Showing papers by "Institute of Cosmology and Gravitation, University of Portsmouth published in 2012"

The clustering of galaxies in the SDSS-III Baryon Oscillation Spectroscopic Survey: Measurements of the growth of structure and expansion rate at z = 0.57 from anisotropic clustering

Abstract: We analyse the anisotropic clustering of massive galaxies from the Sloan Digital Sky Survey III Baryon Oscillation Spectroscopic Survey (BOSS) Data Release 9 (DR9) sample, which consists of 264 283 galaxies in the redshift range 0.43 0.57, and when combined imply ΩΛ = 0.74 ± 0.016, independent of the Universe's evolution at z < 0.57. All of these constraints assume scale-independent linear growth, and assume general relativity to compute both (10 per cent) non-linear model corrections and our errors. In our companion paper, Samushia et al., we explore further cosmological implications of these observations.

Interpreting large-scale redshift-space distortion measurements

Abstract: The simplest theory describing large-scale redshift-space distortions (RSD), based on linear theory and distant galaxies, depends on the growth of cosmological structure, suggesting that strong tests of general relativity can be constructed from galaxy surveys. As data sets become larger and the expected constraints more precise, the extent to which the RSD follow the simple theory needs to be assessed in order that we do not introduce systematic errors into the tests by introducing inaccurate simplifying assumptions. We study the impact of the sample geometry, non-linear processes and biases induced by our lack of understanding of the radial galaxy distribution on RSD measurements. Using Large Suite of Dark Matter Simulations of the Sloan Digital Sky Survey II (SDSS-II) luminous red galaxy data, these effects are shown to be important at the level of 20 per cent. Including them, we can accurately model the recovered clustering in these mock catalogues on scales 30–200 h−1 Mpc. Applying this analysis to robustly measure parameters describing the growth history of the Universe from the SDSS-II data gives f(z= 0.25)σ8(z= 0.25) = 0.3512 ± 0.0583 and f(z= 0.37)σ8(z= 0.37) = 0.4602 ± 0.0378 when no prior is imposed on the growth rate, and the background geometry is assumed to follow a Λ cold dark matter (ΛCDM) model with the Wilkinson Microwave Anisotropy Probe (WMAP)+Type Ia supernova priors. The standard WMAP constrained ΛCDM model with general relativity predicts f(z= 0.25)σ8(z= 0.25) = 0.4260 ± 0.0141 and f(z= 0.37)σ8(z= 0.37) = 0.4367 ± 0.0136, which is fully consistent with these measurements.

Column density distribution and cosmological mass density of neutral gas: Sloan Digital Sky Survey-III Data Release 9

Abstract: We present the first results from an ongoing survey for damped Lyman-α systems (DLAs) in the spectra of z > 2 quasars observed in the course of the Baryon Oscillation Spectroscopic Survey (BOSS), which is part of the Sloan Digital Sky Survey (SDSS) III. Our full (non-statistical) sample, based on Data Release 9, comprises 12 081 systems with log N (H i) ≥ 20, out of which 6839 have log N (H i) ≥ 20.3. This is the largest DLA sample ever compiled, superseding that from SDSS-II by a factor of seven. Using a statistical sub-sample and estimating systematics from realistic mock data, we probe the N (H i) distribution at ⟨z ⟩ = 2.5. Contrary to what is generally believed, the distribution extends beyond 1022 cm-2 with a moderate slope of index ≈−3.5. This result matches the opacity-corrected distribution observed at z = 0 surprisingly well. The cosmological mass density of neutral gas in DLAs is found to be , evolving only mildly over the past 12 billion years.

The clustering of galaxies in the SDSS-III Baryon Oscillation Spectroscopic Survey: analysis of potential systematics

