We investigate the properties of the FLRW flat cosmological models in which the vacuum energy density evolves with time, Λ(t). Using different versions of the Λ(t) model, namely quantum field vacuum, power series vacuum and power law vacuum, we find that the main cosmological functions such as the scale factor of the universe, the Hubble expansion rate H and the energy densities are defined analytically. Performing a joint likelihood analysis of the recent supernovae type Ia data, the Cosmic Microwave Background (CMB) shift parameter and the Baryonic Acoustic Oscillations (BAOs) traced by the Sloan Digital Sky Survey (SDSS) galaxies, we put tight constraints on the main cosmological parameters of the Λ(t) scenarios. Furthermore, we study the linear matter fluctuation field of the above vacuum models. We find that the patterns of the power series vacuum Λ = n1 H + n2 H 2 predict stronger small scale dynamics, which implies a faster growth rate of perturbations with respect to the other two vacuum cases (quantum field and power law), despite the fact that all the cosmological models share the same equation of state (EOS) parameter. In the case of the quantum field vacuum Λ = n0 + n2 H 2 , the corresponding matter fluctuation field resembles that of the traditional Λ cosmology. The power law vacuum (Λ ∝ a −n ) mimics the classical quintessence cosmology, the best fit being tilted in the phantom phase. In this framework, we compare the observed growth rate of clustering measured from the optical galaxies with those predicted by the current Λ(t) models. Performing a Kolmogorov-Smirnov (KS) statistical test we show that the cosmological models which contain a constant vacuum (ΛCDM), quantum field vacuum and power law vacuum provide growth rates that match well with the observed growth rate. However, this is not the case for the power series vacuum models (in particular, the frequently adduced Λ ∝ H model) in which clusters form at significantly earlier times (z ≥ 4) with respect to all other models (z ∼ 2). Finally, we derived the theoretically predicted dark-matter halo mass function and the corresponding distribution of cluster-size halos for all the models studied. Their expected redshift distribution indicates that it will be difficult to distinguish the closely resembling models (constant vacuum, quantum field and power-law vacuum), using realistic future X-ray surveys of cluster abundances. However, cluster surveys based on the Sunayev-Zeldovich detection method give some hope to distinguish the closely resembling models at high redshifts.PACS numbers: 98.80.-k, 95.35.+d, 95.36.+x

The validity of the emission line luminosity vs. ionised gas velocity dispersion (L − σ) correlation for HII galaxies (HIIGx), and its potential as an accurate distance estimator are assessed.For a sample of 128 local (0.02 z 0.2) compact HIIGx with high equivalent widths of their Balmer emission lines we obtained ionized gas velocity dispersion from high S/N high-dispersion spectroscopy (Subaru-HDS and ESO VLT-UVES) and integrated Hβ fluxes from low dispersion wide aperture spectrophotometry.We find that the L(Hβ) − σ relation is strong and stable against restrictions in the sample (mostly based on the emission line profiles). The 'gaussianity' of the profile is important for reducing the rms uncertainty of the distance indicator, but at the expense of substantially reducing the sample.By fitting other physical parameters into the correlation we are able to significantly decrease the scatter without reducing the sample. The size of the starforming region is an important second parameter, while adding the emission line equivalent width or the continuum colour and metallicity, produces the solution with the smallest rms scatter=δ log L(Hβ) = 0.233.The derived coefficients in the best L(Hβ) − σ relation are very close to what is expected from virialized ionizing clusters, while the derived sum of the stellar and ionised gas masses are similar to the dynamical mass estimated using the HST corrected Petrosian radius. These results are compatible with gravity being the main mechanism causing the broadening of the emission lines in these very young and massive clusters. The derived masses range from about 2 ×10 6 M to 10 9 M and their 'corrected' Petrosian radius, from a few tens to a few hundred parsecs. * All the fluxes are given in units of 10 −16 erg s −1 cm −2 . † The instrument flag indicates the origin of the data. 1 : Directly measured using long slit as described in the text. 2: from aperture corrected SDSS DR7 measurements. 3 : from Terlevich et al. (1991), in this case errors in fluxes and EW are taken directly form the cited source.

