Posts Tagged rotation curves

Recent Postings from rotation curves

Constraints on the dark matter sound speed from galactic scales: the cases of the Modified and Extended Chaplygin Gas

We show that the observed rotation curves of spiral galaxies constrain the sound speed of the dark matter to be $c_s < 10^{-4} c$, where $c$ is the speed of light in vacuum. Using the Modified Chaplygin Gas as a representative example of a class of unified dark energy models incorporating an effective dark matter component with a non-zero sound speed, we determine the most stringent constraint to date on the value of the constant contribution to the equation of state parameter in this class of models. Finally, we explain the reason why previous constraints using the Cosmic Microwave Background and Baryonic Acoustic Oscillations were not as competitive as the one presented in this paper and discuss the limitations of the recently proposed Extended Chaplygin Gas.

Constraints on the dark matter sound speed from galactic scales: the cases of the Modified and Extended Chaplygin Gas [Cross-Listing]

We show that the observed rotation curves of spiral galaxies constrain the sound speed of the dark matter to be $c_s < 10^{-4} c$, where $c$ is the speed of light in vacuum. Using the Modified Chaplygin Gas as a representative example of a class of unified dark energy models incorporating an effective dark matter component with a non-zero sound speed, we determine the most stringent constraint to date on the value of the constant contribution to the equation of state parameter in this class of models. Finally, we explain the reason why previous constraints using the Cosmic Microwave Background and Baryonic Acoustic Oscillations were not as competitive as the one presented in this paper and discuss the limitations of the recently proposed Extended Chaplygin Gas.

Constraints on the dark matter sound speed from galactic scales: the cases of the Modified and Extended Chaplygin Gas [Cross-Listing]

We show that the observed rotation curves of spiral galaxies constrain the sound speed of the dark matter to be $c_s < 10^{-4} c$, where $c$ is the speed of light in vacuum. Using the Modified Chaplygin Gas as a representative example of a class of unified dark energy models incorporating an effective dark matter component with a non-zero sound speed, we determine the most stringent constraint to date on the value of the constant contribution to the equation of state parameter in this class of models. Finally, we explain the reason why previous constraints using the Cosmic Microwave Background and Baryonic Acoustic Oscillations were not as competitive as the one presented in this paper and discuss the limitations of the recently proposed Extended Chaplygin Gas.

Searching for IMBHs in Galactic globular clusters through radial velocities of individual stars

I present an overview of our ongoing project aimed at building a new generation of velocity dispersion profiles ad rotation curves for a representative sample of Galactic globular clusters, from the the radial velocity of hundreds individual stars distributed at different distances from the cluster center. The innermost portion of the profiles will be used to constrain the possibile presence of intermediate-mass black holes. The adopted methodology consists in combining spectroscopic observations acquired with three different instruments at the ESO-VLT: the adaptive-optics assisted, integral field unit (IFU) spectrograph SINFONI for the innermost and highly crowded cluster cores, the multi-IFU spectrograph KMOS for the intermediate regions, and the multi-fiber instrument FLAMES/GIRAFFE-MEDUSA for the outskirts. The case of NGC 6388, representing the pilot project that motivated the entire program, is described in some details.

Galaxy rotation curves with log-normal density distribution

The log-normal distribution represents the probability of finding randomly distributed particles in a micro canonical ensemble with high entropy. To a first approximation, a modified form of this distribution with a truncated termination may represent an isolated galactic disk, and this disk density distribution model was therefore run to give the best fit to the observational rotation curves for 37 representative galaxies. The resultant curves closely matched the observational data for a wide range of velocity profiles and galaxy types with rising, flat or descending curves in agreement with Verheijen’s classification of ‘R’, ‘F’ and ‘D’ type curves, and the corresponding theoretical total disk masses could be fitted to a baryonic Tully Fisher relation (bTFR). Nine of the galaxies were matched to galaxies with previously published masses, suggesting a mean excess dynamic disk mass of dex0.61+/-0.26 over the baryonic masses. Although questionable with regard to other measurements of the shape of disk galaxy gravitational potentials, this model can accommodate a scenario in which the gravitational mass distribution, as measured via the rotation curve, is confined to a thin plane without requiring a dark-matter halo or the use of MOND.

High-resolution mass models of dwarf galaxies from LITTLE THINGS

We present high-resolution rotation curves and mass models of 26 dwarf galaxies from LITTLE THINGS. LITTLE THINGS is a high-resolution Very Large Array HI survey for nearby dwarf galaxies in the local volume within 11 Mpc. The rotation curves of the sample galaxies derived in a homogeneous and consistent manner are combined with Spitzer archival 3.6 micron and ancillary optical U, B, and V images to construct mass models of the galaxies. We decompose the rotation curves in terms of the dynamical contributions by baryons and dark matter halos, and compare the latter with those of dwarf galaxies from THINGS as well as Lambda CDM SPH simulations in which the effect of baryonic feedback processes is included. Being generally consistent with THINGS and simulated dwarf galaxies, most of the LITTLE THINGS sample galaxies show a linear increase of the rotation curve in their inner regions, which gives shallower logarithmic inner slopes alpha of their dark matter density profiles. The mean value of the slopes of the 26 LITTLE THINGS dwarf galaxies is alpha =-0.32 +/- 0.24 which is in accordance with the previous results found for low surface brightness galaxies (alpha = -0.2 +/- 0.2) as well as the seven THINGS dwarf galaxies (alpha =-0.29 +/- 0.07). However, this significantly deviates from the cusp-like dark matter distribution predicted by dark-matter-only Lambda CDM simulations. Instead our results are more in line with the shallower slopes found in the Lambda CDM SPH simulations of dwarf galaxies in which the effect of baryonic feedback processes is included. In addition, we discuss the central dark matter distribution of DDO 210 whose stellar mass is relatively low in our sample to examine the scenario of inefficient supernova feedback in low mass dwarf galaxies predicted from recent Lambda SPH simulations of dwarf galaxies where central cusps still remain.

Rotation curves of ultralight BEC dark matter halos with rotation

We study the rotation curves of ultralight BEC dark matter halos. These halos are long lived solutions of initially rotating BEC fluctuations. In order to study the implications of the rotation characterizing these long-lived configurations we consider the particular case of a boson mass $m=10^{-23}\mathrm{eV/c}^2$ and no self-interaction. We find that these halos successfully fit samples of rotation curves (RCs) of LSB galaxies.

