arXiv subjects
Publications and source records attributed to J. A. Peacock.
Some comments are made on the usefulness or otherwise of the concept of `expanding space' in cosmology. These notes are an expanded version of material first published in 2001 but not previously available online except at www.roe.ac.uk/japwww. Since that personal webpage has been referred to in published work, it seems sensible to give these notes a more permanent home.
The Combined EIS-NVSS Survey Of Radio Sources (CENSORS) is a 1.4GHz radio survey selected from the NRAO VLA Sky Survey (NVSS) and complete to a flux-density of 7.2mJy. It targets the ESO Imaging Survey (EIS) Patch D, which is a 3 by 2 square degree field centred on 09 51 36.0, -21 00 00 (J2000). This paper presents the results of spectroscopic observations of 143 of the 150 CENSORS sources. The primary motivation for these observations is to achieve sufficient spectroscopic completeness so that the sample may be used to investigate the evolution of radio sources. The observations result in secure spectroscopic redshifts for 63% of the sample and likely redshifts (based on a single emission line, for example) for a further 8%. Following the identification of the quasars and star-forming galaxies in the CENSORS sample, estimated redshifts are calculated for the remainder of the sample via the K-z relation for radio galaxies. Comparison of the redshift distribution of the CENSORS radio sources to distributions predicted by the various radio luminosity function evolution models of Dunlop & Peacock 1990, results in no good match. This demonstrates that this sample can be used to expand upon previous work in that field.
It has been claimed that the observed magnitude of the vacuum energy density is consistent with the distribution predicted in anthropic models, in which an ensemble of universes is assumed. This calculation is revisited, without making the assumption that the CMB temperature is known, and considering in detail the possibility of a recollapsing universe. New accurate approximations for the growth of perturbations and the mass function of dark haloes are presented. Structure forms readily in the recollapsing phase of a model with negative Lambda, so collapse fraction alone cannot forbid Lambda from being large and negative. A negative Lambda is disfavoured only if we assume that formation of observers can be neglected once the recollapsing universe has heated to T > 8 K. For the case of positive Lambda, however, the current universe does occupy a extremely typical position compared to the predicted distribution on the Lambda-T plane. Contrasting conclusions can be reached if anthropic arguments are applied to the curvature of the universe, and we discuss the falsifiability of this mode of anthropic reasoning.
Determining an accurate position for a submm galaxy (SMG) is the crucial step that enables us to move from the basic properties of an SMG sample - source counts and 2-D clustering - to an assessment of their detailed, multi-wavelength properties, their contribution to the history of cosmic star formation and their links with present-day galaxy populations. In this paper, we identify robust radio and/or IR counterparts, and hence accurate positions, for over two thirds of the SCUBA HAlf-Degree Extragalactic Survey (SHADES) Source Catalogue, presenting optical, 24-um and radio images of each SMG. Observed trends in identification rate have given no strong rationale for pruning the sample. Uncertainties in submm position are found to be consistent with theoretical expectations, with no evidence for significant additional sources of error. Employing the submm/radio redshift indicator, via a parameterisation appropriate for radio-identified SMGs with spectroscopic redshifts, yields a median redshift of 2.8 for the radio-identified subset of SHADES, somewhat higher than the median spectroscopic redshift. We present a diagnostic colour-colour plot, exploiting Spitzer photometry, in which we identify regions commensurate with SMGs at very high redshift. Finally, we find that significantly more SMGs have multiple robust counterparts than would be expected by chance, indicative of physical associations. These multiple systems are most common amongst the brightest SMGs and are typically separated by 2-6", or 15-50/(sin i) kpc at z ~ 2, consistent with early bursts seen in merger simulations.
