L. Moscardini

A test of the nature of cosmic acceleration using galaxy redshift distortions

Observations of distant supernovae indicate that the Universe is now in a phase of accelerated expansion the physical cause of which is a mystery. Formally, this requires the inclusion of a term acting as a negative pressure in the equations of cosmic expansion, accounting for about 75 per cent of the total energy density in the Universe. The simplest option for this ‘dark energy’ corresponds to a ‘cosmological constant’, perhaps related to the quantum vacuum energy. Physically viable alternatives invoke either the presence of a scalar field with an evolving equation of state, or extensions of general relativity involving higher-order curvature terms or extra dimensions. Although they produce similar expansion rates, different models predict measurable differences in the growth rate of large-scale structure with cosmic time. A fingerprint of this growth is provided by coherent galaxy motions, which introduce a radial anisotropy in the clustering pattern reconstructed by galaxy redshift surveys. Here we report a measurement of this effect at a redshift of 0.8. Using a new survey of more than 10,000 faint galaxies, we measure the anisotropy parameter β = 0.70 ± 0.26, which corresponds to a growth rate of structure at that time of f = 0.91 ± 0.36. This is consistent with the standard cosmological-constant model with low matter density and flat geometry, although the error bars are still too large to distinguish among alternative origins for the accelerated expansion. The correct origin could be determined with a further factor-of-ten increase in the sampled volume at similar redshift.

Comparing the temperatures of galaxy clusters from hydrodynamical N-body simulations to Chandra and XMM-Newton observations

Theoretical studies of the physical processes guiding the formation and evolution of galaxies and galaxy clusters in the X-ray region are mainly based on the results of numerical hydrodynamical N-body simulations, which in turn are often directly compared with X-ray observations. Although trivial in principle, these comparisons are not always simple. We demonstrate that the projected spectroscopic temperature of thermally complex clusters obtained from X-ray observations is always lower than the emission-weighed temperature, which is widely used in the analysis of numerical simulations. We show that this temperature bias is mainly related to the fact that the emission-weighted temperature does not reflect the actual spectral properties of the observed source. This has important implications for the study of thermal structures in clusters, especially when strong temperature gradients, such as shock fronts, are present. Because of this bias, in real observations shock fronts appear much weaker than what is predicted by emission-weighted temperature maps, and may not even be detected. This may explain why, although numerical simulations predict that shock fronts are a quite common feature in clusters of galaxies, to date there are very few observations of objects in which they are clearly seen. To fix this problem we propose a new formula, the spectroscopic-like temperature function, and show that, for temperatures higher than 3 keV, it approximates the spectroscopic temperature to better than a few per cent, making simulations more directly comparable to observations.

Systematics in the X-ray Cluster Mass Estimators

We examine the systematics affecting the X-ray mass estimators applied to a set of five galaxy clusters resolved at high resolution in hydrodynamic simulations, including cooling, star formation and feedback processes. These simulated objects are processed through the X-ray Map Simulator, X-MAS, to provide Chandra-like long exposures that are analysed to reconstruct the gas temperature, density and mass profiles used as input. These clusters have different dynamic state: we consider a hot cluster with temperature T = 11.4 keV, a perturbed cluster with T = 3.9 keV, a merging object with T = 3.6 keV, and two relaxed systems with T = 3.3 keV and T = 2.7 keV, respectively. These systems are located at z = 0.175 so that their emission fits within the Chandra ACIS-S3 chip between 0.6 and 1.2 R-500. We find that the mass profile obtained via a direct application of the hydrostatic equilibrium ( HE) equation is dependent upon the measured temperature profile. An irregular radial distribution of the temperature values, with associated large errors, induces a significant scatter on the reconstructed mass measurements. At R-2500, the actual mass is recovered within 1 sigma, although we notice this estimator shows high statistical errors due to high level of Chandra background. Instead, the poorness of the beta-model in describing the gas density profile makes the evaluated masses to be underestimated by similar to 40 per cent with respect to the true mass, both with an isothermal and a polytropic temperature profile. We also test ways to recover the mass by adopting an analytic mass model, such as those proposed by Nvarro, Frenk & White and Rasia, Tormen & Moscardini, and fitting the temperature profile expected from the HE equation to the observed one. We conclude that the methods of the HE equation and those of the analytic fits provide a more robust mass estimation than the ones based on the beta-model. In the present work, the main limitation for a precise mass reconstruction is to ascribe to the relatively high level of the background chosen to reproduce the Chandra one. After artificially reducing the total background by a factor of 100, we find that the estimated mass significantly underestimates the true mass profiles. This is manly due (i) to the neglected contribution of the gas bulk motions to the total energy budget and (ii) to the bias towards lower values of the X-ray temperature measurements because of the complex thermal structure of the emitting plasma.