Rotation curves of ultralight BEC dark matter halos with rotation [Cross-Listing]

We study the rotation curves of ultralight BEC dark matter halos. These halos are long lived solutions of initially rotating BEC fluctuations. In order to study the implications of the rotation characterizing these long-lived configurations we consider the particular case of a boson mass $m=10^{-23}\mathrm{eV/c}^2$ and no self-interaction. We find that these halos successfully fit samples of rotation curves (RCs) of LSB galaxies.

The distribution of dark and luminous matter inferred from extended rotation curves

A better understanding of the formation of mass structures in the universe can be obtained by determining the amount and distribution of dark and luminous matter in spiral galaxies. To investigate such matters a sample of 12 galaxies, most with accurate distances, has been composed of which the luminosities are distributed regularly over a range spanning 2.5 orders of magnitude. Of the observed high quality and extended rotation curves of these galaxies decompositions have been made, for four different schemes, each with two free parameters. For a "maximum disc fit" the rotation curves can be well matched, yet a large range of mass-to-light ratios for the individual galaxies is required. For the alternative gravitational theory of MOND the rotation curves can be explained if the fundamental parameter associated with MOND is allowed as a free parameter. Fixing that parameter leads to a disagreement between the predicted and observed rotation curves for a few galaxies. When cosmologically motivated NFW dark matter halos are assumed, the rotation curves for the least massive galaxies can, by no means, be reproduced; cores are definitively preferred over cusps. Finally, decompositions have been made for a pseudo isothermal halo combined with a universal M/L ratio. For the latter, the light of each galactic disc and bulge has been corrected for extinction and has been scaled by the effect of stellar population. This scheme can successfully explain the observed rotations and leads to sub maximum disc mass contributions. Properties of the resulting dark matter halos are described and a ratio between dark and baryonic mass of approximately 9 for the least, and of approximately 5, for the most luminous galaxies has been determined, at the outermost measured rotation.

Smoothing Rotation Curves and Mass Profiles

We show that spiral activity can erase pronounced features in disk galaxy rotation curves. We present simulations of growing disks, in which the added material has a physically motivated distribution, as well as other examples of physically less realistic accretion. In all cases, attempts to create unrealistic rotation curves were unsuccessful because spiral activity rapidly smoothed away features in the disk mass profile. The added material was redistributed radially by the spiral activity, which was itself provoked by the density feature. In the case of a ridge-like feature in the surface density profile, we show that two unstable spiral modes develop, and the associated angular momentum changes in horseshoe orbits remove particles from the ridge and spread them both inwards and outwards. This process rapidly erases the density feature from the disk. We also find that the lack of a feature when transitioning from disk to halo dominance in the rotation curves of disk galaxies, the so called "disk-halo conspiracy", could also be accounted for by this mechanism. We do not create perfectly exponential mass profiles in the disk, but suggest that this mechanism contributes to their creation.

Smoothing Rotation Curves and Mass Profiles [Replacement]

We show that spiral activity can erase pronounced features in disk galaxy rotation curves. We present simulations of growing disks, in which the added material has a physically motivated distribution, as well as other examples of physically less realistic accretion. In all cases, attempts to create unrealistic rotation curves were unsuccessful because spiral activity rapidly smoothed away features in the disk mass profile. The added material was redistributed radially by the spiral activity, which was itself provoked by the density feature. In the case of a ridge-like feature in the surface density profile, we show that two unstable spiral modes develop, and the associated angular momentum changes in horseshoe orbits remove particles from the ridge and spread them both inwards and outwards. This process rapidly erases the density feature from the disk. We also find that the lack of a feature when transitioning from disk to halo dominance in the rotation curves of disk galaxies, the so called "disk-halo conspiracy", could also be accounted for by this mechanism. We do not create perfectly exponential mass profiles in the disk, but suggest that this mechanism contributes to their creation.

Revisiting galactic rotation curves given a noncommutative-geometry background

It was shown earlier by Rahaman et al. that a noncommutative-geometry background can account for galactic rotation curves without the need for dark matter. The smearing effect that characterizes noncommutative geometry is described by means of a Gaussian distribution intended to replace the Dirac delta function. The purpose of this paper is two-fold: (1) to account for the galactic rotation curves in a more transparent and intuitively more appealing way by replacing the Gaussian function by the simpler Lorentzian distribution proposed by Nozari and Mehdipour and (2) to show that the smearing effect is both a necessary and sufficient condition for meeting the stability criterion.

Cosmological simulations of galaxy formation with cosmic rays

We investigate the dynamical impact of cosmic rays in cosmological simulations of galaxy formation using adaptive-mesh refinement simulations of a $10^{12}$ solar mass halo. In agreement with previous work, a run with only our standard thermal energy feedback model results in a massive spheroid and unrealistically peaked rotation curves. However, the addition of a simple two-fluid model for cosmic rays drastically changes the morphology of the forming disk. We include an isotropic diffusive term and a source term tied to star formation due to (unresolved) supernova-driven shocks. Over a wide range of diffusion coefficients, the CRs generate thin, extended disks with a significantly more realistic (although still not flat) rotation curve. We find that the diffusion of CRs is key to this process, as they escape dense star forming clumps and drive outflows within the more diffuse ISM.

Relational Mechanics as a gauge theory [Replacement]

Absolute space is eliminated from the body of mechanics by gauging translations and rotations in the Lagrangian of a classical system. In this way, the resulting equations of motion are valid in any frame. Even so, there are privileged frames where Newton’s equations are valid (Newtonian frames), but they are determined by the matter distribution of the universe (Machianization). The frame of fixed stars is not exactly Newtonian when it is employed for describing the motion of subsystems having large angular momenta; a dragging effect will be perceived in this frame, which could be detectable in the galactic rotation curves. On the other hand, the absence of an absolute time is known to be a characteristic of parametrized systems. Parametrized systems with potentials that are proportional to inverse square distances are gauge invariant under scalings too (shape-dynamics).