We present measurements of the two-point galaxy angular correlation function w(θ) in the COSMOS field. Independent determinations of w(θ) as a function of magnitude limit are presented for both the HST ACS catalog and also for the ground-based data from Subaru and the CFHT. Despite having significantly different masks, these three determinations agree well. At bright magnitudes (IAB<22), our data generally match very well with existing measurements and with mock catalogs based on semi-analytic galaxy formation calculations of Kitzbichler and White from the Millennium Simulation. The exception is that our result is at the upper end of the expected cosmic variance scatter for θ> 10 arcmin, which we attribute to a particularly rich structure known to exist at z~0.8. For fainter samples, however, the level of clustering is somewhat higher than reported by some previous studies: in all three catalogues we find w(θ=1')~0.014 at a median IAB magnitude of 24. At these very faintest magnitudes, our measurements agree well with the latest determinations from the Canada-France Legacy Survey. This level of clustering is approximately double what is predicted by the semi-analytic catalogs (at all angles). The semi-analytic results allow an estimate of cosmic variance, which is too small to account for the discrepancy. We therefore conclude that the mean amplitude of clustering at this level is higher than previously estimated.
We present imaging data and photometry for the COSMOS survey in 15 photometric bands between 0.3um and 2.4um. These include data taken on the Subaru 8.3m telescope, the KPNO and CTIO 4m telescopes, and the CFHT 3.6m telescope. Special techniques are used to ensure that the relative photometric calibration is better than 1% across the field of view. The absolute photometric accuracy from standard star measurements is found to be 6%. The absolute calibration is corrected using galaxy spectra, providing colors accurate to 2% or better. Stellar and galaxy colors and counts agree well with the expected values. Finally, as the first step in the scientific analysis of these data we construct panchromatic number counts which confirm that both the geometry of the universe and the galaxy population are evolving.
We have developed a method to calculate overdensities in multicolour surveys, facilitating a direct comparison of the local density contrast measured using galaxy samples that have different redshift error distributions, i.e. for red and blue, or bright and faint galaxies, respectively. We calculate overdensities in small redshift slices (Delta z =0.02, which at z=0.3 corresponds roughly to Delta r_comoving=53 h^-1 Mpc) for 9176 galaxies with R<=23.65, M_B(Vega)-5log h<=-18, and z<= 0.7, in three COMBO-17 fields (measuring 31'X 31' each). The mean redshift errors of this sample are approximately sigma_z/(1+z)~0.015. In the Chandra Deep Field South we identify a region that is underdense by almost a factor 2 compared to the other two fields in the same redshift range (0.25<~ z <~0.4). This can be used for an investigation of the variation of the colour-dependent luminosity function with environment: We calculate the luminosity function in this redshift range for red sequence and blue cloud galaxies (as defined by Bell et al. 04) in each of the fields separately. While the luminosity function of the blue galaxies remains unaffected by different density contrasts, the luminosity function of the red galaxies clearly has a more positive faint-end slope in the Chandra Deep Field South as compared to the other two COMBO-17 fields. The underdensity there is thus mainly due to a deficiency of faint red galaxies. This result is in qualitative agreement with the trends seen at z=0.1, e.g. in the 2dFGRS (Croton et al. 05), or in the SDSS (Zandivarez et al. 06).
We present the results of a search for galaxy clusters in the first 36 XMM-Newton pointings on the COSMOS field. We reach a depth for a total cluster flux in the 0.5-2 keV band of 3x10-15 ergs cm-2 s-1, having one of the widest XMM-Newton contiguous raster surveys, covering an area of 2.1 square degrees. Cluster candidates are identified through a wavelet detection of extended X-ray emission. Verification of the cluster candidates is done based on a galaxy concentration analysis in redshift slices of thickness of 0.1-0.2 in redshift, using the multi-band photometric catalog of the COSMOS field and restricting the search to z<1.3 and i_AB < 25. We identify 72 clusters and derive their properties based on the X-ray cluster scaling relations. A statistical description of the survey in terms of the cumulative log(N>S)-lg(S) distribution compares well with previous results, although yielding a somewhat higher number of clusters at similar fluxes. The X-ray luminosity function of COSMOS clusters matches well the results of nearby surveys, providing a comparably tight constraint on the faint end slope of alpha=1.93+/-0.04. For the probed luminosity range of 8x10+42 - 2x10+44 ergs s-1, our survey is in agreement with and adds significantly to the existing data on the cluster luminosity function at high redshifts and implies no substantial evolution at these luminosities to z=1.3.