Abstract: We analyse the density field of galaxies observed by the Sloan Digital Sky Survey (SDSS)-III Baryon Oscillation Spectroscopic Survey (BOSS) included in the SDSS Data Release Nine (DR9). DR9 includes spectroscopic redshifts for over 400 000 galaxies spread over a footprint of 3275 deg2. We identify, characterize and mitigate the impact of sources of systematic uncertainty on large-scale clustering measurements, both for angular moments of the redshift-space correlation function, ξl(s), and the spherically averaged power spectrum, P(k), in order to ensure that robust cosmological constraints will be obtained from these data. A correlation between the projected density of stars and the higher redshift (0.43 120 h−1 Mpc or k < 0.01 h Mpc−1. We find that these errors can be ameliorated by weighting galaxies based on their surface brightness and the local stellar density. The clustering of CMASS galaxies found in the Northern and Southern Galactic footprints of the survey generally agrees to within 2σ. We use mock galaxy catalogues that simulate the CMASS selection function to determine that randomly selecting galaxy redshifts in order to simulate the radial selection function of a random sample imparts the least systematic error on ξl(s) measurements and that this systematic error is negligible for the spherically averaged correlation function, ξ0. We find a peak in ξ0 at s~ 200 h−1 Mpc, with a corresponding feature with period ~0.03 h Mpc−1 in P(k), and find features at least as strong in 4.8 per cent of the mock galaxy catalogues, concluding this feature is likely to be a consequence of cosmic variance. The methods we recommend for the calculation of clustering measurements using the CMASS sample are adopted in companion papers that locate the position of the baryon acoustic oscillation feature, constrain cosmological models using the full shape of ξ0 and measure the rate of structure growth.

Column density distribution and cosmological mass density of neutral gas: Sloan Digital Sky Survey-III Data Release 9

Abstract: We present the first results from an ongoing survey for Damped Lyman-alpha systems (DLAs) in the spectra of z>2 quasars observed in the course of the Baryon Oscillation Spectroscopic Survey (BOSS), which is part of the Sloan Digital Sky Survey (SDSS) III. Our full (non-statistical) sample, based on Data Release 9, comprises 12,081 systems with log N(HI)>=20, out of which 6,839 have log N(HI)>=20.3. This is the largest DLA sample ever compiled, superseding that from SDSS-II by a factor of seven. Using a statistical sub-sample and estimating systematics from realistic mock data, we probe the N(HI) distribution at = 2.5. Contrary to what is generally believed, the distribution extends beyond 10^22 cm^-2 with a moderate slope of index\approx-3.5. This result matches surprisingly well the opacity-corrected distribution observed at z = 0. The cosmological mass density of neutral gas in DLAs is found to be Omega_g_DLA~10^-3, evolving only mildly over the past 12 billion years.

Recovering galaxy stellar population properties from broad-band spectral energy distribution fitting

Abstract: We explore the dependence of galaxy stellar population properties that are derived from broad-band spectral energy distribution (SED) fitting - such as age, stellar mass, dust reddening, etc. - on a variety of parameters, such as star formation histories, age grid, metallicity, initial mass function (IMF), dust reddening and reddening law, filter setup and wavelength coverage. Mock galaxies are used as test particles. We confirm our earlier results based on real z = 2 galaxies, that usually adopted τ-models lead to overestimate the star formation rate and to underestimate the stellar mass. Here, we show that - for star-forming galaxies - galaxy ages, masses and reddening, can be well determined simultaneously only when the correct star formation history is identified. This is the case for inverted-τ models at high-z, for which we find that the mass recovery (at fixed IMF) is as good as ~ 0.04 dex. However, since the right star formation history is usually unknown, we quantify the offsets generated by adopting standard fitting setups. Stellar masses are generally underestimated, which results from underestimating ages. For mixed fitting setups with a variety of star formation histories the median mass recovery at z � 2 − 3 is as decent as ~ 0.1 dex (at fixed IMF), albeit with large scatter. The situation worsens towards lower redshifts, because of the variety of possible star formation histories and ages. At z ~ 0.5 the stellar mass can be underestimated by as much as � 0.6 dex (at fixed IMF). A practical trick to improve upon this figure is to exclude reddening from the fitting parameters, as this helps to avoid unrealistically young and dusty solutions. Stellar masses are underestimated by a smaller amount (~ 0.3 dex at z ~ 0.5). Reddening and the star formation rate should then be determined via a separate fitting. As expected, the recovery of properties is better for passive galaxies, for which e.g. the mass can be fully recovered (within ~ 0.01 dex at fixed IMF) when using a fitting setup including metallicity effects. In both cases of star-forming as well as passive galaxies, the recovery of physical parameters is dependent on the spectral range involved in the fitting. We find that a coverage from the rest-frame UV to the rest-frame near-IR appears to be optimal. We also quantify the effect of narrowing the wavelength coverage or adding and removing filter bands, which can be useful for planning observational surveys. Finally, we provide scaling relations that allow the transformation of stellar masses obtained using different template fitting setups and stellar population models.

ECOSMOG : an efficient code for simulating modified gravity.