Relational Mechanics as a gauge theory: Machianization and dragging effect in galactic halos

The elimination of absolute space from the body of mechanics is achieved by gauging translations and rotations in the kinetic energy of an isolated system. As a result, the gauge invariant Lagrangian leads to equations of motion that can be used in any frame. Nevertheless, there are privileged frames where Newton’s equations are valid, but they are determined by the matter distribution of the universe (Machianization). The relational equations of motion shows that there exists a dragging effect in galactic halos that contributes to the rotation curves. On the other hand, the absence of an absolute time is characteristic of parametrized systems, which are systems possessing an internal time. Parametrized systems with potentials that are proportional to inverse square distances are as well gauge invariant under scalings (shape-dynamics).

Relational Mechanics as a gauge theory [Replacement]

Absolute space is eliminated from the body of mechanics by gauging translations and rotations in the Lagrangian of a classical system. In this way, the resulting equations of motion are valid in any frame. Even so, there are privileged frames where Newton’s equations are valid (Newtonian frames), but they are determined by the matter distribution of the universe (Machianization). The frame of fixed stars is not exactly Newtonian when it is employed for describing the motion of subsystems having large angular momenta; a dragging effect will be perceived in this frame, which could be detectable in the galactic rotation curves. On the other hand, the absence of an absolute time is known to be a characteristic of parametrized systems. Parametrized systems with potentials that are proportional to inverse square distances are gauge invariant under scalings too (shape-dynamics).

Relational Mechanics as a gauge theory: Machianization and dragging effect in galactic halos [Cross-Listing]

The elimination of absolute space from the body of mechanics is achieved by gauging translations and rotations in the kinetic energy of an isolated system. As a result, the gauge invariant Lagrangian leads to equations of motion that can be used in any frame. Nevertheless, there are privileged frames where Newton’s equations are valid, but they are determined by the matter distribution of the universe (Machianization). The relational equations of motion shows that there exists a dragging effect in galactic halos that contributes to the rotation curves. On the other hand, the absence of an absolute time is characteristic of parametrized systems, which are systems possessing an internal time. Parametrized systems with potentials that are proportional to inverse square distances are as well gauge invariant under scalings (shape-dynamics).

Relational Mechanics as a gauge theory [Replacement]

Absolute space is eliminated from the body of mechanics by gauging translations and rotations in the Lagrangian of a classical system. In this way, the resulting equations of motion are valid in any frame. Even so, there are privileged frames where Newton’s equations are valid (Newtonian frames), but they are determined by the matter distribution of the universe (Machianization). The frame of fixed stars is not exactly Newtonian when it is employed for describing the motion of subsystems having large angular momenta; a dragging effect will be perceived in this frame, which could be detectable in the galactic rotation curves. On the other hand, the absence of an absolute time is known to be a characteristic of parametrized systems. Parametrized systems with potentials that are proportional to inverse square distances are gauge invariant under scalings too (shape-dynamics).

Modified gravity models and the central cusp of dark matter halos in galaxies [Cross-Listing]

The N-body dark matter (DM) simulations point that DM density profiles, e.g. the NFW halo, should be cuspy in its center, but observations disfavour this kind of DM profile. Here we consider whether the observed rotation curves "close" to the galactic centre can favour modified gravity models in comparison to the NFW halo, and how to quantify such difference. Two explicit modified gravity models are considered, MOND and a more recent approach called RGGR (in reference to Renormalization Group effects in General Relativity). It is also the purpose of this work to significantly extend the sample on which RGGR has been tested in comparison to other approaches. By analysing 62 galaxies from five samples, we find that: i) there is a radius, given by half the disk scale length, below which RGGR and MOND can match the data about as well or better than NFW, albeit the formers have fewer free parameters; ii) considering the complete rotation curve data, RGGR could achieve fits with better agreement than MOND, and almost as good as a NFW halo with two free parameters (NFW and RGGR have respectively two and one more free parameters than MOND).

Modified gravity models and the central cusp of dark matter halos in galaxies

The N-body dark matter (DM) simulations point that DM density profiles, e.g. the NFW halo, should be cuspy in its center, but observations disfavour this kind of DM profile. Here we consider whether the observed rotation curves "close" to the galactic centre can favour modified gravity models in comparison to the NFW halo, and how to quantify such difference. Two explicit modified gravity models are considered, MOND and a more recent approach called RGGR (in reference to Renormalization Group effects in General Relativity). It is also the purpose of this work to significantly extend the sample on which RGGR has been tested in comparison to other approaches. By analysing 62 galaxies from five samples, we find that: i) there is a radius, given by half the disk scale length, below which RGGR and MOND can match the data about as well or better than NFW, albeit the formers have fewer free parameters; ii) considering the complete rotation curve data, RGGR could achieve fits with better agreement than MOND, and almost as good as a NFW halo with two free parameters (NFW and RGGR have respectively two and one more free parameters than MOND).

On the core-halo distribution of dark matter in galaxies [Replacement]

We investigate the distribution of dark matter in galaxies by solving the equations of equilibrium of a self-gravitating system of massive fermions (`inos’) at selected temperatures and degeneracy parameters within general relativity. Our most general solutions show, as a function of the radius, a segregation of three physical regimes: 1) an inner core of almost constant density governed by degenerate quantum statistics; 2) an intermediate region with a sharply decreasing density distribution followed by an extended plateau, implying quantum corrections; 3) an asymptotic, $\rho\propto r^{-2}$ classical Boltzmann regime fulfilling, as an eigenvalue problem, a fixed value of the flat rotation curves. This eigenvalue problem determines, for each value of the central degeneracy parameter, the mass of the ino as well as the radius and mass of the inner quantum core. Consequences of this alternative approach to the central and halo regions of galaxies, ranging from dwarf to big spirals, for SgrA*, as well as for the existing estimates of the ino mass, are outlined.

On the core-halo distribution of dark matter in galaxies

We investigate the distribution of dark matter in galaxies by solving the equations of equilibrium of a self-gravitating system of massive fermions (`inos’) at selected temperatures and degeneracy parameters within general relativity. The most general solutions present, as a function of the radius, a segregation of three physical regimes: 1) an inner core of almost constant density governed by degenerate quantum statistics; 2) an intermediate region with a sharply decreasing density distribution followed by an extended plateau, implying quantum corrections; 3) a decreasing density distribution $\rho\propto r^{-2}$ leading to flat rotation curves fulfilling the classical Boltzmann statistics. The mass of the inos is determined as an eigenfunction of the mass of the inner quantum cores. We compare and contrast this mass value with the lower limit on the particle mass by Tremaine and Gunn (1979), and show that the latter is approached for the less degenerate quantum cores in agreement with the fixed halo observables. Consequences of this alternative approach to the massive core in SgrA* and to dwarf galaxies are outlined.