We have developed a method to calculate overdensities in multicolour surveys, and compare the local density contrast measured in galaxy samples with different redshift error distributions. We calculated overdensities for three COMBO-17 fields, and identified a region in the CDFS, where the density is lower by a factor of 2 compared to the other two fields. This is mainly due to a deficiency of faint red galaxies. This result is in agreement with local observations in the 2dF.
We present the maps, source catalogue and number counts of the largest, most complete and unbiased extragalactic submillimetre survey ever undertaken: the 850-micron SCUBA HAlf Degree Extragalactic Survey (SHADES). Using the Submillimetre Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope (JCMT), SHADES mapped two separate regions of sky: the Subaru/XMM-Newton Deep Field (SXDF) and the Lockman Hole East (LH). These SCUBA maps cover 720 square arcmin with an RMS noise level of about 2 mJy and have uncovered >100 submillimetre galaxies. In order to ensure the utmost robustness of the resulting source catalogue, data reduction was independently carried out by four sub-groups drawn within the SHADES team, providing an unprecedented degree of reliability with respect to other SCUBA catalogues available from the literature. Individual source lists from the four groups were combined to produce a robust 120-object SHADES catalogue; an invaluable resource for follow-up campaigns aiming to study the properties of a complete and consistent sample of submillimetre galaxies. We present differential and integral source number counts of submillimetre galaxies and find that the differential counts are better fit with a broken power-law or a Schechter function than with a single power-law; the SHADES data alone significantly show that a break is required at several mJy, although the precise position of the break is not well constrained. We also find that an 850-micron survey complete down to 2 mJy would resolve 20-30 per cent of the Far-IR background into point sources. [abridged]
We present measurements of galaxy clustering at redshift =0.6 using 10360 galaxies with photometric redshifts over an area of 0.78 square degree from the COMBO-17 survey. To obtain a result that is unaffected by redshift uncertainties, we calculate the projected correlation function w(r_p), giving results for red sequence and blue cloud galaxies separately. The correlation function of the red galaxies displays clear deviations from a power law at comoving separations around 1 to 3 h^-1 Mpc, and similar but weaker trends are suggested by the data for the blue galaxies. To interpret these results, we fit the correlation functions with analytical predictions derived from a simple halo occupation model. This combines linear clustering of the underlying mass with a description of the number of galaxies occupying each dark-matter halo (the halo occupation distribution). If the occupation numbers are taken to be a simple power law with N proportional to M^alpha, then alpha ~ 0.5 and alpha ~ 0.2 for red and blue galaxies respectively. These figures are little different from the values required to fit present-day clustering data. The power-spectrum shape is assumed to be known in this exercise, but we allow the data to determine the preferred value of sigma_8, the linear power-spectrum normalization. The average normalization inferred from red and blue galaxies at =0.6 is sigma_8=1.02+-0.17 at zero redshift, consistent with independent estimates of this local value. This agreement can be regarded as a verification of the hierarchical growth of the halo mass function.
Deviations from the gravitational inverse-square law would imprint scale-dependent features on the power spectrum of mass density fluctuations. We model such deviations as a Yukawa-like contribution to the gravitational potential and discuss the growth function in a mixed dark matter model with adiabatic initial conditions. Evolution of perturbations is considered in general non-flat cosmological models with a cosmological constant, and an analytical approximation for the growth function is provided. The coupling between baryons and cold dark matter across recombination is negligibly affected by modified gravity physics if the proper cutoff length of the long-range Yukawa-like force is > 10 h^{-1} Mpc. Enhancement of gravity affects the subsequent evolution, boosting large-scale power in a way that resembles the effect of a lower matter density. This phenomenon is almost perfectly degenerate in power-spectrum shape with the effect of a background of massive neutrinos. Back-reaction on density growth from a modified cosmic expansion rate should however also affect the normalization of the power spectrum, with a shape distortion similar to the case of a non-modified background.
