arXiv subjects

C. Tortora

Publications and source records attributed to C. Tortora.

136 records · Page 8Linked to original sources

arXiv2009

Central mass-to-light ratios and dark matter fractions in early-type galaxies

Dynamical studies of local ETGs and the Fundamental Plane point to a strong dependence of M/L ratio on luminosity (and stellar mass) with a relation of the form $M/L \propto L^γ$. The "tilt" $γ$ may be caused by various factors, including stellar population properties, IMF, rotational support, luminosity profile non-homology and dark matter (DM) fraction. We evaluate the impact of all these factors using a large uniform dataset of local ETGs from Prugniel & Simien (1997). We take particular care in estimating the stellar masses, using a general star formation history, and comparing different population synthesis models. We find that the stellar M/L contributes little to the tilt. We estimate the total M/L using simple Jeans dynamical models, and find that adopting accurate luminosity profiles is important but does not remove the need for an additional tilt component, which we ascribe to DM. We survey trends of the DM fraction within one effective radius, finding it to be roughly constant for galaxies fainter than $M_B \sim -20.5$, and increasing with luminosity for the brighter galaxies; we detect no significant differences among S0s and fast- and slow-rotating ellipticals. We construct simplified cosmological mass models and find general consistency, where the DM transition point is caused by a change in the relation between luminosity and effective radius. A more refined model with varying galaxy star formation efficiency suggests a transition from total mass profiles (including DM) of faint galaxies distributed similarly to the light, to near-isothermal profiles for the bright galaxies. These conclusions are sensitive to various systematic uncertainties which we investigate in detail, but are consistent with the results of dynamics studies at larger radii.

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arXiv2009

AGN Jet-induced Feedback in Galaxies. II. Galaxy colours from a multicloud simulation

We study the feedback from an AGN on stellar formation within its host galaxy, mainly using one high resolution numerical simulation of the jet propagation within the interstellar medium of an early-type galaxy. In particular, we show that in a realistic simulation where the jet propagates into a two-phase ISM, star formation can initially be slightly enhanced and then, on timescales of few million years, rapidly quenched, as a consequence both of the high temperatures attained and of the reduction of cloud mass (mainly due to Kelvin-Helmholtz instabilities). We then introduce a model of (prevalently) {\em negative} AGN feedback, where an exponentially declining star formation is quenched, on a very short time scale, at a time t_AGN, due to AGN feedback. Using the Bruzual & Charlot (2003) population synthesis model and our star formation history, we predict galaxy colours from this model and match them to a sample of nearby early-type galaxies showing signs of recent episodes of star formation (Kaviraj et al. 2007). We find that the quantity t_gal - t_AGN, where t_gal is the galaxy age, is an excellent indicator of the presence of feedback processes, and peaks significantly around t_gal - t_AGN \approx 0.85 Gyr for our sample, consistent with feedback from recent energy injection by AGNs in relatively bright (M_{B} \lsim -19) and massive nearby early-type galaxies. Galaxies that have experienced this recent feedback show an enhancement of 3 magnitudes in NUV(GALEX)-g, with respect to the unperturbed, no-feedback evolution. Hence they can be easily identified in large combined near UV-optical surveys.

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arXiv2007

Strong Lensing, dark matter and H_0 estimate

Gravitational lensing represents a powerful tool to estimate the cosmological pa- rameters and the distribution of dark matter. I will describe the main observable quantities, concentrating on strong lensing, that manifests its effect through the formation of spectacular events, like multiple quasars, Einstein rings and arcs in clusters of galaxies. In events where a quasar is lensed by an intervening galaxy, it is possible to give an estimate of the Hubble constant H_0, by choosing a mass density model for the lens, thus allowing to constrain the dark matter content.

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arXiv2006

Dynamical and gravitational lensing properties of a new phenomenological model of elliptical galaxies

Recent observations of the line of sight velocity profile of elliptical galaxies have furnished controversial results with some works favouring the presence of a large amount of dark matter in the outer regions and others arguing in favour of no dark matter at all. In order to shed new light on this controversy, we propose here a new phenomenological description of the total mass profile of galaxies. Under the hypothesis of spherical symmetry, we assume a double power-law expression for the global M/L ratio Upsilon(r)= Upsilon_0(r/r_0) ^{alpha}(1+r/r_0)^{beta}. In particular, Upsilon propto r^{alpha} for r/r_0<<1 so that alpha=0 mimics a constant M/L ratio in the inner regions, while, for (r/r_0>>1), Upsilon propto r^{alpha+beta} thus showing that models with alpha+beta=0 have an asymptotically constant M/L ratio. A wide range of possibilities is obtained by varying the slope parameters in the range we determine on the basis of physical considerations. Choosing a general expression for the luminosity density profile j(r), we work out an effective galaxy model that accounts for all the phenomenology observed in real elliptical galaxies. We derive the main dynamics and lensing properties of such an effective model. We analyze a general class of models, able to take into account different dynamical trends. We are able to obtain analytical expressions for the main dynamical and lensing quantities. We show that constraining the values of alpha+beta makes it possible to analyze the problem of the dark matter in elliptical galaxies. Indeed, positive values of alpha+beta would be a strong evidence for dark matter. Finally we indicate possible future approaches in order to face the observational data, in particular using velocity dispersion profiles and lensed quasar events.

