetime required. Full article

(This article belongs to the Special Issue Tensions, Anomalies, and Challenges in the Standard Cosmological Model)

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20 pages, 1318 KiB

Open AccessArticle

The Galactic Pizza: Flat Rotation Curves in the Context of Cosmological Time-Energy Coupling

by Artur Novais and André L. B. Ribeiro

Galaxies 2025, 13(3), 51; https://doi.org/10.3390/galaxies13030051 - 27 Apr 2025

Viewed by 4730

Abstract

The phenomenon of augmented gravity on the scale of galaxies, conventionally attributed to dark matter halos, is shown to possibly result from the incremental growth of galactic masses and radii over time. This approach elucidates the cosmological origins of the acceleration scale [...] Read more.

The phenomenon of augmented gravity on the scale of galaxies, conventionally attributed to dark matter halos, is shown to possibly result from the incremental growth of galactic masses and radii over time. This approach elucidates the cosmological origins of the acceleration scale

a_{0} \approx c H_{0} / 2 π \approx 10^{- 10}

ms⁻² at which galaxy rotation curves deviate from Keplerian behavior, with no need for new particles or modifications to the laws of gravity, i.e., it constitutes a new explanatory path beyond Cold Dark Matter (CDM) and Modified Newtonian Dynamics (MOND). Once one formally equates the energy density of the universe to the critical value (

ρ = ρ_{c}

) and the cosmic age to the reciprocal of the Hubble parameter (

t = H^{- 1}

), independently of the epoch of observation, the result is the Zero-Energy condition for the cosmic fluid’s equation of state, with key repercussions for the study of dark energy since the observables can be explained in the absence of a cosmological constant. Furthermore, this mass-energy evolution framework is able to reconcile the success of CDM models in describing structure assembly at

z ≲ 6

with the unexpected discovery of massive objects at

z ≳ 10

. Models that feature a strong coupling between cosmic time and energy are favored by this analysis. Full article

(This article belongs to the Special Issue Alternative Interpretations of Observed Galactic Behaviors)

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7 pages, 227 KiB

Open AccessCommunication

Are the Galaxies with Indefinitely Flat Circular Velocities Located Inside Large Dark Matter Haloes?

by Man Ho Chan, Yangzhanhao Zhang and Antonino Del Popolo

Universe 2025, 11(4), 104; https://doi.org/10.3390/universe11040104 - 21 Mar 2025

Viewed by 366

Abstract

A recent study using weak gravitational lensing revealed that the rotation curves of some isolated galaxies are found at a very large distance from their galactic centres. This may provide strong evidence supporting Modified Newtonian Dynamics (MOND) and challenging the standard cold dark [...] Read more.

A recent study using weak gravitational lensing revealed that the rotation curves of some isolated galaxies are found at a very large distance from their galactic centres. This may provide strong evidence supporting Modified Newtonian Dynamics (MOND) and challenging the standard cold dark matter model. In this article, we propose the possibility that these isolated galaxies are possibly located at the centres of corresponding large dark matter haloes. Using the standard gravitational framework of galaxy groups and galaxy clusters, we show that this scenario can reproduce the observed rotation curves, provided the existence of corresponding hot gas haloes extending beyond 1 Mpc. Therefore, MOND may not be the only solution to the observed rotation curves and the cold dark matter interpretation still remains viable. Full article

(This article belongs to the Special Issue The 10th Anniversary of Universe: Standard Cosmological Models, and Modified Gravity and Cosmological Tensions)

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12 pages, 788 KiB

Open AccessArticle

Issues in the Investigations of the Dark Matter Phenomenon in Galaxies: Parcere Personis, Dicere de Vitiis

by Paolo Salucci

Universe 2025, 11(2), 67; https://doi.org/10.3390/universe11020067 - 17 Feb 2025

Cited by 1 | Viewed by 874

Abstract

It is always more evident that the kinematics of galaxies provide us with unique information on the Nature of the dark particles and on the properties of the galaxy Dark Matter (DM) halos. However, in investigating this topic, we have to be very [...] Read more.

