Relativistic Modifications in Galactic Rotation Curves under a Toroidal Galactic Field

Rotational dynamics of galaxies exhibits an increase beyond the Keplerian velocity which corresponds to a missing mass up to six times the dynamic mass in the observable universe. In this paper we show that the observed increase in galactic rotation velocities is a general relativistic effect resulting from the combined effect of toroidal magnetic energy density in galaxies and spacetime dragging due to the rotating compact mass in galactic center. The effect of magnetic energy density on spacetime vorticity is derived from Maxwell equations in axially symmetric spacetime where the dragging effects are shown to extend farther in the galactic disc via the toroidal field, modifying the rotational speed of the galactic matter. This is shown to lead to the diverse rotation curves of spiral galaxies, along with the Tully-Fisher relation for total galactic mass and maximum rotational velocity.

Probing Saraswati’s heart: evaluating the dynamical state of the massive galaxy cluster A2631 through a comprehensive weak-lensing and dynamical analysis

ABSTRACT In this work, we investigate the dynamical state of the galaxy cluster Abell 2631, a massive structure located at the core of the Saraswati supercluster. To do this, we first solve a tension found in the literature regarding the weak-lensing mass determination of the cluster. We do this through a comprehensive weak-lensing analysis, exploring the power of the combination of shear and magnification data sets. We find $M_{200}^{\rm wl} = 8.7_{-2.9}^{+2.5} \times 10^{14}$ M⊙. We also determined the mass based on the dynamics of spectroscopic members, corresponding to $M_{200}^{\rm dy} = 12.2\pm 3.0 \times 10^{14}$ M⊙, consistent within a 68 per cent CL with the weak-lensing estimate. The scenarios provided by the mass distribution and dynamics of galaxies are reconciled with those provided by X-ray observations in a scenario where A2631 is observed at a late stage of merging.

Extreme kinematic misalignment in IllustrisTNG galaxies: the origin, structure, and internal dynamics of galaxies with a large-scale counterrotation

ABSTRACT We provide an in-deep analysis of 25 galaxies with substantial counterrotation from IllustrisTNG100 simulations in the stellar mass range 2×109−3×1010 M⊙. The counterrotation is a result of an external gas infall ≈2–8 Gyr ago. The infall leads to the removal of pre-existing gas, which is captured and mixed together with the infalling component. This mixture ends up in the counterrotating gaseous disc where ${\approx}90{{\ \rm per\ cent}}$ of counterrotating stars formed in-situ. During the early phases of the infall, gas can be found in extended structures which, in some galaxies, are similar to (nearly-) polar ring-like components. We suggest that the AGN activity does not cause the counterrotation, although it is efficiently triggered by the retrograde gas infall, and it correlates well with the misaligned component appearance. We also find the vertical-to-radial velocity dispersion ratio above unity implying the importance of misalignment in shaping the velocity ellipsoids.

Alternative Gravitational Theories

There have been many proposed modifications of gravitational theory, beginning with Einstein’s general relativity, modifying Newtonian gravity, and Weyl’s attempt at unifying gravity and electromagnetism. The standard model of cosmology, the Lambda CDM model, requires dark matter and dark energy to fit experimental data. There is a lack of direct evidence for dark matter and dark energy. An alternative theory called modified gravity (MOG) seeks to fit the observational data for the dynamics of galaxies and clusters of galaxies without dark matter. The MOG gravitational theory has a solution for a black hole that modifies the Schwarzschild and Kerr solutions, and can be tested using the data collected on supermassive black holes by the Event Horizon Telescope. There are many modified gravity theories proposed to explain the accelerating expansion of the universe, generally ascribed to dark energy. However, Einstein’s cosmological constant is the simplest explanation for the accelerating expansion.

Testing MOdified Gravity (MOG) theory and dark matter model in Milky Way using the local observables

ABSTRACT In this paper, we have investigated one of the alternative theories to dark matter named MOdified Gravity (MOG) by testing its ability to describe the local dynamics of the Milky Way (MW) in vertical and transverse directions with the baryonic matter. MOG is designed to interpret the dynamics of galaxies and cluster of galaxies without the need for dark matter. We use local observational data such as the vertical dispersion, rotation curve, surface density, and number density of stars in the Milky Way to obtain the parameters of MOG and the baryonic component of MW by implementing a Bayesian approach to the parameter estimation based on a Markov Chain Monte Carlo method. We compare our results with the dark matter model of MW. The two models of MOG and cold dark matter are able to describe equally well the rotation curve and the vertical dynamics of stars in the local MW. The best values for the free parameters of MOG in this analysis are obtained as α = 8.99 ± 0.02 and μ = 0.054 ± 0.005 kpc−1. Also, we obtain the parameters of the generalized gNFW model in the dark matter model. Our best value of bulge mass from MOG is $(1.06 \pm 0.26)\times 10^{10}\, \rm M_{\odot }$, which is consistent with the estimations form the microlensing observations.

Scale-invariant dynamics of galaxies, MOND, dark matter, and the dwarf spheroidals

ABSTRACT The Scale-Invariant Vacuum (SIV) theory is based on Weyl’s Integrable Geometry, endowed with a gauge scalar field. The main difference between MOND and the SIV theory is that the first considers a global dilatation invariance of space and time, where the scale factor λ is a constant, while the second opens the likely possibility that λ is a function of time. The key equations of the SIV framework are used here to study the relationship between the Newtonian gravitational acceleration due to baryonic matter gbar and the observed kinematical acceleration gobs. The relationship is applied to galactic systems of the same age where the radial acceleration relation (RAR), between the gobs and gbar accelerations, can be compared with observational data. The SIV theory shows an excellent agreement with observations and with MOND for baryonic gravities gbar > 10−11.5 m s−2. Below this value, SIV still fully agrees with the observations, as well as with the horizontal asymptote of the RAR for dwarf spheroidals, while this is not the case for MOND. These results support the view that there is no need for dark matter and that the RAR and related dynamical properties of galaxies can be interpreted by a modification of gravitation.