Abstract: We introduce a new code, ECOSMOG, to run N-body simulations for a wide class of modified gravity and dynamical dark energy theories. These theories generally have one or more new dynamical degrees of freedom, the dynamics of which are governed by their (usually rather nonlinear) equations of motion. Solving these non-linear equations has been a great challenge in cosmology. Our code is based on the RAMSES code, which solves the Poisson equation on adaptively refined meshes to gain high resolutions in the high-density regions. We have added a solver for the extra degree(s) of freedom and performed numerous tests for the f(R) gravity model as an example to show its reliability. We find that much higher efficiency could be achieved compared with other existing mesh/grid-based codes thanks to two new features of the present code: (1) the efficient parallelisation and (2) the usage of the multigrid relaxation to solve the extra equation(s) on both the regular domain grid and refinements, giving much faster convergence even under much more stringent convergence criteria. This code is designed for performing high-accuracy, high-resolution and large-volume cosmological simulations for modified gravity and general dark energy theories, which can be utilised to test gravity and the dark energy hypothesis using the upcoming and future deep and high-resolution galaxy surveys.

Constraints on f(R) gravity from probing the large-scale structure

Abstract: We study cosmological constraints on metric f(R) gravity models that are designed to reproduce the LCDM expansion history with modifications to gravity described by a supplementary cosmological freedom, the Compton wavelength parameter B_0. We conduct a Markov chain Monte Carlo analysis on the parameter space, utilizing the geometrical constraints from supernovae distances, the baryon acoustic oscillation distances, and the Hubble constant, along with all of the cosmic microwave background data, including the largest scales, its correlation with galaxies, and a probe of the relation between weak gravitational lensing and galaxy flows. The strongest constraints, however, are obtained through the inclusion of data from cluster abundance. Using all of the data, we infer a bound of B_0<0.0011 at the 95% C.L.

Interacting dark energy: constraints and degeneracies

Abstract: In standard cosmologies, dark energy (DE) interacts only gravitationally with dark matter (DM). There could be a nongravitational interaction in the dark sector, leading to changes in the effective DE equation of state, in the redshift dependence of the DM density and in structure formation. We use cosmic microwave background, baryon acoustic oscillation and supernova data to constrain a model where the energy transfer in the dark sector is proportional to the DE density. There are two subclasses, defined by the vanishing of momentum transfer either in the DM or the DE frame. We conduct a Markov-Chain Monte Carlo analysis to obtain best-fit parameters. The background evolution allows large interaction strengths, and the constraints from cosmic microwave background anisotropies are weak. The growth of DM density perturbations is much more sensitive to the interaction, and can deviate strongly from the standard case. However, the deviations are degenerate with galaxy bias and thus more difficult to constrain. Interestingly, the integrated Sachs-Wolfe signature is suppressed since the nonstandard background evolution can compensate for high growth rates. We also discuss the partial degeneracy between interacting DE and modified gravity, and how this can be broken.

Haloes and voids in f(R) gravity

Abstract: In this paper, we study the distribution of dark matter halos and voids using high resolution simulations in f(R) gravity models with the chameleon mechanism to screen the fifth force in dense environment. For dark matter halos, we show that the semi-analytic thin shell condition, with a suitably-defined environment, provides a good approximation to describe the mass and environmental dependence of the screening of the fifth force in halos. Due to stronger gravity, there are far more massive halos and large voids in f(R) models compared with the \Lambda CDM model. The numbers of voids with an effective radius of 15Mpc/h are twice and four times as many as those in \Lambda CDM for f(R) models with |f_{R0}|=1e-5 and 1e-4 respectively. This provides a new means to test the models using the upcoming observational data. We also find that halos inside voids are all unscreened in our simulations, which are ideal objects for the gravity test.

Disentangling non-Gaussianity, bias and general relativistic effects in the galaxy distribution

Abstract: Local non-Gaussianity, parametrized by fNL, introduces a scale-dependent bias that is strongest at large scales, precisely where General Relativistic (GR) effects also become significant. With future data, it should be possible to constrain fNL = O(1) with high redshift surveys. GR corrections to the power spectrum and ambiguities in the gauge used to define bias introduce effects similar to fNL = O(1), so it is essential to disentangle these effects. For the first time in studies of primordial non-Gaussianity, we include the consistent GR calculation of galaxy power spectra, highlighting the importance of a proper definition of bias. We present observable power spectra with and without GR corrections, showing that an incorrect definition of bias can mimic non-Gaussianity. However, these effects can be distinguished by their different redshift and scale dependence, so as to extract the true primordial non-Gaussianity