A dynamical model for the distribution of dark matter and gas in galaxy clusters

Using the results of an extended set of high-resolution non-radiative hydrodynamic simulations of galaxy clusters, we obtain simple analytic formulae for the dark matter and hot gas distribution, in the spherical approximation. Starting from the dark matter phase-space radial density distribution, we derive fits for the dark matter density, velocity dispersion and velocity anisotropy. We use these models to test the dynamical equilibrium hypothesis through the Jeans equation: we find that this is satisfied to good accuracy by our simulated clusters inside their virial radii. This result also shows that our fits constitute a self-consistent dynamical model for these systems. We then extend our analysis to the hot gas component, obtaining analytic fits for the gas density, temperature and velocity structure, with no further hypothesis on the gas dynamical status or state equation. Gas and dark matter show similar density profiles down to ≈0.06Rv (with Rv the virial radius), while at smaller radii the gas flattens, producing a central core. Gas temperatures are almost isothermal out to roughly 0.2 Rv, then steeply decrease, reaching at the virial radius a value almost a factor of 2 lower. We find that the gas is not at rest inside Rv: velocity dispersions are increasing functions of the radius, motions are isotropic to slightly tangential, and contribute non-negligibly to the total pressure support. We test this model using a generalization of the hydrostatic equilibrium equation, where the gas motion is properly taken into account. Again we find that the fits provide an accurate description of the system: the hot gas is in equilibrium and is a good tracer of the overall cluster potential if all terms (density, temperature and velocity) are taken into account, while simpler assumptions cause systematic mass underestimates. In particular, we find that using the so-called β-model underestimates the true cluster mass by up to 50 per cent at large radii. We also find that, if gas velocities are neglected, then a simple isothermal model fares better at large radii than a non-isothermal one. The shape of the gas density profile at small radii is at least partially explained by the gas expansion caused by energy transfer from dark matter during the collapse. In fact, when gas bulk energy is also considered, gas and dark matter are in energy equipartition in the final system at radii r > 0.1Rv, while at smaller radii the gas is hotter than the dark matter. This energy imbalance is also probably the reason of the further global halo compression compared with a pure collisionless collapse, which we point out by comparing the dark matter and total density profiles of our hydro-simulated clusters with a set of identical – but pure N-body – ones. The compression has the effect of raising the mean concentration by an amount of roughly 10 per cent.

The VIMOS Public Extragalactic Redshift Survey (VIPERS) - Galaxy clustering and redshift-space distortions at z ≃ 0.8 in the first data release

We present in this paper the general real- and redshift-space clustering properties of galaxies as measured in the first data release of the VIPERS survey. VIPERS is a large redshift survey designed to probe the distant Universe and its large-scale structure at 0.5 < z < 1.2. We describe in this analysis the global properties of the sample and discuss the survey completeness and associated corrections. This sample allows us to measure the galaxy clustering with an unprecedented accuracy at these redshifts. From the redshift-space distortions observed in the galaxy clustering pattern we provide a first measurement of the growth rate of structure at z = 0.8: f\sigma_8 = 0.47 +/- 0.08. This is completely consistent with the predictions of standard cosmological models based on Einstein gravity, although this measurement alone does not discriminate between different gravity models.

Cosmic dynamics in the era of Extremely Large Telescopes

The redshifts of all cosmologically distant sources are expected to experience a small, systematic drift as a function of time due to the evolution of the Universes expansion rate. A measurement of this effect would represent a direct and entirely model-independent determination of the expansion history of the Universe over a redshift range that is inaccessible to other methods. Here we investigate the impact of the next generation of Extremely Large Telescopes on the feasibility of detecting and characterising the cosmological redshift drift. We consider the Lyman alpha forest in the redshift range 2 < z < 5 and other absorption lines in the spectra of high redshift QSOs as the most suitable targets for a redshift drift experiment. Assuming photon-noise limited observations and using extensive Monte Carlo simulations we determine the accuracy to which the redshift drift can be measured from the Ly alpha forest as a function of signal-to-noise and redshift. Based on this relation and using the brightness and redshift distributions of known QSOs we find that a 42-m telescope is capable of unambiguously detecting the redshift drift over a period of ~20 yr using 4000 h of observing time. Such an experiment would provide independent evidence for the existence of dark energy without assuming spatial flatness, using any other cosmological constraints or making any other astrophysical assumption.

