Rotation Curves of Galaxies and Their Dependence on Morphology and Stellar Mass

We study how stellar rotation curves (RCs) of galaxies are correlated on average with morphology and stellar mass (M star) using the final release of Sloan Digital Sky Survey IV MaNGA data. We use the visually assigned T-types for the morphology indicator, and adopt a functional form for the RC that can model non-flat RCs at large radii. We discover that within the radial coverage of the MaNGA data, the popularly known flat rotation curve at large radii applies only to the particular classes of galaxies, i.e., massive late types (T-type ≥ 1, M star ≳ 1010.8 M ⊙) and S0 types (T-type = −1 or 0, M star ≳ 1010.0 M ⊙). The RC of late-type galaxies at large radii rises more steeply as M star decreases, and its slope increases to about +9 km s−1 kpc−1 at M star ≈ 109.7 M ⊙. By contrast, elliptical galaxies (T-type ≤ −2) have descending RCs at large radii. Their slope becomes more negative as M star decreases, and reaches as negative as −15 km s−1 kpc−1 at M star ≈ 1010.2 M ⊙. We also find that the inner slope of the RC is highest for elliptical galaxies with M star ≈ 1010.5 M ⊙, and decreases as T-type increases or M star changes away from 1010.5 M ⊙. The velocity at the turnover radius R t is higher for higher M star, and R t is larger for higher M star and later T-types. We show that the inner slope of the RC is coupled with the central surface stellar mass density, which implies that the gravitational potential of central regions of galaxies is dominated by baryonic matter. With the aid of simple models for matter distribution, we discuss what determines the shapes of RCs.

Is non-particle dark matter equation of state parameter evolving with time?

Recently, the so-called Hubble Tension, i.e. the mismatch between the local and the cosmological measurements of the Hubble parameter, has been resolved when non-particle dark matter is considered which has a negative equation of state parameter ($$\omega \approx -\,0.01$$ ω ≈ - 0.01 ). We investigate if such a candidate can successfully describe the galactic flat rotation curves. It is found that the flat rotation curve feature puts a stringent constraint on the dark matter equation of state parameter $$\omega $$ ω and $$\omega \approx -\,0.01$$ ω ≈ - 0.01 is not consistent with flat rotational curves, observed around the galaxies. However, a dynamic $$\omega $$ ω of non-particle dark matter may overcome the Hubble tension without affecting the flat rotation curve feature.

On Non-Spherical Models for Spiral Galaxy Gravitation Potential Yielding Flat Rotation Curve

A two component model of gravitation potential for spiral galaxies has been proposed which couples a spherically symmetric component with a second component that observes planar radial symmetry on the galactic plane and vanishes outside an annular disk beyond the edge of galaxy's effective radius. It is shown that such a model for potential satisfying Poisson Equation would produce rotation velocity curve towards the edge of the galaxy which is flat over distance from the galactic centre. This relationship, which is experimentally observed in many spiral galaxies, is shown as a consequence of classical understanding of gravity and specific symmetry of the gravitational potential without any extrinsic requirement of dark matter. It is also demonstrated that this potential directly yields a relationship between inner mass of the galaxy and terminal rotation velocity, which has been empirically observed and known as Baryonic Tully-Fisher relations. Furthermore a direct test has been proposed for experimental verification of the proposed theory.

A Derivation of Flat Rotation Curve of Spiral Galaxies and Baryonic Tully-Fisher Relation

Fundamentally for the extended disc region of a spiral galaxy, an alternative solution to Laplace equation has been presented for a potential that is radially symmetric on the disc plane. This potential, unlike newtonian one, is shown to be logarithmic in distance from the centre, which allows for the rotation velocity to be constant along the disc radius.It is also shown that this potential easily manifests into a relationship between inner mass of the galaxy and terminal rotation velocity, which has been empirically observed and known as Baryonic Tully-Fisher relations.

Slicing the cool circumgalactic medium along the major axis of a star-forming galaxy at z = 0.7