Mis)interpreting supernovae observations in a lumpy universe

Abstract: Light from ‘point sources’ such as supernovae is observed with a beam width of the order of the sources’ size – typically less than 1 au. Such a beam probes matter and curvature distributions that are very different from coarse-grained representations in N-body simulations or perturbation theory, which are smoothed on scales much larger than 1 au. The beam typically travels through unclustered dark matter and hydrogen with a mean density much less than the cosmic mean, and through dark matter haloes and hydrogen clouds. Using N-body simulations, as well as a Press–Schechter approach, we quantify the density probability distribution as a function of beam width and show that, even for Gpc-length beams of 500 kpc diameter, most lines of sight are significantly underdense. From this we argue that modelling the probability distribution for au-diameter beams is absolutely critical. Standard analyses predict a huge variance for such tiny beam sizes, and non-linear corrections appear to be non-trivial. It is not even clear whether underdense regions lead to dimming or brightening of sources, owing to the uncertainty in modelling the expansion rate which we show is the dominant contribution. By considering different reasonable approximations which yield very different cosmologies, we argue that modelling ultra-narrow beams accurately remains a critical problem for precision cosmology. This could appear as a discordance between angular diameter and luminosity distances when comparing supernova observations to baryon acoustic oscillations or cosmic microwave background distances.

Evolution of the most massive galaxies to z= 0.6 – I. A new method for physical parameter estimation

Abstract: We use principal component analysis (PCA) to estimate stellar masses, mean stellar ages, star formation histories (SFHs), dust extinctions and stellar velocity dispersions for a set of ∼290 000 galaxies with stellar masses greater than 1011 M⊙ and redshifts in the range 0.4 < z < 0.7 from the Baryon Oscillation Spectroscopic Survey (BOSS). We find that the fraction of galaxies with active star formation first declines with increasing stellar mass, but then flattens above a stellar mass of 1011.5 M⊙ at z∼ 0.6. This is in striking contrast to z∼ 0.1, where the fraction of galaxies with active star formation declines monotonically with stellar mass. At stellar masses of 1012 M⊙, therefore, the evolution in the fraction of star-forming galaxies from z∼ 0.6 to the present day reaches a factor of ∼10. When we stack the spectra of the most massive, star-forming galaxies at z∼ 0.6, we find that half of their [O iii] emission is produced by active galactic nuclei. The black holes in these galaxies are accreting on average at ∼0.01 the Eddington rate. To obtain these results, we use the stellar population synthesis models of Bruzual & Charlot to generate a library of model spectra with a broad range of SFHs, metallicities, dust extinctions and stellar velocity dispersions. The PCA is run on this library to identify its principal components over the rest-frame wavelength range 3700–5500 A. We demonstrate that linear combinations of these components can recover information equivalent to traditional spectral indices such as the 4000-A break strength and HδA, with greatly improved signal-to-noise ratio (S/N). In addition, the method is able to recover physical parameters such as stellar mass-to-light ratio, mean stellar age, velocity dispersion and dust extinction from the relatively low S/N BOSS spectra. We examine in detail the sensitivity of our stellar mass estimates to the input parameters in our model library, showing that almost all changes result in systematic differences in logM* of 0.1 dex or less. The biggest differences are obtained when using different population synthesis models – stellar masses derived using Maraston et al. models are systematically smaller by up to 0.12 dex at young ages.

Beyond the plane-parallel and Newtonian approach: wide-angle redshift distortions and convergence in general relativity

Abstract: We extend previous analyses of wide-angle correlations in the galaxy power spectrum in redshift space to include all general relativistic effects. These general relativistic corrections to the standard approach become important on large scales and at high redshifts, and they lead to new terms in the wide-angle correlations. We show that in principle the new terms can produce corrections of nearly 10% on Gpc scales over the usual Newtonian approximation. General relativistic corrections will be important for future large-volume surveys such as SKA and Euclid, although the problem of cosmic variance will present a challenge in observing this.

The BOSS Emission-Line Lens Survey. II. Investigating mass-density profile evolution in the SLACS+BELLS strong gravitational lens sample