Rotation Curves and Nonextensive Statistics

We investigate the influence of the nonextensive q-statistics and kinetic theory on galactic scales through the analysis of a devised sample of Spiral Rotation Curves. Largely supported by recent developments on the foundations of statistical mechanics and a plethora of astrophysical applications, the theory also provides an alternative interpretation to the empirical cored dark matter profiles observed in galaxies. We show that the observations could well be fitted with reasonable values for the mass model parameters, encouraging further investigation into q-statistcs on the distribution of dark matter from both observational and theoretical points of view.

Galactic Dark Matter: a Dynamical Consequence of Cosmological Expansion

This work wants to show how standard General Relativity is able to explain galactic rotation curves without the need for dark matter, this starting from the idea that when Einstein’s equations are applied to the dynamics of a galaxy embedded in an expanding universe they do not reduce to Poisson’s equation but a generalisation of it taking cosmological expansion into account. A non-linear scheme to perturb Einstein’s field equations around the Robertson-Walker metric is devised in order to find their non-relativistic limit without losing their characteristic non-linearities. The resulting equation is used to numerically study the gravitational potential of a cosmological perturbation and applied to a simple galactic model with an exponentially decreasing baryonic matter distribution in order to obtain rotation curves to be compared with observational data. The non-relativistic limit of General Relativity produces a generalised Poisson equation for the gravitational potential which is non-linear, parabolic and heat-like. It is shown how its non-linearities generate an effective "dark matter" distribution caused by both cosmological expansion and the dynamics of the perturbation’s gravitational potential. It is also shown how this dynamical effect gets completely lost during a linearisation of Einstein’s equations. The equation is then used to successfully fit real galactic rotation curves numerically using a matter distribution following the shape of a simple S\’ersic luminosity profile, common to most galaxies, thus without recourse to dark matter. A few rotation curves with a faster than Newtonian decrease are also presented and successfully fitted, opening the way to a new possible interpretation of these phenomena in terms of an effective "anti-gravitational" dark matter distribution, purely geometrical in origin, not so easily explained using the dark matter conjecture.

Galactic Dark Matter: a Dynamical Consequence of Cosmological Expansion [Cross-Listing]

This work wants to show how standard General Relativity is able to explain galactic rotation curves without the need for dark matter, this starting from the idea that when Einstein’s equations are applied to the dynamics of a galaxy embedded in an expanding universe they do not reduce to Poisson’s equation but a generalisation of it taking cosmological expansion into account. A non-linear scheme to perturb Einstein’s field equations around the Robertson-Walker metric is devised in order to find their non-relativistic limit without losing their characteristic non-linearities. The resulting equation is used to numerically study the gravitational potential of a cosmological perturbation and applied to a simple galactic model with an exponentially decreasing baryonic matter distribution in order to obtain rotation curves to be compared with observational data. The non-relativistic limit of General Relativity produces a generalised Poisson equation for the gravitational potential which is non-linear, parabolic and heat-like. It is shown how its non-linearities generate an effective "dark matter" distribution caused by both cosmological expansion and the dynamics of the perturbation’s gravitational potential. It is also shown how this dynamical effect gets completely lost during a linearisation of Einstein’s equations. The equation is then used to successfully fit real galactic rotation curves numerically using a matter distribution following the shape of a simple S\’ersic luminosity profile, common to most galaxies, thus without recourse to dark matter. A few rotation curves with a faster than Newtonian decrease are also presented and successfully fitted, opening the way to a new possible interpretation of these phenomena in terms of an effective "anti-gravitational" dark matter distribution, purely geometrical in origin, not so easily explained using the dark matter conjecture.

Galactic Dark Matter: a Dynamical Consequence of Cosmological Expansion [Replacement]

This work wants to show how standard General Relativity (GR) is able to explain galactic rotation curves without the need for dark matter, this starting from the idea that when Einstein’s equations are applied to the dynamics of a galaxy embedded in an expanding universe they do not reduce to Poisson’s equation but a generalisation of it taking cosmological expansion into account. A non-linear scheme to perturb Einstein’s field equations around the Robertson-Walker (R-W) metric is devised in order to find their non-relativistic limit without losing their characteristic non-linearities. The resulting equation is used to numerically study the gravitational potential of a cosmological perturbation and applied to a simple galactic model with an exponentially decreasing baryonic matter distribution. The non-relativistic limit of GR in a R-W space-time produces a generalised Poisson equation for the gravitational potential which is non-linear, parabolic and heat-like. It is shown how its non-linearities generate an effective "dark matter" distribution caused by both cosmological expansion and the dynamics of the perturbation’s gravitational potential. It is also shown how this dynamical effect gets completely lost during a linearisation of Einstein’s equations. The equation is then used to successfully fit real galactic rotation curves numerically using a matter distribution following the shape of a simple S\’ersic luminosity profile, common to most galaxies, thus without recourse to dark matter. A relation for the dark to luminous matter ratio is found, explaining the domination of dark matter in low-mass galaxies. A few rotation curves with a faster than Newtonian decrease are also presented and successfully fitted, opening the way to a new possible interpretation of these phenomena in terms of an effective "anti-gravitational" dark matter distribution, purely geometrical in origin.

Galactic Dark Matter: a Dynamical Consequence of Cosmological Expansion [Replacement]

This work wants to show how standard General Relativity (GR) is able to explain galactic rotation curves without the need for dark matter, this starting from the idea that when Einstein’s equations are applied to the dynamics of a galaxy embedded in an expanding universe they do not reduce to Poisson’s equation but a generalisation of it taking cosmological expansion into account. A non-linear scheme to perturb Einstein’s field equations around the Robertson-Walker (R-W) metric is devised in order to find their non-relativistic limit without losing their characteristic non-linearities. The resulting equation is used to numerically study the gravitational potential of a cosmological perturbation and applied to a simple galactic model with an exponentially decreasing baryonic matter distribution. The non-relativistic limit of GR in a R-W space-time produces a generalised Poisson equation for the gravitational potential which is non-linear, parabolic and heat-like. It is shown how its non-linearities generate an effective "dark matter" distribution caused by both cosmological expansion and the dynamics of the perturbation’s gravitational potential. It is also shown how this dynamical effect gets completely lost during a linearisation of Einstein’s equations. The equation is then used to successfully fit real galactic rotation curves numerically using a matter distribution following the shape of a simple S\’ersic luminosity profile, common to most galaxies, thus without recourse to dark matter. A relation for the dark to luminous matter ratio is found, explaining the domination of dark matter in low-mass galaxies. A few rotation curves with a faster than Newtonian decrease are also presented and successfully fitted, opening the way to a new possible interpretation of these phenomena in terms of an effective "anti-gravitational" dark matter distribution, purely geometrical in origin.

