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We report on the discovery of the new gravitational lens system CLASS B0631+519. Imaging with the Very Large Array, the Multi-Element Radio-Linked Interferometer Network (MERLIN) and the Very Long Baseline Array (VLBA) reveals a doubly imaged flat-spectrum radio core, a doubly imaged steep-spectrum radio lobe and possible quadruply imaged emission from a second lobe. The maximum separation between the lensed images is 1.16 arcsec. High-resolution mapping with the VLBA at 5 GHz resolves the most magnified image of the radio core into a number of subcomponents spread across approximately 20 mas. No emission from the lensing galaxy or an odd image is detected down to 0.31 mJy (5σ )a t8.4 GHz. Optical and near-infrared imaging with the Advanced Camera for Surveys and the Near-Infrared Camera and Multi-Object Spectrometer on the Hubble Space Telescope (HST) show that there are two galaxies along the line of sight to the lensed source, as previously discovered by optical spectroscopy. We find that the foreground galaxy at z = 0.0896 is a small irregular, and that the other, at z = 0.6196 is a massive elliptical, which appears to contribute the majority of the lensing effect. The host galaxy of the lensed source is detected in the HST near-infrared imaging as a set of arcs, which form a nearly complete Einstein ring. Mass modelling using non-parametric techniques can reproduce the near-infrared observations and indicates that the small irregular galaxy has a (localized) effect on the flux density distribution in the Einstein ring at the 5‐10 per cent level. Ke yw ords: gravitational lensing ‐ cosmology: miscellaneous.

A prediction of standard inflationary cosmology is that the elemental composition of the medium out of which the earliest stars and galaxies condensed consisted primarily of hydrogen and helium (4)He, with small admixtures of deuterium, lithium (7)Li, and (3)He. The most redshifted quasars, galaxies, and Lyalpha absorbers currently observed, however, all exhibit at least some admixture of heavier elements, as do the most ancient stars in the Galaxy. Here we examine ways in which the abundance of these same elements, if present before the epoch of Population III formation, might be observationally established or ruled out.

In this work we study the properties of protoclusters of galaxies by employing the MUSIC set of hydrodynamical simulations, featuring a mass-limited sample of 282 resimulated clusters with available merger trees up to high redshift, and we trace the cluster formation back to $z$ = 1.5, 2.3 and 4. We study the features and redshift evolution of the mass and the spatial distribution for all the cluster progenitors and for the protoclusters, which we define as the most massive progenitors of the clusters identified at $z$ = 0. A natural extension to redshifts larger than 1 is applied to the estimate of the baryon content also in terms of gas and stars budgets: no remarkable variations with redshift are discovered. Furthermore, motivated by the proven potential of Sunyaev-Zel'dovich surveys to blindly search for faint distant objects, we focus on the scaling relation between total object mass and integrated Compton $y$-parameter, and we check for the possibility to extend the mass-observable paradigm to the protocluster regime, far beyond the redshift of 1, to account for the properties of the simulated objects. We find that the slope of this scaling law is steeper than what expected for a self-similarity assumption among these objects, and it increases with redshift mainly for the synthetic clusters where radiative processes, such as radiative cooling, heating processes of the gas due to UV background, star formation and supernovae feedback, are included. We use three different criteria to account for the dynamical state of the protoclusters, and find no significant dependence of the scaling parameters from the level of relaxation. Based on this, we exclude that the dynamical state is the cause of the observed deviations from self-similarity., 12 pages, 14 figures, submitted to MNRAS

I present constraints on cosmological parameters in the lambda(0)-Omega(0) plane from a joint analysis of gravitational lensing statistics (Helbig et al. 1999b) and the magnitude-redshift relation for Type Ia supernovae (Perlmutter et al. 1999; Riess et al. 1998). I discuss reasons why this particular combination of tests is important and how the constraints can be improved in the future. The lensing statistics and supernova results are not inconsistent, thus it is meaningful to determine joint constraints on lambda(0) and Omega(0) by combining the results from both tests. The quantity measured by the lens statistics and the m-z relation for type Ia supernovae discussed here is approximately lambda(0) - Omega(0). At 95% confidence, the upper limit on lambda(0) - Omega(0) from lensing statistics alone is 0.45 and from supernovae alone is in the range 0.65-0.81 (depending on the data set). For joint constraints, the upper limit on lambda(0) - Omega(0) is in the range .55-0.60 (again depending on the data set). For a flat universe with lambda(0) + Omega(2) = 1, this corresponds to upper limits on lambda(0), taking the top of the range from different data sets, of 0.72, 0.90 and 0.80 for lensing statistics alone, supernovae alone and the joint analysis, respectively. This is perfectly consistent with the current 'standard cosmological model' with lambda(0) approximate to 0.7 and Omega(0) approximate to 0.3 and is consistent with a flat universe but, neglecting other cosmological tests, does riot require it.

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 Universe's 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., Accepted for publication in MNRAS, 27 pages, 19 figures

We have performed a detailed analysis of the local super-massive black-hole (SMBH) mass function based on both kinematic and photometric data and derived an accurate analytical fit in the range 10^6 1.5), while the lower mass BHs responsible for most of the hard X-ray background have mostly grown at z= 10^9 M_sun. The mass accreted during luminous phases is >= 25-30% even if we assume extreme values of \epsilon (\epsilon \~0.3-0.4). An unlikely fine tuning of the parameters would be required to account for the local SMBH mass function accomodating a dominant contribution from 'dark' BH growth (due, e.g., to BH coalescence)., Comment: 12 pages, 14 figures, accepted for publication in MNRAS, minor changes following referee's comments

We present a determination of the bispectrum of the flux in the Lyman-alpha forest of QSO absorption spectra obtained from LUQAS which consists of spectra observed with the high resolution Echelle spectrograph UVES. Typical errors on the observed bispectrum as obtained from a jack-knife estimator are ~ 50%. For wavenumbers in the range 0.03 s/km 2., Comment: 6 pages, 3 figures, 1 table. A table can be found at http://www.ast.cam.ac.uk/~rtnigm/luqas.htm To be published in MNRAS Letters

A prediction of standard inflationary cosmology is that the elemental composition of the medium out of which the earliest stars and galaxies condensed consisted primarily of hydrogen and helium 4He with small admixtures of deuterium, lithium 7Li, and 3He. The most red-shifted quasars, galaxies, and Ly-alpha absorbers currently observed, however, all exhibit at least some admixture of heavier elements, as do the most ancient stars in the Galaxy. Here we examine ways in which the abundance of these same elements, if present before the epoch of population III formation, might be observationally established or ruled out., Accepted by the Astrophysical Journal

I describe here the performance of a parallel treecode with individual particle timesteps. The code is based on the Barnes-Hut algorithm and runs cosmological N-body simulations on parallel machines with a distributed memory architecture using the MPI message-passing library. For a configuration with a constant number of particles per processor the scalability of the code was tested up to P=128 processors on an IBM SP4 machine. In the large $P$ limit the average CPU time per processor necessary for solving the gravitational interactions is $\sim 10 %$ higher than that expected from the ideal scaling relation. The processor domains are determined every large timestep according to a recursive orthogonal bisection, using a weighting scheme which takes into account the total particle computational load within the timestep. The results of the numerical tests show that the load balancing efficiency $L$ of the code is high ($>=90%$) up to P=32, and decreases to $L\sim 80%$ when P=128. In the latter case it is found that some aspects of the code performance are affected by machine hardware, while the proposed weighting scheme can achieve a load balance as high as $L\sim 90%$ even in the large $P$ limit., 30 pages, 3 tables, 9 figures, accepted for publication in New Astronomy