Abstract: We present an analysis of the evolution of the central mass-density profile of massive elliptical galaxies from the SLACS and BELLS strong gravitational lens samples over the redshift interval z Almost-Equal-To 0.1-0.6, based on the combination of strong-lensing aperture mass and stellar velocity-dispersion constraints. We find a significant trend toward steeper mass profiles (parameterized by the power-law density model with {rho}{proportional_to}r {sup -{gamma}}) at later cosmic times, with magnitude d /dz = -0.60 {+-} 0.15. We show that the combined lens-galaxy sample is consistent with a non-evolving distribution of stellar velocity dispersions. Considering possible additional dependence of on lens-galaxy stellar mass, effective radius, and Sersic index, we find marginal evidence for shallower mass profiles at higher masses and larger sizes, but with a significance that is subdominant to the redshift dependence. Using the results of published Monte Carlo simulations of spectroscopic lens surveys, we verify that our mass-profile evolution result cannot be explained by lensing selection biases as a function of redshift. Interpreted as a true evolutionary signal, our result suggests that major dry mergers involving off-axis trajectories play a significant role in the evolution of the average mass-density structure of massive early-type galaxies overmore » the past 6 Gyr. We also consider an alternative non-evolutionary hypothesis based on variations in the strong-lensing measurement aperture with redshift, which would imply the detection of an 'inflection zone' marking the transition between the baryon-dominated and dark-matter halo-dominated regions of the lens galaxies. Further observations of the combined SLACS+BELLS sample can constrain this picture more precisely, and enable a more detailed investigation of the multivariate dependences of galaxy mass structure across cosmic time.« less

Cluster density profiles as a test of modified gravity

Abstract: We present a new test of gravitational interactions at the r≃(0.2–20) Mpc scale, around the virial radius of dark matter halos measured through cluster-galaxy lensing of maxBCG clusters from the Sloan Digital Sky Survey (SDSS). We employ predictions from self-consistent simulations of f(R) gravity to find an upper bound on the background field amplitude of |f_R0|<3.5×10^(-3) at the 1D-marginalized 95% confidence level. As a model-independent assessment of the constraining power of cluster profiles measured through weak gravitational lensing, we also constrain the amplitude F_0 of a phenomenological modification based on the profile enhancement induced by f(R) gravity when not including effects from the increased cluster abundance in f(R). In both scenarios, dark-matter-only simulations of the concordance model corresponding to |fR0|=0 and F0=0 are consistent with the lensing measurements, i.e., at the 68% and 95% confidence level, respectively.

Redshift-space distortions in f(R) gravity

Abstract: We use large-volume N-body simulations to predict the clustering of dark matter in redshift space in f(R) modified gravity cosmologies. This is the first time that the non-linear matter and velocity fields have been resolved to such a high level of accuracy over a broad range of scales in this class of models. We find significant deviations from the clustering signal in standard gravity, with an enhanced boost in power on large scales and stronger damping on small scales in the f(R) models compared to general relativity (GR) at redshifts z < 1. We measure the velocity divergence (Pθθ) and matter (Pδδ) power spectra and find a large deviation in the ratios √Pθθ/Pδδ and Pδθ/Pδδ between the f(R) models and GR for 0.03 < k/(h Mpc−1) < 0.5. In linear theory, these ratios equal the growth rate of structure on large scales. Our results show that the simulated ratios agree with the growth rate for each cosmology (which is scale-dependent in the case of modified gravity) only for extremely large scales, k < 0.06 h Mpc−1 at z = 0. The velocity power spectrum is substantially different in the f(R) models compared to GR, suggesting that this observable is a sensitive probe of modified gravity. We demonstrate how to extract the matter and velocity power spectra from the 2D redshift-space power spectrum, P(k, μ), and can recover the non-linear matter power spectrum to within a few per cent for k < 0.1 h Mpc−1. However, the model fails to describe the shape of the 2D power spectrum, demonstrating that an improved model is necessary in order to reconstruct the velocity power spectrum accurately. The same model can match the monopole moment to within 3 per cent for GR and 10 per cent for the f(R) cosmology at k < 0.2 h Mpc−1 at z = 1. Our results suggest that the extraction of the velocity power spectrum from future galaxy surveys is a promising method to constrain deviations from GR.

