29 research outputs found
A constant dark matter halo surface density in galaxies
We confirm and extend the recent finding that the central surface density
r_0*rho_0 galaxy dark matter halos, where r_0 and rho_0 are the halo core
radius and central density, is nearly constant and independent of galaxy
luminosity. Based on the co-added rotation curves of about 1000 spiral
galaxies, mass models of individual dwarf irregular and spiral galaxies of late
and early types with high-quality rotation curves and, galaxy-galaxy weak
lensing signals from a sample of spiral and elliptical galaxies, we find that
log(r_0*rho_0) = 2.15 +- 0.2, in units of log(Msol/pc^2). We also show that the
observed kinematics of Local Group dwarf spheroidal galaxies are consistent
with this value. Our results are obtained for galactic systems spanning over 14
magnitudes, belonging to different Hubble Types, and whose mass profiles have
been determined by several independent methods. In the same objects, the
approximate constancy of rho_0*r_0 is in sharp contrast to the systematical
variations, by several orders of magnitude, of galaxy properties, including
rho_0 and central stellar surface density.Comment: Accepted for publication in MNRAS. 9 pages, 4 figure
Analysis of Rotation Curves in the framework of R^n gravity
We present an analysis of a devised sample of Rotation Curves (RCs), aimed at
checking the consequences of a modified f(R) gravity on galactic scales.
Originally motivated by the the dark energy mystery, this theory may serve as a
possibility of explaining the observed non-Keplerian profiles of galactic RCs
in terms of a break-down of the Einstein General Relativity. We show that in
general the power-law f(R) version could fit well the observations with
reasonable values for the mass model parameters, encouraging further
investigation on R^n gravity from both observational and theoretical points of
view.Comment: Accepted for publication on Mon.Not.Roy.Astron.So
Probing the dark matter issue in f(R)-gravity via gravitational lensing
For a general class of analytic f(R)-gravity theories, we discuss the weak
field limit in view of gravitational lensing. Though an additional Yukawa term
in the gravitational potential modifies dynamics with respect to the standard
Newtonian limit of General Relativity, the motion of massless particles results
unaffected thanks to suitable cancellations in the post-Newtonian limit. Thus,
all the lensing observables are equal to the ones known from General
Relativity. Since f(R)-gravity is claimed, among other things, to be a possible
solution to overcome for the need of dark matter in virialized systems, we
discuss the impact of our results on the dynamical and gravitational lensing
analyses. In this framework, dynamics could, in principle, be able to reproduce
the astrophysical observations without recurring to dark matter, but in the
case of gravitational lensing we find that dark matter is an unavoidable
ingredient. Another important implication is that gravitational lensing, in the
post-Newtonian limit, is not able to constrain these extended theories, since
their predictions do not differ from General Relativity.Comment: 7 pages, accepted for publication in EPJ
Dark energy and dust matter phases from an exact -cosmology model
We show that dust matter-dark energy combined phases can be achieved by the
exact solution derived from a power law cosmological model. This example
answers the query by which a dust-dominated decelerated phase, before
dark-energy accelerated phase, is needed in order to form large scale
structures.Comment: 6 pages, 5 figures. to appear in Phys. Lett.
The dark matter density at the Sun's location
Aims: We derive the value of the dark matter density at the Sun's location (Ïâ) without fully modeling the mass distribution in the Galaxy. Methods: The proposed method relies on the local equation of centrifugal equilibrium and is independent of i) the shape of the dark matter density profile, ii) knowledge of the rotation curve from the galaxy center out to the virial radius, and iii) the uncertainties and the non-uniqueness of the bulge/disk/dark halo mass decomposition. Results: The result can be obtained in analytic form, and it explicitly includes the dependence on the relevant observational quantities and takes their uncertainties into account. By adopting the reference, state-of-the-art values for these, we find Ï â = 0.43(11)(10) GeV/cm3, where the quoted uncertainties are respectively due to the uncertainty in the slope of the circular-velocity at the Sun location and the ratio between this radius and the length scale of the stellar exponential thin disk. Conclusions: We obtained a reliable estimate of Ïâ, that, in addition has the merit of being ready to take any future change/improvement into account in the measures of the observational quantities it depends on
Dark Matter Universal Properties in Galaxies
In the past years a wealth of observations has unraveled the structural
properties of dark and luminous mass distribution in galaxies, a benchmark for
understanding dark matter and the process of galaxy formation. The study of the
kinematics of over thousand spirals has evidenced a dark-luminous matter
coupling and the presence of a series of scaling laws, pictured by the
Universal Rotation Curve paradigm, an intriguing observational scenario not
easily explained by present theories of galaxy formation.Comment: Proceedings of the VI International Workshop on the Dark side of the
Universe. June 01-06, 2010. Le\'on, M\'exic
f(R) theories
Over the past decade, f(R) theories have been extensively studied as one of
the simplest modifications to General Relativity. In this article we review
various applications of f(R) theories to cosmology and gravity - such as
inflation, dark energy, local gravity constraints, cosmological perturbations,
and spherically symmetric solutions in weak and strong gravitational
backgrounds. We present a number of ways to distinguish those theories from
General Relativity observationally and experimentally. We also discuss the
extension to other modified gravity theories such as Brans-Dicke theory and
Gauss-Bonnet gravity, and address models that can satisfy both cosmological and
local gravity constraints.Comment: 156 pages, 14 figures, Invited review article in Living Reviews in
Relativity, Published version, Comments are welcom