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arXiv2006

Accelerating universe in scalar tensor models - confrontation of theoretical predictions with observations

We consider scalar tensor theories of gravity assuming that the scalar field is non minimally coupled with gravity. We use this theory to study evolution of a flat homogeneous and isotropic universe. In this case the dynamical equations can be derived form a point like Lagrangian. We study the general properties of dynamics of this system and show that for a wide range of initial conditions such models lead in a natural way to an accelerated phase of expansion of the universe. Assuming that the point like Lagrangian admits a Noether symmetry we are able to explicitly solve the dynamical equations. We study one particular model and show that its predictions are compatible with observational data, namely the publicly available data on type Ia supernovae, the parameters of large scale structure determined by the 2-degree Field Galaxy Redshift Survey (2dFGRS), the measurements of cosmological distances with the Sunyaev-Zel'dovich effect and the rate of growth of density perturbations}{It turns out that this model have a very interesting feature of producing in a natural way an epoch of accelerated expansion. With an appropriate choice of parameters our model is fully compatible with several observed characteristics of the universe

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arXiv2005

Beyond the perfect fluid hypothesis for dark energy equation of state

Abandoning the perfect fluid hypothesis, we investigate here the possibility that the dark energy equation of state (EoS) $w$ is a nonlinear function of the energy density $ρ$. To this aim, we consider four different EoS describing classical fluids near thermodynamical critical points and discuss the main features of cosmological models made out of dust matter and a dark energy term with the given EoS. Each model is tested against the data on the dimensionless coordinate distance to Type Ia Supernovae and radio galaxies, the shift and the acoustic peak parameters and the positions of the first three peaks in the anisotropy spectrum of the comic microwave background radation. We propose a possible interpretation of each model in the framework of scalar field quintessence determining the shape of the self interaction potential $V(ϕ)$ that gives rise to each one of the considered thermodynamical EoS. As a general result, we demonstrate that replacing the perfect fluid EoS with more generar expressions gives both the possibility of successfully solving the problem of cosmic acceleration escaping the resort to phantom models.

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arXiv2005

Spherical galaxy models with power law logarithmic slope

We present a new family of spherically symmetric models for the luminous components of elliptical and spiral galaxies and their dark matter haloes. Our starting point is a general expression for the logarithmic slope $α(r) = d\logρ/d\log{r}$ from which most of the cuspy models yet available in literature may be derived. We then dedicate our attention to a particular set of models whose logarithmic slope is a power law function of the radius $r$ investigating in detail their dynamics assuming isotropy in the velocity space. While the basic properties (such as the density profile and the gravitational potential) may be expressed analytically, both the distribution function and the observable quantities (surface brightness and line of sight velocity dispersion) have to be evaluated numerically. We also consider the extension to anisotropic models trying two different parameterization. Since the model recently proposed by Navarro et al. (2004) as the best fit to their sample of numerically simulated haloes belongs to the family we present here, analytical approximations are given for the most useful quantities.

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arXiv2004

Two viable quintessence models of the Universe: confrontation of theoretical predictions with observational data

We use some of the recently released observational data to test the viability of two classes of minimally coupled scalar field models of quintessence with exponential potentials for which exact solutions of the Einstein equations are known. These models are very sturdy, depending on only one parameter - the Hubble constant. To compare predictions of our models with observations we concentrate on the following data: the power spectrum of the CMBR anisotropy as measured by WMAP, the publicly available data on type Ia supernovae, and the parameters of large scale structure determined by the 2-degree Field Galaxy Redshift Survey (2dFGRS). We use the WMAP data on the age of the universe and the Hubble constant to fix the free parameters in our models. We then show that the predictions of our models are consistent with the observed positions and relative heights of the first 3 peaks in the CMB power spectrum, with the energy density of dark energy as deduced from observations of distant type Ia supernovae, and with parameters of the large scale structure as determined by 2dFGRS, in particular with the average density of dark matter. Our models are also consistent with the results of the Sloan Digital Sky Survey (SDSS). Moreover, we investigate the evolution of matter density perturbations in our quintessential models, solve exactly the evolution equation for the density perturbations, and obtain an analytical expression for the growth index $f$. We verify that the approximate relation f ~ Omega_M^(alpha) also holds in our models.

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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.

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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).

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