Galaxy Zoo: the environmental dependence of bars and bulges in disc galaxies

Abstract: We present an analysis of the environmental dependence of bars and bulges in disc galaxies, using a volume-limited catalogue of 15 810 galaxies at z < 0.06 from the Sloan Digital Sky Survey with visual morphologies from the Galaxy Zoo 2 project. We find that the likelihood of having a bar, or bulge, in disc galaxies increases when the galaxies have redder (optical) colours and larger stellar masses, and observe a transition in the bar and bulge likelihoods at M∗ = 2 × 10 10 M� , such that massive disc galaxies are more likely to host bars and bulges. In addition, while some barred and most bulge-dominated galaxies are on the ‘red sequence’ of the colour–magnitude diagram, we see a wider variety of colours for galaxies that host bars. We use galaxy clustering methods to demonstrate statistically significant environmental correlations of barred, and bulge-dominated, galaxies, from projected separations of 150 kpch −1 to 3 Mpch −1 . These environmental correlations appear to be independent of each other: i.e. bulge-dominated disc galaxies exhibit a significant bar–environment correlation, and barred disc galaxies show a bulge–environment correlation. As a result of sparse sampling tests – our sample is nearly 20 times larger than those used previously – we argue that previous studies that did not detect a bar–environment correlation were likely inhibited by small number statistics. We demonstrate that approximately half of the bar–environment correlation can be explained by the fact that more massive dark matter haloes host redder disc galaxies, which are then more likely to have bars; this fraction is estimated to be 50 ± 10 per cent from a mock catalogue analysis and 60 ± 5 per cent from the data. Likewise, we show that the environmental dependence of stellar mass can only explain a smaller fraction (25 ± 10 per cent) of the bar–environment correlation. Therefore, a significant fraction of our observed environmental dependence of barred galaxies is not due to colour or stellar mass dependences, and hence must be due to another galaxy property, such as gas content, or to environmental influences.

The significance of the integrated Sachs–Wolfe effect revisited

Abstract: We revisit the state of the integrated Sachs–Wolfe (ISW) effect measurements in light of newly available data and address criticisms about the measurements which have recently been raised. We update the data set previously assembled by Giannantonio et al. to include new data releases for both the cosmic microwave background and the large-scale structure of the Universe. We find that our updated results are consistent with previous measurements. By fitting a single template amplitude, we now obtain a combined significance of the ISW detection at the 4.4σ level, which fluctuates by ∼0.4σ when alternative data cuts and analysis assumptions are considered. We also make new tests for systematic contaminations of the data, focusing in particular on the issues raised by Sawangwit et al. Amongst them, we address the rotation test, which aims at checking for possible systematics by correlating pairs of randomly rotated maps. We find results consistent with the expected data covariance, no evidence for enhanced correlation on any preferred axis of rotation, and therefore no indication of any additional systematic contamination. We publicly release the results, the covariance matrix and the sky maps used to obtain them.

Characterizing Vainshtein solutions in massive gravity

Abstract: We study static, spherically symmetric solutions in a recently proposed ghost-free model of nonlinear massive gravity We focus on a branch of solutions where the helicity-0 mode can be strongly coupled within certain radial regions, giving rise to the Vainshtein effect We truncate the analysis to scales below the gravitational Compton wavelength, and consider the weak field limit for the gravitational potentials, while keeping all nonlinearities of the helicity-0 mode We determine analytically the number and properties of local solutions that exist asymptotically on large scales, and of local (inner) solutions that exist on small scales We find two kinds of asymptotic solutions, one of which is asymptotically flat, while the other one is not, and also two types of inner solutions, one of which displays the Vainshtein mechanism, while the other exhibits a self-shielding behavior of the gravitational field We analyze in detail in which cases the solutions match in an intermediate region The asymptotically flat solutions connect only to inner configurations displaying the Vainshtein mechanism, while the nonasymptotically flat solutions can connect with both kinds of inner solutions We show furthermore that there are some regions in the parameter space where global solutions do not exist, and characterize precisely in which regions of the phase space the Vainshtein mechanism takes place

Chameleon f(R) gravity in the virialized cluster

Abstract: Current constraints on $f(R)$ gravity from the large-scale structure are at the verge of penetrating into a region where the modified forces become nonlinearly suppressed. For a consistent treatment of observables at these scales, we study cluster quantities produced in chameleon and linearized Hu-Sawicki $f(R)$ gravity dark matter $N$-body simulations. We find that the standard Navarro-Frenk-White halo density profile and the radial power law for the pseudo-phase-space density provide equally good fits for $f(R)$ clusters as they do in the Newtonian scenario. We give qualitative arguments for why this should be the case. For practical applications, we derive analytic relations, e.g., for the $f(R)$ scalar field, the gravitational potential, and the velocity dispersion as seen within the virialized clusters. These functions are based on three degrees of freedom fitted to simulations, i.e., the characteristic density, scale, and velocity dispersion. We further analyze predictions for these fitting parameters from the gravitational collapse and the Jeans equation, which are found to agree well with the simulations. Our analytic results can be used to consistently constrain chameleon $f(R)$ gravity with future observations on virialized cluster scales without the necessity of running a large number of simulations.