Despite a history that dates back at least a quarter of a century, studies of voids in the large-scale structure of the Universe are bedevilled by a major problem: there exist a large number of quite different void-finding algorithms, a fact that has so far got in the way of groups comparing their results without worrying about whether such a comparison in fact makes sense. Because of the recent increased interest in voids, both in very large galaxy surveys and in detailed simulations of cosmic structure formation, this situation is very unfortunate. We here present the first systematic comparison study of 13 different void finders constructed using particles, haloes, and semi-analytical model galaxies extracted from a subvolume of the Millennium simulation. This study includes many groups that have studied voids over the past decade. We show their results and discuss their differences and agreements. As it turns out, the basic results of the various methods agree very well with each other in that they all locate a major void near the centre of our volume. Voids have very underdense centres, reaching below 10 per cent of the mean cosmic density. In addition, those void finders that allow for void galaxies show that those galaxies follow similar trends. For example, the overdensity of void galaxies brighter than m B = −20 is found to be smaller than about −0.8 by all our void finding algorithms.

A new class of Friedmann-Lemaître-Robertson-Walker (FLRW) cosmological models with time-evolving fundamental parameters should emerge naturally from a description of the expansion of the universe based on the first principles of quantum field theory and string theory. Within this general paradigm, one expects that both the gravitational Newton's coupling G and the cosmological term Λ should not be strictly constant but appear rather as smooth functions of the Hubble rate H(t). This scenario ("running FLRW model") predicts, in a natural way, the existence of dynamical dark energy without invoking the participation of extraneous scalar fields. In this paper, we perform a detailed study of some of these models in the light of the latest cosmological data, which serves to illustrate the phenomenological viability of the new dark energy paradigm as a serious alternative to the traditional scalar field approaches. By performing a joint likelihood analysis of the recent supernovae type Ia data (SNIa), the Cosmic Microwave Background (CMB) shift parameter, and the Baryonic Acoustic Oscillations (BAOs) traced by the Sloan Digital Sky Survey (SDSS), we put tight constraints on the main cosmological parameters. Furthermore, we derive the theoretically predicted dark-matter halo mass function and the corresponding redshift distribution of cluster-size halos for the "running" models studied. Despite the fact that these models closely reproduce the standard ΛCDM Hubble expansion, their normalization of the perturbation's power-spectrum varies, imposing, in many cases, a significantly different cluster-size halo redshift distribution. This fact indicates that it should be relatively easy to distinguish between the "running" models and the ΛCDM using realistic future X-ray and Sunyaev-Zeldovich cluster surveys.

We report the first results of a programme aimed at studying the properties of high redshift galaxies with on-going massive and dominant episodes of star formation (HII galaxies). We use the L(Hβ) − σ distance estimator based on the correlation between the ionized gas velocity dispersions and Balmer emission line luminosities of HII galaxies and Giant HII regions to trace the expansion of the Universe up to z ∼ 2.33. This approach provides an independent constraint on the equation of state of dark energy and its possible evolution with look-back time.Here we present high-dispersion (8,000 to 10,000 resolution) spectroscopy of HII galaxies at redshifts between 0.6 and 2.33, obtained at the VLT using XShooter. Using six of these HII galaxies we obtain broad constraints on the plane Ω m − w 0 . The addition of 19 high-z HII galaxies from the literature improves the constraints and highlights the need for high quality emission line profiles, fluxes and reddening corrections. The 25 high-z HII galaxies plus our local compilation of 107 H ii galaxies up to z = 0.16 were used to impose further constraints. Our results are consistent with recent studies, although weaker due to the as yet small sample and low quality of the literature data of high-z HII galaxies.We show that much better and competitive constraints can be obtained using a larger sample of high redshift HII galaxies with high quality data that can be easily obtained with present facilities like KMOS at the VLT.Partly based on observations obtained at the European Southern Observatory, programme ID 091.A-0413(A) †