Large‐scale non‐Gaussian mass function and halo bias: tests on N‐body simulations

A B ST R A C T Thedescription oftheabundanceand clusteringofhalosfornon-Gaussian initialconditions has recently received renewed interest,m otivated by the forthcom ing large galaxy and clustersurveys,which can potentially yield constraintsoforderunity on thenon-Gaussianity param eter fN L.W epresenttestson N-body sim ulationsofanalyticalform ulaedescribing thehalo abundanceand clusteringfornon-Gaussian initial conditions.W e calibratethe analyticnon-Gaussian m assfunction ofM atarreseetal. (2000) and LoVerdeetal.(2008) and the analytic description ofclustering ofhalos

Modelling the cosmological co-evolution of supermassive black holes and galaxies – I. BH scaling relations and the AGN luminosity function

We model the cosmological co-evolution of galaxies and their central supermassive black holes (BHs) within a semi-analytical framework developed on the outputs of the Millennium Simulation. This model, described in detail in Croton et al. (2006) and De Lucia & Blaizot (2007), introduces a ‘radio mode’ feedback from Active Gala ctic Nuclei (AGN) at the centre of X-ray emitting atmospheres in galaxy groups and clusters. Thanks to this mechanism, the model can simultaneously explain: (i) the low observed mass drop-out rate in cooling flows; (ii) the exponential cut-off in the bright end of the galaxy l uminosity function; and (iii) the bulge-dominated morphologies and old stellar ages of the most massive galaxies in clusters. This paper is the first of a series in which we investigate how w ell this model can also reproduce the physical properties of BHs and AGN. Here we analyze the scaling relations, the fundamental plane and the mass function of BHs, and compare them with the most recent observational data. Moreover, we extend the semi-analytic model to follow the evolution of the BH mass accretion and its conversion into radiation, and compare the derived AGN bolometric luminosity function with the observed one. While we fi nd for the most part a very good agreement between predicted and observed BH properties, the semi-analytic model underestimates the number density of luminous AGN at high redshifts, independently of the adopted Eddington factor and accretion efficiency. However, an agre ement with the observations is possible within the framework of our model, provided it is assumed that the cold gas fraction accreted by BHs at high redshifts is larger than at low redshifts.

Simulated X-ray galaxy clusters at the virial radius: slopes of the gas density, temperature and surface brightness profiles

Using a set of hydrodynamical simulations of nine galaxy clusters with masses in the range 1.5 x 10 14 < M vir < 3.4 x 10 15 M ⊙ , we have studied the density, temperature and X-ray surface brightness profiles of the intracluster medium in the regions around the virial radius. We have analysed the profiles in the radial range well above the cluster core, the physics of which are still unclear and matter of tension between simulated and observed properties, and up to the virial radius and beyond, where present observations are unable to provide any constraints. We have modelled the radial profiles between 0.3R 200 and 3R 200 with power laws with one index, two indexes and a rolling index. The simulated temperature and [0.5-2] keV surface brightness profiles well reproduce the observed behaviours outside the core. The shape of all these profiles in the radial range considered depends mainly on the activity of the gravitational collapse, with no significant difference among models including extraphysics. The profiles steepen in the outskirts, with the slope of the power-law fit that changes from -2.5 to -3.4 in the gas density, from -0.5 to -1.8 in the gas temperature and from -3.5 to -5.0 in the X-ray soft surface brightness. We predict that the gas density, temperature and [0.5-2] keV surface brightness values at R 200 are, on average, 0.05, 0.60, 0.008 times the measured values at 0.3R 200 . At 2R 200 , these values decrease by an order of magnitude in the gas density and surface brightness, by a factor of 2 in the temperature, putting stringent limits on the detectable properties of the intracluster-medium (ICM) in the virial regions.

Modelling the cosmological co-evolution of supermassive black holes and galaxies – II. The clustering of quasars and their dark environment

We use semi-analytic modelling on top of the Millennium simulation to study the joint formation of galaxies and their embedded supermassive black holes. Our goal is to test scenarios in which black hole accretion and quasar activity are triggered by galaxy mergers, and to constrain different models for the light curves associated with individual quasar events. In the present work, we focus on studying the spatial distribution of simulated quasars. At all luminosities, we find that the simulated quasar two-point correlation function is fit well by a single power law in the range 0.5 ≤ r ≤ 20 h ―1 Mpc, but its normalization is a strong function of redshift. When we select only quasars with luminosities within the range typically accessible by todays quasar surveys, their clustering strength depends only weakly on luminosity, in agreement with observations. This holds independently of the assumed light-curve model, since bright quasars are black holes accreting close to the Eddington limit, and are hosted by dark matter haloes with a narrow mass range of a few 10 12 h ―1 M ⊙ . Therefore, the clustering of bright quasars cannot be used to disentangle light-curve models, but such a discrimination would become possible if the observational samples can be pushed to significantly fainter limits. Overall, our clustering results for the simulated quasar population agree rather well with observations, lending support to the conjecture that galaxy mergers could be the main physical process responsible for triggering black hole accretion and quasar activity.