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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 369

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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10 pages, 316 KiB

Open AccessArticle

Surface Density of Disk Galaxies in MOND

by Antonino Del Popolo and Morgan Le Delliou

Universe 2023, 9(1), 32; https://doi.org/10.3390/universe9010032 - 1 Jan 2023

Cited by 2 | Viewed by 2187

Abstract

In this paper, we extend a paper by Milgrom (2009, MNRAS 398, 1023) dealing with the existence of a quasi-universal surface density for object of all mass and structure, if they are in the Newtonian regime, i.e., that their mean acceleration is larger [...] Read more.

In this paper, we extend a paper by Milgrom (2009, MNRAS 398, 1023) dealing with the existence of a quasi-universal surface density for object of all mass and structure, if they are in the Newtonian regime, i.e., that their mean acceleration is larger than MOND typical acceleration

a_{0}

. This result is in agreement with Donato et al. (2009)’s results, claiming the existence of a quasi-universal surface density in all masses in galaxies. The Milgrom paper also predicts that objects with mean inner acceleration smaller than the values discussed do not show the quasi-universal behavior of the surface density discussed. In the present paper, we extend the result of Milgrom’s paper, based on a point mass model, considering spiral galaxies, modelled with a double exponential disk. Similar to Milgrom’s results, we find the existence of a universal surface density for galaxies with large surface density, and a different behavior for galaxies having small surface density. Full article

(This article belongs to the Section Cosmology)

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14 pages, 800 KiB

Open AccessArticle

Splashback Radius in a Spherical Collapse Model

by Antonino Del Popolo and Morgan Le Delliou

Universe 2022, 8(9), 462; https://doi.org/10.3390/universe8090462 - 6 Sep 2022

Viewed by 1651

Abstract

It was shown several years ago that dark matter halo outskirts are characterized by very steep density profiles in a very small radial range. This feature has been interpreted as a pile-up of different particle orbits at a similar location, namely, splashback material [...] Read more.

It was shown several years ago that dark matter halo outskirts are characterized by very steep density profiles in a very small radial range. This feature has been interpreted as a pile-up of different particle orbits at a similar location, namely, splashback material at half an orbit after collapse. Adhikari et al. (2014) obtained the location of the splashback radius through a very simple model by calculating a dark matter shell trajectory in the secondary infall model while it crosses a growing NFW profile-shaped dark matter halo. Because they imposed a halo profile instead of calculating it from the trajectories of the shells of dark matter, they were not able to find the dark matter profile around the splashback radius. In the present paper, we use an improved spherical infall model taking into account shell crossing as well as several physical effects such as ordered and random angular momentum, dynamical friction, adiabatic contraction, etc. This allows us to determine the density profile from the inner to the outer region and to study the behavior of the outer density profile. We compare the density profiles and their logarithmic slope of with the simulation results of Diemer and Kravtsov (2014), finding a good agreement between the prediction of the model and the simulations. Full article

(This article belongs to the Special Issue Modified Gravity and Dark Matter at the Scale of Galaxies)

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20 pages, 984 KiB

Open AccessArticle

A Mass Dependent Density Profile from Dwarfs to Clusters

by Antonino Del Popolo and Morgan Le Delliou

Galaxies 2022, 10(3), 69; https://doi.org/10.3390/galaxies10030069 - 18 May 2022

Viewed by 2835

Abstract

In this paper, we extend the work of Freundlich et al. 2020 who showed how to obtain a Dekel–Zhao density profile with mass dependent shape parameters in the case of galaxies. In the case of Freundlich et al. 2020, the baryonic dependence was [...] Read more.

In this paper, we extend the work of Freundlich et al. 2020 who showed how to obtain a Dekel–Zhao density profile with mass dependent shape parameters in the case of galaxies. In the case of Freundlich et al. 2020, the baryonic dependence was obtained using the NIHAO set of simulations. In our case, we used simulations based on a model of ours. Following Freundlich et al. 2020, we obtained the dependence from baryon physics of the two shape parameters, obtaining in this way a mass dependent Dekel–Zhao profile describing the dark matter profiles from galaxies to clusters of galaxies. The extension to the Dekel–Zhao mass dependent profile to clusters of galaxies is the main result of the paper. In the paper, we show how the Dekel–Zhao mass dependent profile gives a good description of the density profiles of galaxies, already shown by Freundlich et al. 2020, but also to a set of clusters of galaxies. Full article