It is always more evident that the kinematics of galaxies provide us with unique information on the Nature of the dark particles and on the properties of the galaxy Dark Matter (DM) halos. However, in investigating this topic, we have to be very careful about certain issues related to the assumptions that we take or to the practices that we follow. Here, we critically discuss such issues, that, today, result of fundamental importance, in that we have realized that the Nature of the DM will be not provided by “The Theory” but, has to be inferred by reverse engineering the observational scenario. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2024—"Galaxies and Clusters")

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12 pages, 1136 KiB

Open AccessArticle

Prediction of Individual Halo Concentrations Across Cosmic Time Using Neural Networks

by Tianchi Zhang, Tianxiang Mao, Wenxiao Xu and Guan Li

Universe 2025, 11(2), 37; https://doi.org/10.3390/universe11020037 - 26 Jan 2025

Viewed by 828

Abstract

The concentration of dark matter haloes is closely linked to their mass accretion history. We utilize the halo mass accretion histories from large cosmological N-body simulations as inputs for our neural networks, which we train to predict the concentration of individual haloes [...] Read more.

The concentration of dark matter haloes is closely linked to their mass accretion history. We utilize the halo mass accretion histories from large cosmological N-body simulations as inputs for our neural networks, which we train to predict the concentration of individual haloes at a given redshift. The trained model performs effectively in other cosmological simulations, achieving the root mean square error between the actual and predicted concentrations that significantly lower than that of the model by Zhao et al. and Giocoli et al. at any redshift. This model serves as a valuable tool for rapidly predicting halo concentrations at specified redshifts in large cosmological simulations. Full article

(This article belongs to the Special Issue Advances in Studies of Galaxies at High Redshift)

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18 pages, 714 KiB

Open AccessArticle

Implications of the Intriguing Constant Inner Mass Surface Density Observed in Dark Matter Halos

by Jorge Sánchez Almeida

Galaxies 2025, 13(1), 6; https://doi.org/10.3390/galaxies13010006 - 9 Jan 2025

Viewed by 992

Abstract

It has long been known that the observed mass surface density of cored dark matter (DM) halos is approximately constant, independently of the galaxy mass (i.e.,

ρ_{c} r_{c} ≃ constant

, with

ρ_{c}

and

r_{c}

being the central volume [...] Read more.

It has long been known that the observed mass surface density of cored dark matter (DM) halos is approximately constant, independently of the galaxy mass (i.e.,

ρ_{c} r_{c} ≃ constant

, with

ρ_{c}

and

r_{c}

being the central volume density and the radius of the core, respectively). Here, we review the evidence supporting this empirical fact as well as its theoretical interpretation. It seems to be an emergent law resulting from the concentration–halo mass relation predicted by the current cosmological model, where the DM is made of collisionless cold DM particles (CDM). We argue that the prediction

ρ_{c} r_{c} ≃ constant

is not specific to this particular model of DM but holds for any other DM model (e.g., self-interacting) or process (e.g., stellar or AGN feedback) that redistributes the DM within halos conserving its CDM mass. In addition, the fact that

ρ_{c} r_{c} ≃ constant

is shown to allow the estimate of the core DM mass and baryon fraction from stellar photometry alone is particularly useful when the observationally expensive conventional spectroscopic techniques are unfeasible. Full article

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12 pages, 535 KiB

Open AccessEditor’s ChoiceArticle

Reanalysis of the MACHO Constraints on PBH in the Light of Gaia DR3 Data

by Juan García-Bellido and Michael Hawkins

Universe 2024, 10(12), 449; https://doi.org/10.3390/universe10120449 - 6 Dec 2024

Cited by 6 | Viewed by 1148

Abstract

The recent astrometric data of hundreds of millions of stars from Gaia DR3 has allowed for a precise determination of the Milky Way rotation curve up to 28 kpc. The data suggest a rapid decline in the density of dark matter beyond 19 [...] Read more.