ABSTRACT We present spatially resolved Echelle spectroscopy of an intervening Mg ii–Fe ii–Mg i absorption-line system detected at zabs = 0.73379 towards the giant gravitational arc PSZ1 G311.65–18.48. The absorbing gas is associated with an inclined disc-like star-forming galaxy, whose major axis is aligned with the two arc-segments reported here. We probe in absorption the galaxy’s extended disc continuously, at ≈3 kpc sampling, from its inner region out to 15× the optical radius. We detect strong ($W_0^{2796}\gt 0.3$Å) coherent absorption along 13 independent positions at impact parameters D = 0–29 kpc on one side of the galaxy, and no absorption at D = 28–57 kpc on the opposite side (all de-lensed distances at zabs). We show that (1) the gas distribution is anisotropic; (2) $W_0^{2796}$, $W_0^{2600}$, $W_0^{2852}$, and the ratio $W_0^{2600}\!/W_0^{2796}$, all anticorrelate with D; (3) the $W_0^{2796}$–D relation is not cuspy and exhibits significantly less scatter than the quasar-absorber statistics; (4) the absorbing gas is co-rotating with the galaxy out to D ≲ 20 kpc, resembling a ‘flat’ rotation curve, but at D ≳ 20 kpc velocities decline below the expectations from a 3D disc-model extrapolated from the nebular [O ii] emission. These signatures constitute unambiguous evidence for rotating extra-planar diffuse gas, possibly also undergoing enriched accretion at its edge. Arguably, we are witnessing some of the long-sought processes of the baryon cycle in a single distant galaxy expected to be representative of such phenomena.

Plasticity of spacetime as an alternative to dark matter

We propose an alternative to the dark matter hypothesis that would explain why the effects of a curvature of spacetime are measured in regions where no electromagnetic radiation is observed. The problem could be solved, assuming a plasticity of spacetime that, in some conditions, can allow a region to remain locally curved even when the mass density has gone away from that region. This way we can observe curvature also in the absence of a local mass density. We apply this idea in a very simple way to the rotation curve of Milky Way showing an agreement between the prediction of the theory and the experimental data. We propose also a different scenario that gives as a result a flat rotation curve. At this stage, our approach is just an idea, not an already complete theory, so many problems remain still unsolved.

Origin of dark mass apparentfrom gravitationally bound extended ordinary material systems

<p>To grow up a gravitationally bound system with fixed proper mass requires sufficient energy to overcome the gravitational pull. Following General relativity both matter energy and field energy act on the source of gravity. Apart from the field energy if our concern is just for the bound system of mass only, then we can easily attribute the gain of matter energy as the gain of gravitational mass. This gain of matter energy can be backed immediately if the system is allowed to collapse to its initial state. So in the case of expansion there will be no possibility in creation of normal or proper mass. But the concern regarding the gain of matter energy must have realistic effect in increase of gravitational mass. This article explains how gravitational mass of the gravitationally bound large material system like galaxy exceeds its ordinary mass with the size of the system. Here presence of the dark mass and flat rotation curve are given without considering MOND theory, not even distorting any accepted paradigm of post Newtonian gravity. The findings are truly consistent with the recent observed data.</p>

Explaining the density profile of self-gravitating systems by statistical mechanics

A self-gravitating system usually shows a quasi-universal density profile, such as the NFW profile of a simulated dark matter halo, the flat rotation curve of a spiral galaxy, the Sérsic profile of an elliptical galaxy, the King profile of a globular cluster and the exponential law of the stellar disk. It will be interesting if all of the above can be obtained from first principles. Based on the original work of White & Narayan (1987), we propose that if the self-bounded system is divided into infinite infinitesimal subsystems, the entropy of each subsystem can be maximized, but the whole system's gravity may just play the role of the wall, which may not increase the whole system's entropy St, and finally St may be the minimum among all of the locally maximized entropies (He & Kang 2010). For spherical systems with isotropic velocity dispersion, the form of the equation of state will be a hybrid of isothermal and adiabatic (Kang & He 2011). Hence this density profile can be approximated by a truncated isothermal sphere, which means that the total mass must be finite and our results can be consistent with observations (Kang & He 2011b). Our method requires that the mass and energy should be conserved, so we only compare our results with simulations of mild relaxation (i.e. the virial ratio is close to -1) of dissipationless collapse (Kang 2014), and the fitting also is well. The capacity can be calculated and is found not to be always negative as in previous works, and combining with calculations of the second order variation of the entropy, we find that the thermodynamical stability still can be true (Kang 2012) if the temperature tends to be zero. However, the cusp in the center of dark matter halos can not be explained, and more works will continue.The above work can be generalized to study the radial distribution of the disk (Kang 2015). The energy constraint automatically disappears in our variation, because angular momentum is much more important than energy for the disk-shape system. To simplify this issue, a toy model is taken: 2D gravity is adopted, then at large scale it will be consistent with a flat rotation curve; the bulge and the stellar disk are studied together. Then with constraints of mass and angular momentum, the calculated surface density can be consistent with the truncated, up-bended or standard exponential law. Therefore the radial distribution of the stellar disk may be determined by both the random and orbital motions of stars. In our fittings the central gravity is set to be nonzero to include the effect of asymmetric components.