(This article belongs to the Special Issue Dark Cosmology: Shedding Light on Our Current Universe)

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14 pages, 584 KiB

Open AccessArticle

Non-Thermal Emission from Radio-Loud AGN Jets: Radio vs. X-rays

by Elena Fedorova, Bohdan Hnatyk, Antonino Del Popolo, Anatoliy Vasylenko and Vadym Voitsekhovskyi

Galaxies 2022, 10(1), 6; https://doi.org/10.3390/galaxies10010006 - 4 Jan 2022

Cited by 8 | Viewed by 3225

Abstract

We consider the sample of 55 blazars and Seyferts cross-correlated from the Planck all-sky survey based on the Early Release Compact Source Catalog (ERCSC) and Swift BAT 105-Month Hard X-ray Survey. The radio Planck spectra vs. X-ray Swift/XRT+BAT spectra of the active galactic [...] Read more.

We consider the sample of 55 blazars and Seyferts cross-correlated from the Planck all-sky survey based on the Early Release Compact Source Catalog (ERCSC) and Swift BAT 105-Month Hard X-ray Survey. The radio Planck spectra vs. X-ray Swift/XRT+BAT spectra of the active galactic nuclei (AGN) sample were fitted with the simple and broken power law (for the X-ray spectra taking into account also the Galactic neutral absorption) to test the dependencies between the photon indices of synchrotron emission (in radio range) and synchrotron self-Compton (SSC) or inverse-Compton emission (in X-rays). We show that for the major part of the AGN in our sample there is a correspondence between synchrotron and SSC photon indices (one of two for broken power-law model) compatible within the error levels. For such objects, this can give a good perspective for the task of distinguishing between the jet base counterpart from that one emitted in the disk+corona AGN “central engine”. Full article

(This article belongs to the Special Issue Global Understanding of Accretion and Ejection around Black Holes)

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33 pages, 1307 KiB

Open AccessArticle

Review of Solutions to the Cusp-Core Problem of the ΛCDM Model

by Antonino Del Popolo and Morgan Le Delliou

Galaxies 2021, 9(4), 123; https://doi.org/10.3390/galaxies9040123 - 18 Dec 2021

Cited by 28 | Viewed by 7274

Abstract

This review aims at proposing to the field an overview of the Cusp-core problem, including a discussion of its advocated solutions, assessing how each can satisfactorily provide a description of central densities. Whether the Cusp-core problem reflects our insufficient grasp on the nature [...] Read more.

This review aims at proposing to the field an overview of the Cusp-core problem, including a discussion of its advocated solutions, assessing how each can satisfactorily provide a description of central densities. Whether the Cusp-core problem reflects our insufficient grasp on the nature of dark matter, of gravity, on the impact of baryonic interactions with dark matter at those scales, as included in semi-analytical models or fully numerical codes, the solutions to it can point either to the need for a paradigm change in cosmology, or to to our lack of success in ironing out the finer details of the ΛCDM paradigm. Full article

(This article belongs to the Special Issue Challenges in Our Understanding of Dwarf Galaxies)

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15 pages, 540 KiB

Open AccessArticle

Neutron Stars and Dark Matter

by Antonino Del Popolo, Morgan Le Delliou and Maksym Deliyergiyev

Universe 2020, 6(12), 222; https://doi.org/10.3390/universe6120222 - 26 Nov 2020

Cited by 17 | Viewed by 3888

Abstract

Neutron stars change their structure with accumulation of dark matter. We study how their mass is influenced from the environment. Close to the sun, the dark matter accretion from the neutron star does not have any effect on it. Moving towards the galactic [...] Read more.