We derive constraints on cosmological parameters using the power spectrum of galaxy clustering measured from the final two-degree field galaxy redshift survey (2dFGRS) and a compilation of measurements of the temperature power spectrum and temperature-polarization cross-correlation of the cosmic microwave background radiation. We analyse a range of parameter sets and priors, allowing for massive neutrinos, curvature, tensors and general dark energy models. In all cases, the combination of datasets tightens the constraints, with the most dramatic improvements found for the density of dark matter and the energy-density of dark energy. If we assume a flat universe, we find a matter density parameter of $Ω_{\rm m}=0.237 \pm 0.020$, a baryon density parameter of $Ω_{\rm b} = 0.041 \pm 0.002$, a Hubble constant of $H_{0}=74\pm2 {\rm kms}^{-1}{\rm Mpc}^{-1}$, a linear theory matter fluctuation amplitude of $σ_{8}=0.77\pm0.05$ and a scalar spectral index of $n_{\rm s}=0.954 \pm 0.023$ (all errors show the 68% interval). Our estimate of $n_{\rm s}$ is only marginally consistent with the scale invariant value $n_{\rm s}=1$; this spectrum is formally excluded at the 95% confidence level. However, the detection of a tilt in the spectrum is sensitive to the choice of parameter space. If we allow the equation of state of the dark energy to float, we find $w_{\rm DE}= -0.85_{-0.17}^{+0.18}$, consistent with a cosmological constant. We also place new limits on the mass fraction of massive neutrinos: $f_ν < 0.105$ at the 95% level, corresponding to $\sum m_ν < 1.2$ eV.
Extracting sources with low signal-to-noise from maps with structured background is a non-trivial task which has become important in studying the faint end of the submillimetre number counts. In this article we study source extraction from submillimetre jiggle-maps from the Submillimetre Common-User Bolometer Array (SCUBA) using the Mexican Hat Wavelet (MHW), an isotropic wavelet technique. As a case study we use a large (11.8 arcmin^2) jiggle-map of the galaxy cluster Abell 2218, with a 850um 1sigma r.m.s. sensitivity of 0.6-1mJy. We show via simulations that MHW is a powerful tool for reliable extraction of low signal-to-noise sources from SCUBA jiggle-maps and nine sources are detected in the A2218 850um image. Three of these sources are identified as images of a single background source with an unlensed flux of 0.8mJy. Further, two single-imaged sources also have unlensed fluxes <2mJy, below the blank-field confusion limit. In this ultradeep map, the individual sources detected resolve nearly all of the extragalactic background light at 850um, and the deep data allow to put an upper limit of 44 sources per arcmin^2 to 0.2mJy at 850um.
We present a power spectrum analysis of the final 2dF Galaxy Redshift Survey, employing a direct Fourier method. The sample used comprises 221,414 galaxies with measured redshifts. We investigate in detail the modelling of the sample selection. A new angular mask is derived, based on revisions to the photometric calibration. The redshift selection function is determined by dividing the survey according to rest-frame colour, and deducing a self-consistent treatment of k-corrections and evolution for each population. The covariance matrix for the power-spectrum estimates is determined using two different approaches to the construction of mock surveys which are used to demonstrate that the input cosmological model can be correctly recovered. We are confident that the 2dFGRS power spectrum can be used to infer the matter content of the universe. On large scales, our estimated power spectrum shows evidence for the `baryon oscillations' that are predicted in CDM models. Fitting to a CDM model, assuming a primordial $n_{s}=1$ spectrum, $h=0.72$ and negligible neutrino mass, the preferred parameters are $Ω_{M} h = 0.168 \pm 0.016$ and a baryon fraction $Ω_{b} /Ω_{M} = 0.185\pm0.046$ (1$σ$ errors). The value of $Ω_{M} h$ is $1σ$ lower than the $0.20 \pm 0.03$ in our 2001 analysis of the partially complete 2dFGRS. This shift is largely due to the signal from the newly-sampled regions of space, rather than the refinements in the treatment of observational selection. This analysis therefore implies a density significantly below the standard $Ω_{M} =0.3$: in combination with CMB data from WMAP, we infer $Ω_{M} =0.231\pm 0.021$. (Abridged.)