The recent astrometric data of hundreds of millions of stars from Gaia DR3 has allowed for a precise determination of the Milky Way rotation curve up to 28 kpc. The data suggest a rapid decline in the density of dark matter beyond 19 kpc. We fit the whole rotation curve with four components (gas, disk, bulge, and halo), and compute the microlensing optical depth to the Large Magellanic Cloud. With this model of the galaxy we reanalyse the microlensing events of the MACHO and EROS-2 Collaborations. Using the published MACHO efficiency function for the duration of their survey, together with the rate of expected events according to the new density profile, we find that the Dark Matter halo could be composed of up to 20% of massive compact halo objects for any mass between

0.001

to

1 M_{⊙}

. For the EROS-2 survey, using a modified efficiency curve for consistency with the MACHO analysis, we also find compatibility with a MACHO halo, but with a tighter constraint around

0.005 M_{⊙}

where the halo fraction cannot be larger than ∼10%. This result assumes that all the lenses have the same mass. If these were distributed in an extended mass function like that of the Thermal History Model, the constraints are weakened, allowing 100% of all DM in the form of Primordial Black Holes. Full article

(This article belongs to the Section Cosmology)

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21 pages, 899 KiB

Open AccessArticle

The Shape of the Chameleon Fifth-Force on the Mass Components of Galaxy Clusters

by Lorenzo Pizzuti, Valentina Amatori, Alexandre M. Pombo and Sandeep Haridasu

Universe 2024, 10(12), 443; https://doi.org/10.3390/universe10120443 - 30 Nov 2024

Viewed by 913

Abstract

In the context of chameleon gravity, we present a semi-analytical solution of the chameleon field profile in an accurately modelled galaxy cluster’s mass components, namely the stellar mass of the Brightest Cluster Galaxy (BCG), the baryonic mass in galaxies other than the BCG, [...] Read more.

In the context of chameleon gravity, we present a semi-analytical solution of the chameleon field profile in an accurately modelled galaxy cluster’s mass components, namely the stellar mass of the Brightest Cluster Galaxy (BCG), the baryonic mass in galaxies other than the BCG, the mass of the Intra-Cluster Medium (ICM) and the diffuse cold dark matter (CDM). The obtained semi-analytic profile is validated against the numerical solution of the chameleon field equation and implemented in the MG-MAMPOSSt code for kinematic analyses of galaxy clusters in modified gravity scenarios. By means of mock halos, simulated both in GR and in modified gravity, we show that the combination of the velocities and positions of cluster member galaxies, along with the data of the stellar velocity dispersion profile of the BCG, can impose constraints on the parameter space of the chameleon model; for a cluster generated in GR, these constraints are at the same level as a joint lensing+kinematics analysis of a cluster modelled with a single mass profile, without the BCG data. Full article

(This article belongs to the Special Issue Universe: Feature Papers 2024—'Cosmology')

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13 pages, 320 KiB

Open AccessArticle

Structure of the Baryon Halo Around a Supermassive Primordial Black Hole

by Boris Murygin, Viktor Stasenko and Yury Eroshenko

Particles 2024, 7(4), 1004-1016; https://doi.org/10.3390/particles7040061 - 13 Nov 2024

Viewed by 1172

Abstract

According to some theoretical models, primordial black holes with masses of more than 10⁸ solar masses could be born in the early universe, and their possible observational manifestations have been investigated in a number of works. Dense dark matter and baryon halos [...] Read more.

According to some theoretical models, primordial black holes with masses of more than 10⁸ solar masses could be born in the early universe, and their possible observational manifestations have been investigated in a number of works. Dense dark matter and baryon halos could form around such primordial black holes even at the pre-galactic stage (in the cosmological Dark Ages epoch). In this paper, the distribution and physical state of the gas in the halo are calculated, taking into account the radiation transfer from the central accreting primordial black hole. This made it possible to find the ionization radius, outside of which there are regions of neutral hydrogen absorption in the 21 cm line. The detection of annular absorption regions at high redshifts in combination with a central bright source may provide evidence of the existence of supermassive primordial black holes. We also point out the fundamental possibility of observing absorption rings with strong gravitational lensing on galaxy clusters, which weakens the requirements for the angular resolution of radio telescopes. Full article