Existence of wormholes with a barotropic equation of state

This paper examines the effect of the linear barotropic equation of state $p=\omega\rho$ on the existence or theoretical construction of traversable wormholes. If either the energy density or the closely related shape function is known, then the resulting redshift function $\Phi$ will almost always lead to an event horizon. Specifying the redshift function avoids this problem but only by relinquishing some of the control over the physics. Moving to a cosmological setting, we assume that $\Phi$ is such that $e^{2\Phi}=[(r+a)/b_0]^l$, $a\ge 0$, based on the existence of galactic rotation curves. Here the wormhole structure can only be maintained if $\omega<-1$. This condition is independent of $l$. The scope of this model can therefore be extended to zero and negative $l$ by taking $l$ to be a convenient parameter that is not necessarily related to the tangential velocity. The main reason is that if $a=0$, then the two special cases $l=0$ and $l<0$ correspond to the only exactly solvable models for wormholes supported by phantom energy, while simultaneously avoiding an event horizon, thereby providing an additional motivation for the form of the redshift function. A final topic is a possible unification of these cases by means of teleparallel gravity.

The Luminous Convolution Model as an alternative to dark matter in spiral galaxies

The Luminous Convolution Model (LCM) demonstrates that it is possible to predict the rotation curves of spiral galaxies directly from estimates of the luminous matter. We consider two frame-dependent effects on the light observed from other galaxies: relative velocity and relative curvature. With one free parameter, we predict the rotation curves of twenty-three (23) galaxies represented in forty-two (42) data sets. Relative curvature effects rely upon knowledge of both the gravitational potential from luminous mass of the emitting galaxy and the receiving galaxy, and so each emitter galaxy is compared to four (4) different Milky Way luminous mass models. On average in this sample, the LCM is more successful than either dark matter or modified gravity models in fitting the observed rotation curve data. Implications of LCM constraints on populations synthesis modeling are discussed in this paper. This paper substantially expands the results in arXiv:1309.7370.

Scalar Field Dark Matter mass model and evolution of rotation curves for Lsb galaxies

We study the evolution of gas rotation curves within the scalar field dark matter (SFDM) model. In this model the galactic haloes are astronomical Bose-Einstein Condensate drops of scalar field. These haloes are characterized by a constant-density core and are consistent with observed rotation curves of dark matter dominated galaxies, a missing feature in CDM haloes resulting from DM-only simulations. We add the baryonic component to the SFDM haloes and simulate the evolution of the dark matter tracer in a set of grid-based hydrodynamic simulations aimed to analyse the evolution of the rotation curves and the gas density distribution in the case of dark matter dominated galaxies. Previous works had found that when considering an exact analytic solution for a static SF configuration, the free parameters of the model allows for good fits to the rotation curves, we confirm that in our simulations but now taking into account the evolution of the baryonic component in a static dark matter and stellar disk potential. Including live gas is a step forward from the previous work using SFDM, as for example, the rotation velocity of the gas is not always exactly equal to the circular velocity of a test particle on a circular orbit. Contrasting with the data the cored mass model presented here is preferred instead of a cuspy one.

Astrophysical Tests of Modified Gravity: Stellar and Gaseous Rotation Curves in Dwarf Galaxies

Chameleon theories of gravity predict that the gaseous component of isolated dwarf galaxies rotates with a faster velocity than the stellar component. In this paper, we exploit this effect to obtain new constraints on the model parameters using the measured rotation curves of six low surface brightness galaxies. For $f(R)$ theories, we rule out values of $f_{R0}>10^{-6}$. For more general theories, we find that the constraints from Cepheid variable stars are currently more competitive than the bounds we obtain here but we are able to rule out self-screening parameters $\chi_c>10^{-6}$ for fifth-force strengths (coupling of the scalar to matter) as low as $0.05$ the Newtonian force. This region of parameter space has hitherto been inaccessible to astrophysical probes. We discuss the future prospects for improving these bounds.

Generalized curvature-matter couplings in modified gravity [Replacement]

In this work, we review a plethora of modified theories of gravity with generalized curvature-matter couplings. The explicit nonminimal couplings, for instance, between an arbitrary function of the scalar curvature $R$ and the Lagrangian density of matter, induces a non-vanishing covariant derivative of the energy-momentum tensor, implying non-geodesic motion and consequently leads to the appearance of an extra force. Applied to the cosmological context, these curvature-matter couplings lead to interesting phenomenology, where one can obtain a unified description of the cosmological epochs. We also consider the possibility that the behavior of the galactic flat rotation curves can be explained in the framework of the curvature-matter coupling models, where the extra-terms in the gravitational field equations modify the equations of motion of test particles, and induce a supplementary gravitational interaction. In addition to this, these models are extremely useful for describing dark energy-dark matter interactions, and for explaining the late-time cosmic acceleration.

Generalized curvature-matter couplings in modified gravity [Cross-Listing]

In this work, we review a plethora of modified theories of gravity with generalized curvature-matter couplings. The explicit nonminimal couplings, for instance, between an arbitrary function of the scalar curvature $R$ and the Lagrangian density of matter, induces a non-vanishing covariant derivative of the energy-momentum tensor, implying non-geodesic motion and consequently leads to the appearance of an extra force. Applied to the cosmological context, these curvature-matter couplings lead to interesting phenomenology, where one can obtain a unified description of the cosmological epochs. We also consider the possibility that the behavior of the galactic flat rotation curves can be explained in the framework of the curvature-matter coupling models, where the extra-terms in the gravitational field equations modify the equations of motion of test particles, and induce a supplementary gravitational interaction. In addition to this, these models are extremely useful for describing dark energy-dark matter interactions, and for explaining the late-time cosmic acceleration.