The 2dFGRS is used in conjunction with the 2MASSXSC to study the near-IR light and stellar mass content of the local Universe. Mock galaxy catalogues, constructed from cosmological N-body simulations and semi-analytical galaxy formation models, are used to gauge the accuracy with which quantities can be recovered. The mean luminosity densities of the Universe are found to be rho_J=(3.57+/-0.11)*10^8 h Lsol/Mpc^3 and rho_KS=(7.04+/-0.23)*10^8 h Lsol/Mpc^3 (statistical uncertainty). Using the 2PIGG catalogue, the group dynamical mass-to-light ratio in the K_S band is found to increase by a factor of ~3 when going from groups with total bJ-band luminosities of 3*10^10 h^-2 Lsol to rich clusters, which have typical values of Upsilon_K~80 h Upsilon_sol. Taking into account the bias introduced by uncertainties in estimating galaxy stellar masses from luminosities, a value of Omega_* h=(0.99+/-0.03)*10^-3 is measured, assuming that a Kennicutt stellar IMF is applicable to all galaxies. The 2PIGGs are then used to study the distribution of the stellar content of the local Universe. The three main conclusions are: (1) a slowly rising stellar M/L_KS is found with the clusters having the largest value of ~0.6 Upsilon_sol, (2) in contrast, the fraction of mass in stars decreases with increasing group size, reaching ~5*10^-3 h for the rich clusters, and (3) in answer to the question posed in the title, most stellar mass is contained in Local Group-sized objects (M~2*10^12 h^-1 Msol) with only ~2% in clusters with M>5*10^14 h^-1 Msol.
We measure the redshift space reduced void probability function (VPF) for 2dFGRS volume limited galaxy samples covering the absolute magnitude range M_bJ-5logh=-18 to -22. Theoretically, the VPF connects the distribution of voids to the moments of galaxy clustering of all orders, and can be used to discriminate clustering models in the weakly non-linear regime. The reduced VPF measured from the 2dFGRS is in excellent agreement with the paradigm of hierarchical scaling of the galaxy clustering moments. The accuracy of our measurement is such that we can rule out, at a very high significance, popular models for galaxy clustering, including the lognormal distribution. We demonstrate that the negative binomial model gives a very good approximation to the 2dFGRS data over a wide range of scales, out to at least 20h-1Mpc. Conversely, the reduced VPF for dark matter in a LambdaCDM universe does appear to be lognormal on small scales but deviates significantly beyond \approx 4h-1Mpc. We find little dependence of the 2dFGRS reduced VPF on galaxy luminosity. Our results hold independently in both the north and south Galactic pole survey regions.
We measure moments of the galaxy count probability distribution function in the two-degree field galaxy redshift survey (2dFGRS). The survey is divided into volume limited subsamples in order to examine the dependence of the higher order clustering on galaxy luminosity. We demonstrate the hierarchical scaling of the averaged p-point galaxy correlation functions, xibar_p, up to p=6. The hierarchical amplitudes, S_p = xibar_p/xibar_2^{p-1}, are approximately independent of the cell radius used to smooth the galaxy distribution on small to medium scales. On larger scales we find the higher order moments can be strongly affected by the presence of rare, massive superstructures in the galaxy distribution. The skewness S_3 has a weak dependence on luminosity, approximated by a linear dependence on log luminosity. We discuss the implications of our results for simple models of linear and non-linear bias that relate the galaxy distribution to the underlying mass.