Neutron stars change their structure with accumulation of dark matter. We study how their mass is influenced from the environment. Close to the sun, the dark matter accretion from the neutron star does not have any effect on it. Moving towards the galactic center, the density increase in dark matter results in increased accretion. At distances of some fraction of a parsec, the neutron star acquire enough dark matter to have its structure changed. We show that the neutron star mass decreases going towards the galactic centre, and that dark matter accumulation beyond a critical value collapses the neutron star into a black hole. Calculations cover cases varying the dark matter particle mass, self-interaction strength, and ratio between the pressure of dark matter and ordinary matter. This allow us to constrain the interaction cross section,

σ_{dm}

, between nucleons and dark matter particles, as well as the dark matter self-interaction cross section. Full article

(This article belongs to the Special Issue Neutron Star Astrophysics)

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18 pages, 398 KiB

Open AccessArticle

An Anisotropic Model for the Universe

by Morgan Le Delliou, Maksym Deliyergiyev and Antonino del Popolo

Symmetry 2020, 12(10), 1741; https://doi.org/10.3390/sym12101741 - 21 Oct 2020

Cited by 8 | Viewed by 2900

Abstract

Motivated by the back-reaction debate, and some unexplained characteristics of the CMB, we investigate the possibility of some anisotropy in the universe observed around us. To this aim, we build up a novel prediction for the Hubble law for the late universe from [...] Read more.

Motivated by the back-reaction debate, and some unexplained characteristics of the CMB, we investigate the possibility of some anisotropy in the universe observed around us. To this aim, we build up a novel prediction for the Hubble law for the late universe from a Bianchi type I model, taken as proof of concept, transcribing the departure of such model from a ΛCDM model. We dicussed the redshift measurement in this universe, and finally formalized the Hubble diagram. Full article

(This article belongs to the Special Issue New Advances of Cosmology and Astrophysics)

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12 pages, 359 KiB

Open AccessArticle

On the Dwarf Galaxy Rotation Curve Diversity Problem

by Antonino Del Popolo, Morgan Le Delliou and Xiguo Lee

Galaxies 2018, 6(3), 67; https://doi.org/10.3390/galaxies6030067 - 29 Jun 2018

Cited by 7 | Viewed by 3685

Abstract

In this paper, we show how baryonic physics can solve the problem of the striking diversity in dwarf galaxy rotation curves shapes. To this aim, we compare the distribution of galaxies of the SPARC sample, in the plane

V_{2 kpc}

- [...] Read more.

In this paper, we show how baryonic physics can solve the problem of the striking diversity in dwarf galaxy rotation curves shapes. To this aim, we compare the distribution of galaxies of the SPARC sample, in the plane

V_{2 kpc}

-

V_{Rlast}

(

V_{2 kpc}

being the galaxy rotation velocity at 2 kpc, and

V_{Rlast}

being the outermost one), with that of galaxies that we simulated, taking account of baryonic effects. The scatter in the rotation curves in the

V_{2 kpc}

-

V_{Rlast}

plane, as well as the trend of the SPARC sample, and our simulated galaxy distribution is in good agreement. The solution of the “diversity” problem lies in the ability of the baryonic process to produce non-self-similar haloes, contrary to DM-only simulations. We show also that baryonic effects can reproduce the rotation curves of galaxies such as IC2574, which are characterized by a slow rise in radius. A solution to the diversity problem can be obtained taking the baryon physics effects appropriately into account. Full article

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46 pages, 2624 KiB

Open AccessArticle

Small Scale Problems of the ΛCDM Model: A Short Review

by Antonino Del Popolo and Morgan Le Delliou

Galaxies 2017, 5(1), 17; https://doi.org/10.3390/galaxies5010017 - 20 Feb 2017

Cited by 223 | Viewed by 15926

Abstract

The ΛCDM model, or concordance cosmology, as it is often called, is a paradigm at its maturity. It is clearly able to describe the universe at large scale, even if some issues remain open, such as the cosmological constant problem, the small-scale problems [...] Read more.

The ΛCDM model, or concordance cosmology, as it is often called, is a paradigm at its maturity. It is clearly able to describe the universe at large scale, even if some issues remain open, such as the cosmological constant problem, the small-scale problems in galaxy formation, or the unexplained anomalies in the CMB. ΛCDM clearly shows difficulty at small scales, which could be related to our scant understanding, from the nature of dark matter to that of gravity; or to the role of baryon physics, which is not well understood and implemented in simulation codes or in semi-analytic models. At this stage, it is of fundamental importance to understand whether the problems encountered by the ΛDCM model are a sign of its limits or a sign of our failures in getting the finer details right. In the present paper, we will review the small-scale problems of the ΛCDM model, and we will discuss the proposed solutions and to what extent they are able to give us a theory accurately describing the phenomena in the complete range of scale of the observed universe. Full article

(This article belongs to the Special Issue Dark Matter: Large versus Small Scale Structures)

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