Generalized curvature-matter couplings in modified gravity [Cross-Listing]

In this work, we review a plethora of modified theories of gravity with generalized curvature-matter couplings. The explicit nonminimal couplings, for instance, between an arbitrary function of the scalar curvature $R$ and the Lagrangian density of matter, induces a non-vanishing covariant derivative of the energy-momentum tensor, implying non-geodesic motion and consequently leads to the appearance of an extra force. Applied to the cosmological context, these curvature-matter couplings lead to interesting phenomenology, where one can obtain a unified description of the cosmological epochs. We also consider the possibility that the behavior of the galactic flat rotation curves can be explained in the framework of the curvature-matter coupling models, where the extra-terms in the gravitational field equations modify the equations of motion of test particles, and induce a supplementary gravitational interaction. In addition to this, these models are extremely useful for describing dark energy-dark matter interactions, and for explaining the late-time cosmic acceleration.

Generalized curvature-matter couplings in modified gravity [Replacement]

In this work, we review a plethora of modified theories of gravity with generalized curvature-matter couplings. The explicit nonminimal couplings, for instance, between an arbitrary function of the scalar curvature $R$ and the Lagrangian density of matter, induces a non-vanishing covariant derivative of the energy-momentum tensor, implying non-geodesic motion and consequently leads to the appearance of an extra force. Applied to the cosmological context, these curvature-matter couplings lead to interesting phenomenology, where one can obtain a unified description of the cosmological epochs. We also consider the possibility that the behavior of the galactic flat rotation curves can be explained in the framework of the curvature-matter coupling models, where the extra-terms in the gravitational field equations modify the equations of motion of test particles, and induce a supplementary gravitational interaction. In addition to this, these models are extremely useful for describing dark energy-dark matter interactions, and for explaining the late-time cosmic acceleration.

Galactic space-times in modified theories of gravity [Cross-Listing]

We study Bertrand space-times (BSTs), which have been proposed as viable models of space-times seeded by galactic dark matter, in modified theories of gravity. We first critically examine the issue of galactic rotation curves in General Relativity, and establish the usefulness of BSTs to fit experimental data in this context. We then study BSTs in metric $f(R)$ gravity and in Brans-Dicke theories. For the former, the nature of the Newtonian potential is established, and we also compute the effective equation of state and show that it can provide good fits to some recent experimental results. For the latter, we calculate the Brans-Dicke scalar analytically in some limits and numerically in general, and find interesting constraints on the parameters of the theory. Our results provide evidence for the physical nature of Bertrand space-times in modified theories of gravity.

Galactic space-times in modified theories of gravity [Cross-Listing]

We study Bertrand space-times (BSTs), which have been proposed as viable models of space-times seeded by galactic dark matter, in modified theories of gravity. We first critically examine the issue of galactic rotation curves in General Relativity, and establish the usefulness of BSTs to fit experimental data in this context. We then study BSTs in metric $f(R)$ gravity and in Brans-Dicke theories. For the former, the nature of the Newtonian potential is established, and we also compute the effective equation of state and show that it can provide good fits to some recent experimental results. For the latter, we calculate the Brans-Dicke scalar analytically in some limits and numerically in general, and find interesting constraints on the parameters of the theory. Our results provide evidence for the physical nature of Bertrand space-times in modified theories of gravity.

On the rotation curves for axially symmetric disk solutions of the Vlasov-Poisson system [Replacement]

A large class of flat axially symmetric solutions to the Vlasov-Poisson system is constructed with the property that the corresponding rotation curves are approximately flat, slightly decreasing or slightly increasing. The rotation curves are compared with measurements from real galaxies and satisfactory agreement is obtained. These facts raise the question whether the observed rotation curves for disk galaxies may be explained without introducing dark matter. Furthermore, it is shown that for the ansatz we consider stars on circular orbits do not exist in the neighborhood of the boundary of the steady state.

On the rotation curves for axially symmetric disk solutions of the Vlasov-Poisson system

A large class of flat axially symmetric solutions to the Vlasov-Poisson system is constructed with the property that the corresponding rotation curves are approximately flat, slightly decreasing or slightly increasing. The rotation curves are compared with measurements from real galaxies and satisfactory agreement is obtained. These facts raise the question whether the observed rotation curves for disk galaxies may be explained without introducing dark matter. Furthermore, it is shown that for the ansatz we consider stars on circular orbits do not exist in the neighborhood of the boundary of the steady state.

On the rotation curves for axially symmetric disk solutions of the Vlasov-Poisson system [Replacement]

A large class of flat axially symmetric solutions to the Vlasov-Poisson system is constructed with the property that the corresponding rotation curves are approximately flat, slightly decreasing or slightly increasing. The rotation curves are compared with measurements from real galaxies and satisfactory agreement is obtained. These facts raise the question whether the observed rotation curves for disk galaxies may be explained without introducing dark matter. Furthermore, it is shown that for the ansatz we consider stars on circular orbits do not exist in the neighborhood of the boundary of the steady state.

VADER: A Flexible, Robust, Open-Source Code for Simulating Viscous Thin Accretion Disks

The evolution of thin axisymmetric viscous accretion disks is a classic problem in astrophysics. While such models provide only approximations to the true processes of instability-driven mass and angular momentum transport, their simplicity makes them invaluable tools for both semi-analytic modeling and simulations of long-term evolution where two- or three-dimensional calculations are too computationally costly. Despite the utility of these models, there is no publicly-available framework for simulating them. Here we describe a highly flexible, general numerical method for simulating viscous thin disks with arbitrary rotation curves, viscosities, boundary conditions, grid spacings, equations of state, and rates of gain or loss of mass (e.g., through winds) and energy (e.g., through radiation). Our method is based on a conservative, finite-volume, second-order accurate discretization of the equations, which we solve using an unconditionally-stable implicit scheme. We implement Anderson acceleration to speed convergence of the scheme, and show that this leads to factor of ~5 speed gains over non-accelerated methods in realistic problems. We have implemented our method in the new code Viscous Accretion Disk Evolution Resource (VADER), which is freely available for download from https://bitbucket.org/krumholz/vader/ under the terms of the GNU General Public License.