Inhomogeneous vacuum energy

Abstract: Vacuum energy remains the simplest model of dark energy which could drive the accelerated expansion of the Universe without necessarily introducing any new degrees of freedom. Inhomogeneous vacuum energy is necessarily interacting in general relativity. Although the 4-velocity of vacuum energy is undefined, an interacting vacuum has an energy transfer and the vacuum energy defines a particular foliation of spacetime. In particular we will discuss cosmological solutions where the background vacuum energy is spatially homogeneous. It is possible to give a consistent description of vacuum dynamics and in particular the relativistic equations of motion for spatially inhomogeneous perturbations given a covariant prescription for the vacuum energy, or equivalently the energy transfer 4-vector, and we construct gauge-invariant vacuum perturbations. We show that any dark energy cosmology can be decomposed into an interacting vacuum+matter cosmology whose inhomogeneous perturbations obey simple first-order equations.

A young massive stellar population around the intermediate-mass black hole ESO 243-49 HLX-1

Abstract: We present Hubble Space Telescope and simultaneous Swift X-ray Telescope observations of the strongest candidate intermediate-mass black hole (IMBH) ESO 243-49 HLX-1. Fitting the spectral energy distribution from X-ray to near-infrared wavelengths showed that the broadband spectrum is not consistent with simple and irradiated disk models, but is well described by a model comprised of an irradiated accretion disk plus a ~106 M ☉ stellar population. The age of the population cannot be uniquely constrained, with both young and old stellar populations allowed. However, the old solution requires excessive disk reprocessing and an extremely small disk, so we favor the young solution (~13 Myr). In addition, the presence of dust lanes and the lack of any nuclear activity from X-ray observations of the host galaxy suggest that a gas-rich minor merger may have taken place less than ~200 Myr ago. Such a merger event would explain the presence of the IMBH and the young stellar population.

Astrophysical tests of gravity: a screening map of the nearby universe

Abstract: Astrophysical tests of modified gravity theories in the nearby universe have been emphasized recently by Hui 2009 and Jain 2011. A key element of such tests is the screening mechanism whereby general relativity is restored in massive halos or high density environments like the Milky Way. In chameleon theories of gravity, including all f(R) models, field dwarf galaxies may be unscreened and therefore feel an extra force, as opposed to screened galaxies. The first step to study differences between screened and unscreened galaxies is to create a 3D screening map. We use N-body simulations to test and calibrate simple approximations to determine the level of screening in galaxy catalogs. Sources of systematic errors in the screening map due to observational inaccuracies are modeled and their contamination is estimated. We then apply our methods to create a map out to 200 Mpc in the Sloan Digital Sky Survey footprint using data from the Sloan survey and other sources. In two companion papers this map will be used to carry out new tests of gravity using distance indicators and the disks of dwarf galaxies. We also make our screening map publicly available.

Natural quintessence in string theory

Abstract: We introduce a natural model of quintessence in string theory where the light rolling scalar is radiatively stable and couples to Standard Model matter with weaker-than-Planckian strength. The model is embedded in an anisotropic type IIB compactification with two exponentially large extra dimensions and TeV-scale gravity. The bulk turns out to be nearly supersymmetric since the scale of the gravitino mass is of the order of the observed value of the cosmological constant. The quintessence field is a modulus parameterising the size of an internal four-cycle which naturally develops a potential of the order (gravitino mass)4, leading to a small dark energy scale without tunings. The mass of the quintessence field is also radiatively stable since it is protected by supersymmetry in the bulk. Moreover, this light scalar couples to ordinary matter via its mixing with the volume mode. Due to the fact that the quintessence field is a flat direction at leading order, this mixing is very small, resulting in a suppressed coupling to Standard Model particles which avoids stringent fifth-force constraints. On the other hand, if dark matter is realised in terms of Kaluza-Klein states, unsuppressed couplings between dark energy and dark matter can emerge, leading to a scenario of coupled quintessence within string theory. We study the dynamics of quintessence in our set-up, showing that its main features make it compatible with observations.

Astrophysical Tests of Modified Gravity: A Screening Map of the Nearby Universe

Abstract: Astrophysical tests of modified modified gravity theories in the nearby universe have been emphasized recently by Hui, Nicolis and Stubbs (2009) and Jain and VanderPlas (2011). A key element of such tests is the screening mechanism whereby general relativity is restored in massive halos or high density environments like the Milky Way. In chameleon theories of gravity, including all f(R) models, field dwarf galaxies may be unscreened and therefore feel an extra force, as opposed to screened galaxies. The first step to study differences between screened and unscreened galaxies is to create a 3D screening map. We use N-body simulations to test and calibrate simple approximations to determine the level of screening in galaxy catalogs. Sources of systematic errors in the screening map due to observational inaccuracies are modeled and their contamination is estimated. We then apply our methods to create a map out to 200 Mpc in the Sloan Digital Sky Survey footprint using data from the Sloan survey and other sources. In two companion papers this map will be used to carry out new tests of gravity using distance indicators and the disks of dwarf galaxies. We also make our screening map publicly available.

