Institute of Cosmology and Gravitation, University of Portsmouth

About: Institute of Cosmology and Gravitation, University of Portsmouth is a based out in . It is known for research contribution in the topics: Galaxy & Redshift. The organization has 297 authors who have published 1207 publications receiving 76919 citations.

Galaxy Zoo: multimergers and the Millennium Simulation

Abstract: We present a catalogue of 39 multiple mergers, found using the mergers catalogue of the Galaxy Zoo project for z < 0.1, and compare them to corresponding semi-analytical galaxies from the Millennium Simulation. We estimate the (volume-limited) multimerger fraction of the local Universe using our sample and find it to be at least 2 orders of magnitude less than binary mergers – in good agreement with the simulations (especially the Munich group). We then investigate the properties of galaxies in binary mergers and multimergers (morphologies, colours, stellar masses and environment) and compare these results with those predicted by the semi-analytical galaxies. We find that multimergers favour galaxies with properties typical of elliptical morphologies and that this is in qualitative agreement with the models. Studies of multimergers thus provide an independent (and largely corroborating) test of the Millennium semi-analytical models.

Stacking a 4D geometry into an Einstein-Gauss-Bonnet bulk

Abstract: In Einstein gravity there is a simple procedure to build D-dimensional spacetimes starting from (D-1)-dimensional ones, by stacking any (D-1)-dimensional Ricci-flat metric into the extra dimension We analyze this procedure in the context of Einstein-Gauss-Bonnet gravity, and find that it can be applied only to metrics with a constant Krestschmann scalar For instance, we show that solutions of the black-string type are not allowed in this framework

Non-local bias in the halo bispectrum with primordial non-Gaussianity

Abstract: Primordial non-Gaussianity can lead to a scale-dependent bias in the density of collapsed halos relative to the underlying matter density. The galaxy power spectrum already provides constraints on local-type primordial non-Gaussianity complementary those from the cosmic microwave background (CMB), while the bispectrum contains additional shape information and has the potential to outperform CMB constraints in future. We develop the bias model for the halo density contrast in the presence of local-type primordial non-Gaussianity, deriving a bivariate expansion up to second order in terms of the local linear matter density contrast and the local gravitational potential in Lagrangian coordinates. Non-linear evolution of the matter density introduces a non-local tidal term in the halo model. Furthermore, the presence of local-type non-Gaussianity in the Lagrangian frame leads to a novel non-local convective term in the Eulerian frame, that is proportional to the displacement field when going beyond the spherical collapse approximation. We use an extended Press-Schechter approach to evaluate the halo mass function and thus the halo bispectrum. We show that including these non-local terms in the halo bispectra can lead to corrections of up to $25\%$ for some configurations, on large scales or at high redshift.

Dark Energy Survey Year 1 results: measurement of the galaxy angular power spectrum

Abstract: We use data from the first-year observations of the DES collaboration to measure the galaxy angular power spectrum (APS), and search for its BAO feature. We test our methodology in a sample of 1800 DES Y1-like mock catalogues. We use the pseudo-C_l method to estimate the APS and the mock catalogues to estimate its covariance matrix. We use templates to model the measured spectra and estimate template parameters firstly from the C_l’s of the mocks using two different methods, a maximum likelihood estimator and a Markov Chain Monte Carlo, finding consistent results with a good reduced χ^2. Robustness tests are performed to estimate the impact of different choices of settings used in our analysis. Finally, we apply our method to a galaxy sample constructed from DES Y1 data specifically for LSS studies. This catalogue comprises galaxies within an effective area of 1318 deg^2 and 0.6 < z < 1.0. We find that the DES Y1 data favour a model with BAO at the |$2.6 \sigma$| C.L. However, the goodness of fit is somewhat poor, with χ^2/(d.o.f.) = 1.49. We identify a possible cause showing that using a theoretical covariance matrix obtained from C_l’s that are better adjusted to data results in an improved value of χ^2/(dof) = 1.36 which is similar to the value obtained with the real-space analysis. Our results correspond to a distance measurement of (z_eff = 0.81)/r_d = 10.65 ± 0.49, consistent with the main DES BAO findings. This is a companion paper to the main DES BAO article showing the details of the harmonic space analysis.

Flexion measurement in simulations of Hubble Space Telescope data.

Abstract: We present a simulation analysis of weak gravitational lensing flexion and shear measurement using shapelet decomposition, and identify differences between flexion and shear measurement noise in deep survey data. Taking models of galaxies from the Hubble Space Telescope Ultra Deep Field (HUDF) and applying a correction for the HUDF point spread function, we generate lensed simulations of deep, optical imaging data from Hubble's Advanced Camera for Surveys, with realistic galaxy morphologies. We find that flexion and shear estimates differ in our measurement pipeline: whereas intrinsic galaxy shape is typically the dominant contribution to noise in shear estimates, pixel noise due to finite photon counts and detector read noise is a major contributor to uncertainty in flexion estimates, across a broad range of galaxy signal-to-noise. This pixel noise also increases more rapidly as galaxy signal-to-noise decreases than is found for shear estimates. We provide simple power-law fitting functions for this behaviour, for both flexion and shear, allowing the effect to be properly accounted for in future forecasts for flexion measurement. Using the simulations, we also quantify the systematic biases of our shapelet flexion and shear measurement pipeline for deep Hubble data sets such as Galaxy Evolution from Morphology and SEDs, Space Telescope A901/902 Galaxy Evolution Survey or Cosmic Evolution Survey. Flexion measurement biases are found to be significant but consistent with previous studies.