VADER: A Flexible, Robust, Open-Source Code for Simulating Viscous Thin Accretion Disks [Replacement]

The evolution of thin axisymmetric viscous accretion disks is a classic problem in astrophysics. While models based on this simplified geometry provide only approximations to the true processes of instability-driven mass and angular momentum transport, their simplicity makes them invaluable tools for both semi-analytic modeling and simulations of long-term evolution where two- or three-dimensional calculations are too computationally costly. Despite the utility of these models, the only publicly-available frameworks for simulating them are rather specialized and non-general. Here we describe a highly flexible, general numerical method for simulating viscous thin disks with arbitrary rotation curves, viscosities, boundary conditions, grid spacings, equations of state, and rates of gain or loss of mass (e.g., through winds) and energy (e.g., through radiation). Our method is based on a conservative, finite-volume, second-order accurate discretization of the equations, which we solve using an unconditionally-stable implicit scheme. We implement Anderson acceleration to speed convergence of the scheme, and show that this leads to factor of $\sim 5$ speed gains over non-accelerated methods in realistic problems, though the amount of speedup is highly problem-dependent. We have implemented our method in the new code Viscous Accretion Disk Evolution Resource (VADER), which is freely available for download from https://bitbucket.org/krumholz/vader/ under the terms of the GNU General Public License.

Spiral Galaxies - classical description of spiral arms and rotational velocity pattern - toy model [Replacement]

We propose an explanation of features of spiral galaxies: spiral arms and observed flat rotation curves, without the presence of an exotic form of matter. The formalism is based on Boltzmanns transport equation for the collisional matter and the very-low-velocity post-Newtonian approximation of the general relativity equations expressed in the Maxwell-like form. The Maxwell-like formulation provides the base for the explanation of the above phenomena in the language of dynamically created gravitoelectromagnetic fields by the movement of mass streams in the plane of the galaxy disc. In the model we use radical simplifications expressed as neglect of the gravitational interaction between neighbors and approximation of the incompressible mass flow. In this frame we show that if the galaxy disc is fuelled constantly by collisional mass carriers, then the amplification of the gravitomagnetic field can be large enough to create the rotational velocity pattern and spiral arms. In this framework the collisional part of the mass gas in the galaxy disc plane, i.e. molecules and atoms, is responsible for the creation of the gravitomagnetic field. According to the model the spiral pattern of arms is static and determined by a direction of the mass flow. The model reproduces qualitatively the observed spiral arms and reproduces well the shape of the rotational velocity pattern. As an example of the usability of the proposed mechanism, we reproduce qualitatively the above features for the IC 342 and NGC 4321 galaxies.

Spiral Galaxies - classical description of spiral arms and rotational velocity pattern - toy model

We propose an explanation of features of spiral galaxies: spiral arms and observed flat rotation curves, without the presence of an exotic form of matter. The formalism is based on Boltzmanns transport equation for the collisional matter and the very-low-velocity post-Newtonian approximation of the general relativity equations expressed in the Maxwell-like form. The Maxwell-like formulation provides the base for the explanation of the above phenomena in the language of dynamically created gravitoelectromagnetic fields by the movement of mass streams in the plane of the galaxy disc. In the model we use radical simplifications expressed as neglect of the gravitational interaction between neighbors and approximation of the incompressible mass flow. In this frame we show that if the galaxy is fuelled constantly by mass from an external gas reservoir, then the amplification of the gravitomagnetic field can be large enough to create the rotational velocity pattern and spiral arms without the necessity of introducing exotic matter or any other new phenomena. In this framework the collisional part of the mass gas in the galaxy disc plane, i.e. molecules and atoms, is responsible for the creation of the gravitomagnetic field. According to the model the spiral pattern of arms is static and determines the mass flow, identified by the galaxy spiral structure, which moves towards the galaxy center from the peripheries of the galaxy disc. The model reproduces qualitatively the observed spiral arms and reproduces well the shape of the rotational velocity pattern. Prediction for the radial velocity is below 10 $km\mbox{ }s^{-1}$, in agreement with observations. As an example of the usability of the proposed mechanism, we reproduce qualitatively the above features for the IC 342 and NGC 4321 galaxies.

Extremely Flat Haloes and the Shape of the Galaxy

We present a set of highly flattened galaxy models with asymptotically constant rotation curves. The mass density in the equatorial plane falls like (distance)$^{-1}$ at large radii. Although the inner equidensity contours may be spherical, oblate or prolate, the outer parts are always severely flattened. The elongated shape is supported by rotation or tangential velocity anisotropy. The models are thickened Mestel discs, and form a previously undiscovered part of the Miyamoto & Nagai sequence of flattened galaxies. The properties of the models — axis ratios, velocity dispersions, streaming velocities and distribution functions — are all discussed in some detail. We pose the question: are extremely flattened or disk-like haloes possible for the Milky Way galaxy? This has never been examined before, as very flattened halo models were not available. We fit the rotation curve and the vertical kinematics of disc stars in the solar neighbourhood to constrain the overall shape of the Galaxy. Denoting the ratio of polar axis to major axis by $q$, we show that models with $q\lesssim 0.57$ cannot simultaneously reproduce the in-plane and out-of-plane constraints. The kinematics of the Sagittarius galaxy also strongly disfavour models with high flattening, as the orbital plane precession is too great and the height reached above the Galactic plane is too small. At least for our Galaxy, the dark halo cannot be flatter than E4 (or axis ratio $q \sim 0.57$) at the Solar circle. Models in which the dark matter is accounted for by a massive baryonic disc or by decaying neutrinos are therefore ruled out by constraints from the rotation curve and the vertical kinematics.

Newtonian explanation of galaxy rotation curves based on distribution of baryonic matter

Circular velocities of stars and gas in galaxies generally do not decline in accordance with widely expected Keplerian fall-off in velocities further from the galactic nucleus. Two main groups of theories were proposed to explain the supposed discrepancy–first, the most commonly accepted, is the suggestion of the existence of large non-baryonic dark matter halo, and, second are theories advocating some modification to the law of gravity. So far however, there is no empirical evidence for either dark matter or modified gravity. Here we show that a broad range of galaxy rotation curves can be explained solely in accordance with Newton’s law of gravity by modeling the distribution of baryonic matter in a galaxy. We demonstrate that the expectation of Keplerian fall-off is incorrect, and that a large number of likely galaxy mass distribution profiles should in fact produce flat or accelerating rotation curves similar to those observed in reality. We further support our theoretical findings with the model fit of 47 rotation curves of real galaxies, representing a broad range of galactic types and sizes, and achieving correlation of expected and observed velocities of over 0.995 for all cases. Our results make theories of exotic dark matter or modified gravity unnecessary for the explanation of galaxy rotation curves.