(This article belongs to the Special Issue Selected Papers from XXVIII Workshop “What Comes Beyond the Standard Models?”: New Trends in Particle Cosmology)

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32 pages, 1586 KiB

Open AccessArticle

The Magellanic Clouds Are Very Rare in the IllustrisTNG Simulations

by Moritz Haslbauer, Indranil Banik, Pavel Kroupa, Hongsheng Zhao and Elena Asencio

Universe 2024, 10(10), 385; https://doi.org/10.3390/universe10100385 - 1 Oct 2024

Cited by 3 | Viewed by 1123

Abstract

The Large and Small Magellanic Clouds (LMC and SMC) form the closest interacting galactic system to the Milky Way, therewith providing a laboratory to test cosmological models in the local Universe. We quantify the likelihood for the Magellanic Clouds (MCs) to be observed [...] Read more.

The Large and Small Magellanic Clouds (LMC and SMC) form the closest interacting galactic system to the Milky Way, therewith providing a laboratory to test cosmological models in the local Universe. We quantify the likelihood for the Magellanic Clouds (MCs) to be observed within the

Λ

CDM model using hydrodynamical simulations of the IllustrisTNG project. The orbits of the MCs are constrained by proper motion measurements taken by the Hubble Space Telescope and Gaia. The MCs have a mutual separation of

d_{MCs} = 24.5 kpc

and a relative velocity of

v_{MCs} = 90.8 km s^{- 1}

, implying a specific phase-space density of

f_{MCs, obs} \equiv {(d_{MCs} \cdot v_{MCs})}^{- 3} = 9.10 \times 10^{- 11} {km}^{- 3} s^{3} {kpc}^{- 3}

. We select analogues to the MCs based on their stellar masses and distances in MW-like halos. None of the selected LMC analogues have a higher total mass and lower Galactocentric distance than the LMC, resulting in >3.75

σ

tension. We also find that the

f_{MCs}

distribution in the highest resolution TNG50 simulation is in

3.95 σ

tension with observations. Thus, a hierarchical clustering of two massive satellites like the MCs in a narrow phase-space volume is unlikely in

Λ

CDM, presumably because of short merger timescales due to dynamical friction between the overlapping dark matter halos. We show that group infall led by an LMC analogue cannot populate the Galactic disc of satellites (DoS), implying that the DoS and the MCs formed in physically unrelated ways in

Λ

CDM. Since the

20^{\circ}

alignment of the LMC and DoS orbital poles has a likelihood of

P = 0.030

(

2.17 σ

), adding this

χ^{2}

to that of

f_{MCs}

gives a combined likelihood of

P = 3.90 \times 10^{- 5}

(

4.11 σ

). Full article

(This article belongs to the Special Issue Universe: Feature Papers 2024—"Galaxies and Clusters")

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28 pages, 2303 KiB

Open AccessArticle

Probing the Nonlinear Density Wave Theory of Spiral Galaxies by Baryonic Tully–Fisher Relation

by Miroslava Vukcevic, Djordje Savic and Predrag Jovanović

Universe 2024, 10(9), 359; https://doi.org/10.3390/universe10090359 - 6 Sep 2024

Cited by 1 | Viewed by 1188

Abstract

The baryonic mass–velocity relation provides an important test of different galaxy dynamics models such as Lambda–cold dark matter (

Λ

CDM) and alternatives like Modified Newtonian Dynamics (MOND). Novel nonlinear density wave theory with a soliton solution gives an opportunity to test whether [...] Read more.

