arXiv subjects

V. F. Cardone

Publications and source records attributed to V. F. Cardone.

157 records · Page 9Linked to original sources

arXiv2004

f(R) theories of gravity in Palatini approach matched with observations

We investigate the viability of f(R) theories in the framework of the Palatini approach as solutions to the problem of the observed accelerated expansion of the universe. Two physically motivated popular choices for f(R) are considered: power law, f(R) = βR^n, and logarithmic, f(R) = α\ln{R}. Under the Palatini approach, both Lagrangians give rise to cosmological models comprising only standard matter and undergoing a present phase of accelerated expansion. We use the Hubble diagram of type Ia Supernovae and the data on the gas mass fraction in relaxed galaxy clusters to see whether these models are able to reproduce what is observed and to constrain their parameters. It turns out that they are indeed able to fit the data with values of the Hubble constant and of the matter density parameter in agreement with some model independent estimates, but the today deceleration parameter is higher than what is measured in the concordance LambdaCDM model.

astro-ph↗

arXiv2004

Some astrophysical implications of dark matter and gas profiles in a new galaxy clusters model

The structure of the dark matter and the thermodynamical status of the hot gas in galaxy clusters is an interesting and widely discussed topic in modern astrophysics. Recently, Rasia et al. (2004) have proposed a new dynamical model for the mass density profile of clusters of galaxies as a result of a set of high resolution hydrodynamical simulations of structure formation. We investigate the lensing properties of this model evaluating the deflection angle, the lensing potential and the amplification of the images. We reserve particular attention to the structure and position of the critical curves in order to see whether this model is able to produce radial and tangential arcs. To this aim, we also investigate the effect of taking into account the brightest cluster galaxy in the lensing potential and the deviations from spherical symmetry mimicked by an external shear. We also analyze the implication of the gas density and temperature profiles of the Rasia et al.(2004) model on the properties of the X - ray emission and the comptonization parameter that determines the CMBR temperature decrement due to the Sunyaev - Zel'dovich effect.

astro-ph↗

arXiv2004

Lens modelling and H_0 estimate in quadruply lensed systems

We present a numerical method to estimate the lensing parameters and the Hubble constant H_0 from quadruply imaged gravitational lens systems. The lens galaxy is modeled using both separable deflection potentials and constant mass-to-light ratio profiles, while possible external perturbations have been taken into account introducing an external shear. The model parameters are recovered inverting the lens and the time delay ratio equations and imposing a set of physically motivated selection criteria. We investigate correlations among the model parameters and the Hubble constant. Finally, we apply the codes to the real lensed quasars PG 1115+080 and RX J0911+0551, and combine the results from these two systems to get H_0 = 56 +/- 23 km/(s Mpc). In addition, we are able to fit to the single systems a general elliptical potential with a non fixed angular part, and then we model the two lens systems with the same potential and a shared H_0: in this last case we estimate H_0=49_(-11)^(+6) km/(s Mpc).

astro-ph↗

arXiv2004

Can higher order curvature theories explain rotation curves of galaxies?

Higher order curvature gravity has recently received a lot of attention due to the fact that it gives rise to cosmological models which seem capable of solving dark energy and quintessence issues without using "ad hoc" scalar fields. In this letter, a gravitational potential is obtained which differs from the Newtonian one because of a repulsive correction increasing with distance. We evaluate the rotation curve of our Galaxy and compare it with the observed data in order both to test the viability of these theories and to estimate the scalelength of the correction. It is remarkable that the Milky Way rotation curve is well fitted without the need of any dark matter halo and a similar result tentatively holds also for other galaxies.

gr-qc↗

arXiv2004

Unified dark energy models : a phenomenological approach

A phenomenological approach is proposed to the problem of universe accelerated expansion and of the dark energy nature. A general class of models is introduced whose energy density depends on the redshift $z$ in such a way that a smooth transition among the three main phases of the universe evolution (radiation era, matter domination, asymptotical de Sitter state) is naturally achieved. We use the estimated age of the universe, the Hubble diagram of Type Ia Supernovae and the angular size - redshift relation for compact and ultracompact radio structures to test whether the model is in agreement with astrophysical observation and to constrain its main parameters. Although phenomenologically motivated, the model may be straightforwardly interpreted as a two fluids scenario in which the quintessence is generated by a suitably chosen scalar field potential. On the other hand, the same model may also be read in the context of unified dark energy models or in the framework of modified Friedmann equation theories.

astro-ph↗

arXiv2003

Curvature quintessence matched with observational data

Quintessence issues can be achieved by taking into account higher order curvature invariants into the effective action of gravitational field. Such an approach is naturally related to fundamental theories of quantum gravity which predict higher order terms in loop expansion of quantum fields in curved space-times. In this framework, we obtain a class of cosmological solutions which are fitted against cosmological data. We reproduce encouraging results able to fit high redshift supernovae and WMAP observations. The age of the universe and other cosmological parameters are discussed in this context.