Constraints on Bose-Einstein-condensed Axion Dark Matter from The HI Nearby Galaxy Survey data [Replacement]

One of the leading candidates for dark matter is axion or axion-like particle in a form of Bose-Einstein condensate (BEC). In this paper, we present an analysis of 17 high-resolution galactic rotation curves from "The H{\footnotesize I} Nearby Galaxy Survey (THINGS)" data [F. Walter et al., Astron. J. 136, 2563 (2008)] in the context of the axionic Bose-Einstein condensed dark matter model. Assuming a repulsive two-body interaction, we solve the non-relativistic Gross-Pitaevskii equation for $N$ gravitationally trapped bosons in the Thomas-Fermi approximation. We obtain the maximum possible radius $R$ and the mass profile $M(r)$ of a dilute axionic Bose-Einstein condensed gas cloud. A standard least-$\chi^2$ method is employed to find the best-fit values of the total mass $M$ of the axion BEC and its radius $R$. The local mass density of BEC axion dark-matter is $\rho_{a}\simeq 0.02~{\rm GeV/cm}^3$, which agrees with that presented by Beck [C. Beck, Phys. Rev. Lett. 111, 231801 (2013)]. The axion mass $m_a$ we obtain depends not only on the best-fit value of $R$ but also on the $s$-wave scattering length $a$ ($m_a \propto a^{1/3}$). The transition temperature $T_a$ of axion BEC on galactic scales is also estimated. Comparing the calculated $T_a$ with the ambient temperature of galaxies and galaxy clusters implies that $a\sim 10^{-3}$ fm. The corresponding axion mass is $m_a\simeq 0.58$ meV. We compare our results with others.

Constraints on Bose-Einstein-condensed Axion Dark Matter from The HI Nearby Galaxy Survey data

One of the leading candidates for dark matter is axion or axion-like particle in a form of Bose-Einstein condensate (BEC). In this paper, we present an analysis of 17 high-resolution galactic rotation curves from "The H{\footnotesize I} Nearby Galaxy Survey (THINGS)" data [F. Walter et al., Astron. J. 136, 2563 (2008)] in the context of the axionic Bose-Einstein condensed dark matter model. Assuming a repulsive two-body interaction, we solve the non-relativistic Gross-Pitaevskii equation for $N$ gravitationally trapped bosons in the Thomas-Fermi approximation. We obtain the maximum possible radius $R$ and the mass profile $M(r)$ of a dilute axionic Bose-Einstein condensed gas cloud. A standard least-$\chi^2$ method is employed to find the best-fit values of the total mass $M$ of the axion BEC and its radius $R$. The local mass density of BEC axion dark-matter is $\rho_{a}\simeq 0.02~{\rm GeV/cm}^3$, which agrees with that presented by Beck [C. Beck, Phys. Rev. Lett. 111, 231801 (2013)]. The axion mass $m_a$ we obtain depends not only on the best-fit value of $R$ but also on the $s$-wave scattering length $a$ ($m_a \propto a^{1/3}$). The transition temperature $T_a$ of axion BEC on galactic scales is also estimated. Comparing the calculated $T_a$ with the ambient temperature of galaxies and galaxy clusters implies that $a\sim 10^{-3}$ fm. The corresponding axion mass is $m_a\simeq 0.58$ meV. We compare our results with others.

The H$\alpha$ kinematics of interacting galaxies in 12 compact groups

We present new Fabry-Perot observations for a sample of 42 galaxies located in twelve compact groups of galaxies: HCG 1, HCG 14, HCG 25, HCG 44, HCG 53, HCG 57, HCG 61, HCG 69, HCG 93, VV 304, LGG 455 and Arp 314. From the 42 observed galaxies, a total of 26 objects are spiral galaxies, which range from Sa to Im morphological types. The remaining 16 objects are E, S0 and S0a galaxies. Using these observations, we have derived velocity maps, monochromatic and velocity dispersion maps for 24 galaxies, where 18 are spiral, three are S0a, two are S0 and one is an Im galaxy. From the 24 velocity fields obtained, we could derive rotation curves for 15 galaxies; only two of them exhibit rotation curves without any clear signature of interactions. Based on kinematic information, we have evaluated the evolutionary stage of the different groups of the current sample. We identify groups that range from having no H$\alpha$ emission to displaying an extremely complex kinematics, where their members display strongly perturbed velocity fields and rotation curves. In the case of galaxies with no H$\alpha$ emission, we suggest that past galaxy interactions removed their gaseous components, thereby quenching their star formation. However, we can not discard that the lack of H$\alpha$ emission is linked with the detection limit for some of our observations.

Bose-Einstein Condensate Dark Matter Halos confronted with galactic observations

We present a comparative confrontation of both the Bose-Einstein Condensate (BEC) and the Navarro-Frenk-White (NFW) dark halo models with galactic rotation curves and velocity dispersion data. We conclude that the BEC model fits better the dwarf galaxy dark matter distribution, but suffers from sharp cut-off in larger galaxies, where the NFW model performs better. In more detail, we employ 6 High Surface Brightness (HSB), 6 Low Surface Brightness (LSB) and 7 dwarf galaxies with rotation curves falling into two classes, based on their shapes. In the first class the rotational velocities increase with radius over the whole observed range, the BEC and NFW models giving comparable fits for both HSB and LSB galaxies, while significantly improving over the NFW fit for dwarf galaxies. This improvement is due to the central density cusp avoidance property of the BEC model. The rotational velocity of HSB and LSB galaxies falling into the second class exhibit long flat plateaus, resulting in a better fit of the NFW model for HSB galaxies, and comparable fits for LSB galaxies. The weaker performance of the BEC model for the HSB type II galaxies is due to the BEC density profiles dropping rapidly to zero outside a nearly constant density core. Finally we confront both models with the projected velocity dispersion profiles of 6 Virgo cluster galaxies, which after a steep rising, remain flat over the sampled region. The two models gave comparable combined $\tilde{\chi}_{\min}^{2}$ values for these galaxies but both model fits remained outside the 3$\sigma$ confidence level, pointing out the need for a better modelling of the velocity dispersion of galaxies that both the BEC and NFW models could provide.

 

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