The baryonic mass–velocity relation provides an important test of different galaxy dynamics models such as Lambda–cold dark matter (

Λ

CDM) and alternatives like Modified Newtonian Dynamics (MOND). Novel nonlinear density wave theory with a soliton solution gives an opportunity to test whether the derived rotational velocity expression is able to support the well known Tully–Fisher empirical relation between mass and rotation velocity in disk galaxies. Initial assumptions do not involve any larger dark matter halo that supports the stability of the very thin galactic disk nor any modified gravitational acceleration acting on galactic scales. It rather follows an important gravitational interaction between constituents of disk mass in the outer part of the disk via gravitational potential. Data are obtained by a fitting procedure applied on the sample of 81 rotational curves of late type spirals using expressions for the rotational velocity derived as an exact, a self-consistent solution of the nonlinear Schrodinger (NLS) equation for galactic surface mass density. The location of these selected objects in the baryonic mass–rotation velocity plane follows the relation

log M_{b} = (3.7 \pm 0.2) log V_{f l a t} + (2.7 \pm 0.4)

in marginal agreement with the findings in the literature. Full article

(This article belongs to the Special Issue Recent Advances in Gravitational Lensing and Galactic Dynamics)

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29 pages, 429 KiB

Open AccessEditor’s ChoiceReview

A Review of Stable, Traversable Wormholes in f(R) Gravity Theories

by Ramesh Radhakrishnan, Patrick Brown, Jacob Matulevich, Eric Davis, Delaram Mirfendereski and Gerald Cleaver

Symmetry 2024, 16(8), 1007; https://doi.org/10.3390/sym16081007 - 7 Aug 2024

Cited by 8 | Viewed by 6330

Abstract

It has been proven that in standard Einstein gravity, exotic matter (i.e., matter violating the pointwise and averaged Weak and Null Energy Conditions) is required to stabilize traversable wormholes. Quantum field theory permits these violations due to the quantum coherent effects found in [...] Read more.

It has been proven that in standard Einstein gravity, exotic matter (i.e., matter violating the pointwise and averaged Weak and Null Energy Conditions) is required to stabilize traversable wormholes. Quantum field theory permits these violations due to the quantum coherent effects found in any quantum field. Even reasonable classical scalar fields violate the energy conditions. In the case of the Casimir effect and squeezed vacuum states, these violations have been experimentally proven. It is advantageous to investigate methods to minimize the use of exotic matter. One such area of interest is extended theories of Einstein gravity. It has been claimed that in some extended theories, stable traversable wormholes solutions can be found without the use of exotic matter. There are many extended theories of gravity, and in this review paper, we first explore

f (R)

theories and then explore some wormhole solutions in

f (R)

theories, including Lovelock gravity and Einstein Dilaton Gauss–Bonnet (EdGB) gravity. For completeness, we have also reviewed ‘Other wormholes’ such as Casimir wormholes, dark matter halo wormholes, thin-shell wormholes, and Nonlocal Gravity (NLG) wormholes, where alternative techniques are used to either avoid or reduce the amount of exotic matter that is required. Full article

(This article belongs to the Special Issue Symmetry in Gravity Theories and Cosmology)

17 pages, 1175 KiB

Open AccessArticle

The Response of the Inner Dark Matter Halo to Stellar Bars

by Daniel A. Marostica, Rubens E. G. Machado, E. Athanassoula and T. Manos

Galaxies 2024, 12(3), 27; https://doi.org/10.3390/galaxies12030027 - 28 May 2024

Cited by 3 | Viewed by 1283

Abstract

Barred galaxies constitute about two-thirds of observed disc galaxies. Bars affect not only the mass distribution of gas and stars but also that of the dark matter. An elongation of the inner dark matter halo is known as the halo bar. We aim [...] Read more.

Barred galaxies constitute about two-thirds of observed disc galaxies. Bars affect not only the mass distribution of gas and stars but also that of the dark matter. An elongation of the inner dark matter halo is known as the halo bar. We aim to characterize the structure of the halo bars, with the goal of correlating them with the properties of the stellar bars. We use a suite of simulated galaxies with various bar strengths, including gas and star formation. We quantify the strengths, shapes, and densities of these simulated stellar bars. We carry out numerical experiments with frozen and analytic potentials in order to understand the role played by a live responsive stellar bar. We find that the halo bar generally follows the trends of the disc bar. The strengths of the halo and stellar bars are tightly correlated. Stronger bars induce a slight increase in dark matter density within the inner halo. Numerical experiments show that a non-responsive frozen stellar bar would be capable of inducing a dark matter bar, but it would be weaker than the live case by a factor of roughly two. Full article