astro-ph↗

arXiv2003

Gravitomagnetic corrections to the lensing deflection angle for spiral galaxy models

We investigate the effects of the gravitomagnetic corrections to the usual gravitational lens quantities for a specific lensing mass distribution modelled after spiral galaxies. An exponential disk is embedded into two different spherical halo models where disk and haloes parameters are fixed according to the observed mass to light ratios, galaxy magnitudes and rotation curves. The general expressions for the lensing deflection angle are given also taking into account the orientation of the galaxy disk plane with respect to the lens plane. It is found that the gravitomagnetic term changes the deflection angle by a typical amount of the order of ten microarcseconds.

astro-ph↗

arXiv2003

Pixel lensing observations towards globular clusters

It has been suggested that a monitoring program employing the pixel lensing method to search for microlensing events towards galactic globular clusters may increase the statistics and discriminate among different halo models. Stimulated by this proposal, we evaluate an upper limit to the pixel lensing event rate for such a survey. Four different dark halo models have been considered changing both the flattening and the slope of the mass density profile. The lenses mass function has been modelled as a homogenous power - law for $μ\in (μ_l, μ_u)$ and both the mass limits and the slope of the mass function have been varied to investigate their effect on the rate. The target globular clusters have been selected in order to minimize the disk contribution to the event rate. We find that a pixel lensing survey towards globular clusters is unable to discriminate among different halo models since the number of detectable events is too small to allow any reliable statistical analysis.

astro-ph↗

arXiv2002

Matching torsion Lambda - term with observations

Taking into account a torsion field gives rise to a negative pressure contribution in cosmological dynamics and then to an accelerated behaviour of Hubble fluid. The presence of torsion has the same effect of a Lambda - term. We obtain a general exact solution which well fits the data coming from high - redshift supernovae and Sunyaev - Zeldovich/X - ray method for the determination of the cosmological parameters. On the other hand, it is possible to obtain observational constraints on the amount of torsion density. A dust dominated Friedmann behaviour is recovered as soon as torsion effects are not relevant.

astro-ph↗

arXiv2001

A new method for the estimate of H_0 from quadruply imaged gravitational lens systems

We present a new method to estimate the Hubble constant H_0 from the measured time delays in quadruply imaged gravitational lens systems. We show how it is possible to get an estimate of H_0 without the need to completely reconstruct the lensing potential thus avoiding any a priori hypotheses on the expression of the galaxy lens model. Our method only needs to assume that the lens potential may be expressed as r^α F(θ), whatever the shape function F(θ) is, and it is thus able to fully explore the degeneracy in the mass models taking also into account the presence of an external shear. We test the method on simulated cases and show that it does work well in recovering the correct value of the slope αof the radial profile and of the Hubble constant H_0. Then, we apply the same method to the real quadruple lenses PG1115+080 and B1422+231 obtaining H_0 = 58_{-15}^{+17} km/s/Mpc (68% CL).

astro-ph↗

arXiv2001

Gravitational lensing potential reconstruction in quadruply imaged systems

We develop a semi - analytical method to reconstruct the lensing potential in quadruply imaged gravitational lens systems. Assuming that the potential belongs to a broad class of boxy non - elliptical models, we show how it is possible to write down a system of equations which can be numerically solved to recover the potential parameters directly from image positions and using physical constraints. We also describe a code developed to search for solutions of the system previously found and test it on simulated cases. Finally, we apply the method to the quadruple lens PG1115+080 which allows us to get also an estimate of the Hubble constant H_0 from the measured time delay as H_0 = 56_{-11}^{+17} km/s/Mpc.

astro-ph↗

arXiv2001

Constraints on the slope of the dark halo mass function by microlensing observables

We investigate the dark halo lens mass function (MF) for a wide class of spheroidal non singular isothermal models comparing observed and observable microlensing quantities for MACHO observations towards LMC and taking into account the detection efficiency. We evaluate the microlensing observable quantities, i.e. observable optical depth, number of events and mean duration, for models with homogenous power - law MF changing the upper and lower mass limits and the flattening of the dark halo. By applying the simple technique of the inverse problem method we are then able to get some interesting constraints on the slope $α$ of the MF and on the dark halo mass fraction f made out by MACHOs consistently with previous results.

astro-ph↗

arXiv1998

The inverse problem in microlensing: from the optical depth to the galaxy models parameters

We present in this paper a simple method to obtain informations on galaxy models parameters using the measured value of the microlensing optical depth. Assuming a 100 percent MACHO's dark halo, we ask the predicted optical depth for a given model to be the same as the observed one, in a given direction. Writing the optical depth in terms of the given halo model parameters and inverting this relation with respect to one of them, it is possible to get information on it, fixing under reasonable hypothesis the other parameters. This is what we call the "inverse problem in microlensing. We apply this technique to the class of power-law models with flat rotation curves, determining the range for the core radius R_c compatible with the constraints on the halo flattening q and the measures of the optical depth towards LMC. Next, we apply the same method to a simple triaxial model, evaluating the axial ratios.

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