We present a detailed study of the morphological features of 22 rich galaxy clusters. Our sample is constructed from a cross-correlation of optical (Abell+APM) data with X-ray (0.1 - 2.4) keV ROSAT pointed observations. We systematically compare cluster images and morphological parameters in an attempt to reliably identify possible substructure in both optical and the X-ray images. To this end, we compute various moments of the optical and X-ray surface-brightness distribution such as the ellipticities, center-of-mass shifts and ellipsoidal orientations. We assess the significance of our results using Monte Carlo simulations. We find significant correlations between the optical and X-ray morphological parameters, indicating that in both parts of the spectrum it is possible to identify correctly the dynamical state of a cluster. Most of our clusters (17/22) have a good 1-to-1 correspondence between the optical and the X-ray images and about 10 appear to have strong indications of substructure. This corresponds to a minimum percentage of order 45 per cent which is in very good accordance with other similar analyses. Finally, 5 out of 22 systems seem to have distinct subclumps in the optical which are not verified in the X-ray images, and thus are suspect of being due to optical projection effects. These results will serve as a useful guide in interpreting subsequent analyses of large optical cluster catalogues.Comment: 15 pages, including 9 figures, MNRAS in press, revised versio

We present the spatial clustering properties of 1466 X-ray selected AGN compiled from the Chandra CDF-N, CDF-S, eCDF-S, COSMOS and AEGIS fields in the 0.5 − 8 keV band. The X-ray sources span the redshift interval 0 < z < 3 and have a median value ofz = 0.976. We employ the projected two-point correlation function to infer the spatial clustering and find a clustering length of r 0 = 7.2 ± 0.6h −1 Mpc and a slope of γ = 1.48 ± 0.12, which corresponds to a bias of b(z) = 2.26 ± 0.16. Using two different halo bias models, we consistently estimate an average dark-matter host halo mass of M h ≃ 1.3(±0.3) × 10 13 h −1 M ⊙ . The Xray AGN bias and the corresponding dark-matter host halo mass, are significantly higher than the corresponding values of optically selected AGN (at the same redshifts). The redshift evolution of the X-ray selected AGN bias indicates, in agreement with other recent studies, that a unique dark-matter halo mass does not fit well the bias at all the different redshifts probed. Furthermore, we investigate if there is a dependence of the clustering strength on X-ray luminosity. To this end we consider only 650 sources around z ∼ 1 and we apply a procedure to disentangle the dependence of clustering on redshift. We find indications for a positive dependence of the clustering length on X-ray luminosity, in the sense that the more luminous sources have a larger clustering length and hence a higher dark-matter halo mass. In detail we find for an average luminosity difference of δ log 10 L x ≃ 1 a halo mass difference of a factor of ∼3.These findings appear to be consistent with a galaxy-formation model where the gas accreted onto the supermassive black hole in intermediate luminosity AGN comes mostly from the hot-halo atmosphere around the host galaxy.

We report the first results of a long-term programme aiming to provide accurate independent estimates of the Hubble constant (H 0 ) using the L(Hβ)-σ distance estimator for giant extragalactic H II regions (GEHR) and H II galaxies.We have used Very Large Telescope and Subaru high-dispersion spectroscopic observations of a local sample of H II galaxies, identified in the Sloan Digital Sky Survey Data Release 7 (SDSS DR7) catalogue in order to redefine and improve the L(Hβ)-σ distance indicator and to determine the Hubble constant. To this end, we utilized as local calibration or 'anchor' of this correlation GEHR in nearby galaxies which have accurate distance measurements determined via primary indicators. Using our best sample of 69 nearby H II galaxies and 23 GEHR in nine galaxies, we obtain H 0 = 74.3 ± 3.1 (statistical) ± 2.9 (systematic) km s −1 Mpc −1 , in excellent agreement with, and independently confirming, the most recent Type Ia supernovae based results.