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16 pages, 14681 KiB

Open AccessFeature PaperArticle

Galaxy Groups as the Ultimate Probe of AGN Feedback

by Dominique Eckert, Fabio Gastaldello, Ewan O’Sullivan, Alexis Finoguenov, Marisa Brienza and the X-GAP Collaboration

Galaxies 2024, 12(3), 24; https://doi.org/10.3390/galaxies12030024 - 13 May 2024

Cited by 6 | Viewed by 2195

Abstract

The co-evolution between supermassive black holes and their environment is most directly traced by the hot atmospheres of dark matter halos. The cooling of the hot atmosphere supplies the central regions with fresh gas, igniting active galactic nuclei (AGN) with long duty cycles. [...] Read more.

The co-evolution between supermassive black holes and their environment is most directly traced by the hot atmospheres of dark matter halos. The cooling of the hot atmosphere supplies the central regions with fresh gas, igniting active galactic nuclei (AGN) with long duty cycles. The outflows from the central engine tightly couple with the surrounding gaseous medium and provide the dominant heating source, preventing runaway cooling. Every major modern hydrodynamical simulation suite now includes a prescription for AGN feedback to reproduce the realistic populations of galaxies. However, the mechanisms governing the feeding/feedback cycle between the central black holes and their surrounding galaxies and halos are still poorly understood. Galaxy groups are uniquely suited to constrain the mechanisms governing the cooling–heating balance, as the energy supplied by the central AGN can exceed the gravitational binding energy of halo gas particles. Here, we provide a brief overview of our knowledge of the impact of AGN on the hot atmospheres of galaxy groups, with a specific focus on the thermodynamic profiles of the groups. We then present our on-going efforts to improve on the implementation of AGN feedback in galaxy evolution models by providing precise measurements of the properties of galaxy groups. We introduce the XMM-Newton Group AGN Project (X-GAP), a large program on XMM-Newton targeting a sample of 49 galaxy groups out to

R_{500 c}

. Full article

(This article belongs to the Special Issue Multi-Phase Fueling and Feedback Processes in Jetted AGN)

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19 pages, 755 KiB

Open AccessEditor’s ChoiceArticle

The Relevance of Dynamical Friction for the MW/LMC/SMC Triple System

by Wolfgang Oehm and Pavel Kroupa

Universe 2024, 10(3), 143; https://doi.org/10.3390/universe10030143 - 14 Mar 2024

Cited by 7 | Viewed by 2138

Abstract

Simulations of structure formation in the standard cold dark matter cosmological model quantify the dark matter halos of galaxies. Taking into account dynamical friction between dark matter halos, we investigate the past orbital dynamical evolution of the Magellanic Clouds in the presence of [...] Read more.

Simulations of structure formation in the standard cold dark matter cosmological model quantify the dark matter halos of galaxies. Taking into account dynamical friction between dark matter halos, we investigate the past orbital dynamical evolution of the Magellanic Clouds in the presence of the Galaxy. Our calculations are based on a three-body model of rigid Navarro–Frenk–White profiles for dark matter halos but were verified in a previous publication by comparison to high-resolution N-body simulations of live self-consistent systems. Under the requirement that the LMC and SMC had an encounter within 20 kpc between 1 and 4 Gyr ago in order to allow the development of the Magellanic Stream, using the latest astrometric data, the dynamical evolution of the MW/LMC/SMC system is calculated backwards in time. With the employment of the genetic algorithm and a Markov-Chain Monte-Carlo method, the present state of this system is unlikely, with a probability of

< 10^{- 9}

(

6 σ

complement), because the solutions found do not fit into the error bars for the observed plane-of-sky velocity components of the Magellanic Clouds. This implies that orbital solutions that assume dark matter halos, according to cosmological structure formation theory, to exist around the Magellanic Clouds and the Milky Way are not possible with a confidence of more than 6 sigma. Full article

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