Redshift and distances in a ΛCDM cosmology with non-linear inhomogeneities

Abstract: Motivated by the dawn of precision cosmology and the wealth of forthcoming high-precision and volume galaxy surveys, in this paper we study the effects of inhomogeneities on light propagation in a flat Λ cold dark matter (ΛCDM) background. To this end we use exact solutions of Einstein’s equations as derived by Meures & Bruni where, starting from small fluctuations, inhomogeneities arise from a standard growing mode and become non-linear. While the matter distribution in these models is necessarily idealized, there is still enough freedom to assume an arbitrary initial density profile along the line of sight. We can therefore model overdensities and voids of various sizes and distributions, e.g. single harmonic sinusoidal modes, coupled modes and more general distributions in a ΛCDM background. Our models allow for an exact treatment of the light-propagation problem, so that the results are unaffected by approximations and unambiguous. Along lines of sight with density inhomogeneities which average out on scales less than the Hubble radius, we find the distance–redshift relation to diverge negligibly from the Friedmann–Lemaitre–Robertson–Walker (FLRW) result. On the contrary, if we observe along lines of sight which do not have the same average density as the background, we find large deviations from the FLRW distance–redshift relation. Hence, a possibly large systematic might be introduced into the analysis of cosmological observations, e.g. supernovae, if we observe along lines of sight which are typically more or less dense than the average density of the Universe. In turn, this could lead to wrong parameter estimation: even if the cosmological principle is valid, the identification of the true FLRW background in an inhomogeneous universe may be more difficult than usually assumed.

Cosmological Measurements with Forthcoming Radio Continuum Surveys

Abstract: We present forecasts for constraints on cosmological models which can be obtained by forthcoming radio continuum surveys: the wide surveys with the LOw Frequency ARray (LOFAR), Australian Square Kilometre Array Pathfinder (ASKAP) and th e Westerbork Observations of the Deep APERTIF Northern sky (WODAN). We use simulated catalogues appropriate to the planned surveys to predict measurements obtained with the source auto-correlation, the crosscorrelation between radio sources and CMB maps (the Integrated Sachs-Wolfe effect), the cross-correlation of radio sources with foreground object s due to cosmic magnification, and a joint analysis together with the CMB power spectrum and supernovae. We show that near future radio surveys will bring complementary measurements to other experiments, probing different cosmological volumes, and having different systematics. Our results show that the unprecedented sky coverage of these surveys combined should provide the most significant measurement yet of the Integrated Sachs-Wolfe effect. In addition, we show that using the ISW effect will significantly tighten constraints on modifie d gravity parameters, while the best measurements of dark energy models will come from galaxy auto-correlation function analyses. Using the combination of EMU and WODAN to provide a full sky survey, it will be possible to measure the dark energy parameters with an uncertainty of {σ(w0) = 0.05, σ(wa) = 0.12} and the modified gravity parameters {σ(η0) = 0.10, σ(µ0) = 0.05}, assuming Planck CMB+SN(current data) priors. Finally, we show that radio surveys would detect a primordial non-Gaussianity of fNL = 8 at 1-σ and we briefly discuss other promising probes.

Galaxy Zoo : Dust lane early-type galaxies are tracers of recent, gas-rich minor mergers

Abstract: We present the second of two papers concerning the origin and evolution of local early-type galaxies exhibiting dust features. We use optical and radio data to examine the nature of AGN activity in these objects, and compare these with carefully constructed control samples. We find that dust lane early-type galaxies are much more likely to host emission-line AGN than the control samples. Moreover, there is a strong correlation between radio and emission-line AGN activity in dust lane early-types, but not the control samples. Dust lane early-type galaxies show the same distribution of AGN properties in rich and poor environments, suggesting a similar triggering mechanism. By contrast, this is not the case for early-types with no dust features. These findings strongly suggest that dust lane early-type galaxies are starburst systems formed in gas-rich mergers. Further evidence in support of this scenario is provided by enhanced star formation and black hole accretion rates in these objects. We derive radio AGN ages and show that these are younger in dust lane galaxies than in the control sample. Dust lane early-types therefore represent an evolutionary stage between starbursting and quiescent galaxies. In these objects, the AGN has already been triggered but has not as yet completely destroyed the gas reservoir required for star formation.