Showing papers by "Giovanni Covone published in 2009"

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

Abstract: Dynamical studies of local elliptical galaxies and the Fundamental Plane point to a strong dependence of the total mass-to-light ratio (M/L) on luminosity with a relation of the form M/L ∝ L γ . The ‘tilt’ γ may be caused by various factors, including stellar population properties (metallicity, age and star formation history), initial mass function, rotational support, luminosity profile non-homology and dark matter (DM) fraction. We evaluate the impact of all these factors using a large uniform data set of local early-type galaxies from Prugniel & Simien. 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 ∼− 20.5, and increasing with luminosity for the brighter galaxies; we detect no significant differences between 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 dynamical studies at larger radii.

Extending the Hubble diagram by gamma ray bursts

Abstract: Aims. A new method to constrain the cosmological equation of state is proposed by using combined samples of gamma-ray bursts (GRBs) and supernovae (SNeIa). Methods. The Chevallier-Polarski-Linder parameterization is adopted for the equation of state in order to find a realistic approach to achieve the deceleration/acceleration transition phase of dark energy models. Results. We find that GRBs, calibrated by SNeIa, could be good distance indicators capable of discriminating between cosmological models and ACDM model at high redshift.

On the contribution of large-scale structure to strong gravitational lensing

Abstract: We study the correlation between the locations of galaxy-galaxy strong-lensing candidates and tracers of large-scale structure from both weak lensing (WL) or X-ray emission. The Cosmological Evolution Survey (COSMOS) is a unique data set, combining deep, high resolution and contiguous imaging in which strong lenses have been discovered, plus unparalleled multiwavelength coverage. To help interpret the COSMOS data, we have also produced mock COSMOS strong- and WL observations, based on ray-tracing through the Millennium Simulation. In agreement with the simulations, we find that strongly lensed images with the largest angular separations are found in the densest regions of the COSMOS field. This is explained by a prevalence among the lens population in dense environments of elliptical galaxies with high total-to-stellar mass ratios, which can deflect light through larger angles. However, we also find that the overall fraction of elliptical galaxies with strong gravitational lensing is independent of the local mass density; this observation is not true of the simulations, which predict an increasing fraction of strong lenses in dense environments. The discrepancy may be a real effect, but could also be explained by various limitations of our analysis. For example, our visual search of strong lens systems could be incomplete and suffer from selection bias; the luminosity function of elliptical galaxies may differ between our real and simulated data; or the simplifying assumptions and approximations used in our lensing simulations may be inadequate. Work is therefore ongoing. Automated searches for strong lens systems will be particularly important in better constraining the selection function.

Redshifts and lens profile for the double quasar QJ 0158-4325

Abstract: Aims We report on the redshift of the lensing galaxy and of the quasar QJ 0158-4325 and on the lens model of the system

GAUGING THE DARK MATTER FRACTION IN AN L * S0 GALAXY AT z = 0.47 THROUGH GRAVITATIONAL LENSING FROM DEEP HUBBLE SPACE TELESCOPE/ADVANCED CAMERA FOR SURVEYS IMAGING*

Abstract: We analyze a new gravitational lens OAC-GL J1223-1239, serendipitously found in a deep I814-band image of the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS). The lens is an L*, edge-on S0 galaxy at zl = 0.4656. The gravitational arc has a radius of 0farcs42 sime 1.74 h^ –1 kpc. We have determined the total mass and the dark matter (DM) fraction within the Einstein radius R E as a function of the lensed source redshift, which is presently unknown. For z s ~ 1.3, which is in the middle of the redshift range plausible for the source according to some external constraints, we find the central velocity dispersion to be ~180 km s^–1. With this value, close to that obtained by means of the Faber-Jackson relation at the lens redshift, we compute a 30% DM fraction within RE (given the uncertainty in the source redshift, the allowed range for the DM fraction is 25%-35% in our lensing model). When compared with the galaxies in the local universe, the lensing galaxy OAC-GL J1223-1239 seems to fall in the transition regime between massive, DM-dominated galaxies and lower-mass, DM-deficient systems.

Cluster and cluster galaxy evolution history from IR to X-ray observations of the young cluster RX J1257.2+4738 at z=0.866

Abstract: Context. The cosmic time around the z ~ 1 redshift range appears crucial in the cluster and galaxy evolution, since it is probably the epoch of the first mature galaxy clusters. Our knowledge of the properties of the galaxy populations in these clusters is limited because only a handful of z ~ 1 clusters are presently known. Aims. In this framework, we report the discovery of a z ~ 0.87 cluster and study its properties at various wavelengths. Methods. We gathered X-ray and optical data (imaging and spectroscopy), and near and far infrared data (imaging) in order to confirm the cluster nature of our candidate, to determine its dynamical state, and to give insight on its galaxy population evolution. Results. Our candidate structure appears to be a massive z ~ 0.87 dynamically young cluster with an atypically high X-ray temperature as compared to its X-ray luminosity. It exhibits a significant percentage (~90%) of galaxies that are also detected in the 24 m band. Conclusions. The cluster RXJ1257.2+4738 appears to be still in the process of collapsing. Its relatively high temperature is probably the consequence of significant energy input into the intracluster medium besides the regular gravitational infall contribution. A significant part of its galaxies are red objects that are probably dusty with on-going star formation.

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

Abstract: 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 $M/L \propto L^{\gamma}$. The "tilt" $\gamma$ 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.

Testing Dark Energy models vs $\Lambda$CDM Cosmology by Supernovae and Gamma Ray Bursts

Abstract: A new method to constrain the cosmological equation of state is proposed by using combined samples of gamma-ray bursts (GRBs) and supernovae (SNeIa). The Chevallier-Polarski-Linder parameterization is adopted for the equation of state in order to find out a realistic approach to achieve the deceleration/acceleration transition phase of dark energy models. As results, we find that GRBs, calibrated by SNeIa, could be, at least, good distance indicators capable of discriminating cosmological models with respect to $\Lambda$CDM at high redshift. Besides, GRBs+SNeIa combined redshift-distance diagram puts better in evidence the change of slope around redshift $z\sim 0.5$ which is usually addressed as the "signature" of today observed acceleration. This feature could be interpreted, in more standard way, by the red sequence in galaxy clusters.

Extending the Hubble diagram by gamma ray bursts

Abstract: A new method to constrain the cosmological equation of state is proposed by using combined samples of gamma-ray bursts (GRBs) and supernovae (SNeIa). The Chevallier-Polarski-Linder parameterization is adopted for the equation of state in order to find out a realistic approach to achieve the deceleration/acceleration transition phase of dark energy models. We find that GRBs, calibrated by SNeIa, could be good distance indicators capable of discriminating between cosmological models and $\Lambda$CDM model at high redshift.