arXiv subjects

J. A. Peacock

Publications and source records attributed to J. A. Peacock.

At least 145 records · Page 8Linked to original sources

arXiv2000

Starburst galaxies and structure in the submillimetre background towards the Hubble Deep Field

We use an 850-micron SCUBA map of the HDF to study the dust properties of optically-selected starburst galaxies at high redshift. Although the 850-micron map does not allow detection of the majority of individual sources, we show that the galaxies with the highest UV star-formation rates are detected statistically, with a flux density of about S_{850}=0.2 mJy for an apparent UV star-formation rate of 1 h^{-2} msun yr^{-1}. This level of submillimetre output indicates that the total star-forming activity is on average a factor of approximately 6 times larger than the rate inferred from the UV output of these galaxies. The general population of optical starbursts is then predicted to contribute at least 25 per cent of the 850-micron background. We carry out a power-spectrum analysis of the map, which yields some evidence for angular clustering of the background source population, but at a level lower than that seen in Lyman-break galaxies. Together with other lines of argument, particularly from the NICMOS HDF data, this suggests that the 850-micron background originates over an extremely wide range of redshifts -- perhaps 1 < z < 6.

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arXiv2000

Halo occupation numbers and galaxy bias

We propose a heuristic model that displays the main features of realistic theories for galaxy bias. We show that the low-order clustering statistics of the dark-matter distribution depend almost entirely on the locations and density profiles of dark-matter haloes. A hypothetical galaxy catalogue depends on (i) the efficiency of galaxy formation, as manifested by the halo occupation number -- the number of galaxies brighter than some sample limit contained in a halo of a given mass; (ii) the location of these galaxies within their halo. The first factor is constrained by the empirical luminosity function of groups. For the second factor, we assume that one galaxy marks the halo centre, with any remaining galaxies acting as satellites that trace the halo mass. These simple assumptions amount to a recipe for non-local bias, in which the probability of finding a galaxy is not a simple function of its local mass density. We have applied this prescription to some CDM models of current interest, and find that the predictions are close to the observed galaxy correlations for a flat $Ω=0.3$ model ($Λ$CDM), but not for an $Ω=1$ model with the same power spectrum ($τ$CDM). This is an inevitable consequence of cluster normalization for the power spectra: cluster-scale haloes of given mass have smaller core radii for high $Ω$, and hence display enhanced small-scale clustering. Finally, the pairwise velocity dispersion of galaxies in the $Λ$CDM model is lower than that of the mass, allowing cluster-normalized models to yield a realistic Mach number for the peculiar velocity field. This is largely due to the strong variation of galaxy-formation efficiency with halo mass that is required in this model.

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arXiv2000

The LBDS Hercules sample of millijansky radio sources at 1.4 GHz: I. Multi-colour photometry

The results are presented of an extensive programme of optical and infrared imaging of radio sources in a complete subsample of the Leiden-Berkeley Deep Survey. The LBDS Hercules sample consists of 72 sources observed at 1.4 GHz, with flux densities S(1.4) >= 1.0 mJy, in a 1.2 deg^2 region of Hercules. This sample is almost completely identified in the g, r, i and K bands, with some additional data available at J and H. The magnitude distributions peak at r ~= 22 mag, K ~= 16 mag and extend down to r ~= 26 mag, K ~= 21 mag. The K-band magnitude distributions for the radio galaxies and quasars are compared with those of other radio surveys. At S(1.4) <~ 1 Jy, the K-band distribution does not change significantly with radio flux density. The sources span a broad range of colours, with several being extremely red (r-K >~ 6). Although small, this is the most optically complete sample of millijansky radio sources available at 1.4 GHz, and is ideally suited to study the evolution of the radio luminosity function out to high redshifts.

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arXiv2000

An analytic model for the epoch of halo creation

In this paper we describe the Bayesian link between the cosmological mass function and the distribution of times at which isolated halos of a given mass exist. By assuming that clumps of dark matter undergo monotonic growth on the time-scales of interest, this distribution of times is also the distribution of `creation' times of the halos. This monotonic growth is an inevitable aspect of gravitational instability. The spherical top-hat collapse model is used to estimate the rate at which clumps of dark matter collapse. This gives the prior for the creation time given no information about halo mass. Applying Bayes' theorem then allows any mass function to be converted into a distribution of times at which halos of a given mass are created. This general result covers both Gaussian and non-Gaussian models. We also demonstrate how the mass function and the creation time distribution can be combined to give a joint density function, and discuss the relation between the time distribution of major merger events and the formula calculated. Finally, we determine the creation time of halos within three N-body simulations, and compare the link between the mass function and creation rate with the analytic theory.

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arXiv2000

Clustering of mass and galaxies

These lectures cover various aspects of the statistical description of cosmological density fields. Observationally, this consists of the point process defined by galaxies, and the challenge is to relate this to the continuous density field generated by gravitational instability in dark matter. The main topics discussed are (1) nonlinear structure in CDM models; (2) statistical measures of clustering; (3) redshift-space distortions; (4) small-scale clustering and bias. The overall message is optimistic, in that simple assumptions for where galaxies should form in the mass density field allow one to understand the systematic differences between galaxy data and the predictions of CDM models.

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arXiv1999

Cosmological galaxy formation

We discuss the first results from two successful simulations of galaxy formation within a cosmological volume. With over 2000 large objects forming in each we have sufficient numbers to reliably produce both galaxy correlation and luminosity functions. We find that the observed galaxy counts are well fitted by these models and that the galaxies display an almost un-evolving correlation function back to a redshift of 3 which closely resembles the featureless observed form and amplitude.

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arXiv1999

Galaxy clustering determined from numerical cosmological simulations

We have simulated the growth of structure in two 100 Mpc boxes for LCDM and SCDM universes. These N-body/SPH simulations include a gaseous component which is able to cool radiatively. A fraction of the gas cools into cold dense objects which we identify as galaxies. In this article we give a preliminary analysis of the clustering behaviour of these galaxies concentrating on the LCDM model. We find a galaxy correlation function which is very close to a power law and which evolves relatively little with redshift. The pairwise dispersions of the galaxies are significantly lower than the dark matter. The LCDM model gives a surprisingly good match to the observational determinations of the galaxy correlation function and pairwise dispersions.

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arXiv1999

A simulation of galaxy formation and clustering

We discuss early results from the first large N-body/hydrodynamical simulation to resolve the formation of galaxies in a cold dark matter universe. The simulation follows the formation of galaxies by gas cooling within dark halos of mass a few times $10^{11}\Msun$ and above, in a flat universe with a positive cosmological constant. Over 2200 galaxies form in our simulated volume of $(100 \Mpc)^3$. Assigning luminosities to the model galaxies using a spectral population synthesis model results in a K-band luminosity function in excellent agreement with observations. The two-point correlation function of galaxies in the simulation evolves very little since $z=3$ and has a shape close to a power-law over four orders of magnitude in amplitude. At the present day, the galaxy correlation function in the simulation is antibiased relative to the mass on small scales and unbiased on large scales. It provides a reasonable match to observations.

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arXiv1999

The 2dF Galaxy Redshift Survey: Spectral Types and Luminosity Functions

We describe the 2dF Galaxy Redshift Survey (2dFGRS), and the current status of the observations. In this exploratory paper, we apply a Principal Component Analysis to a preliminary sample of 5869 galaxy spectra and use the two most significant components to split the sample into five spectral classes. These classes are defined by considering visual classifications of a subset of the 2dF spectra, and also by comparing to high quality spectra of local galaxies. We calculate a luminosity function for each of the different classes and find that later-type galaxies have a fainter characteristic magnitude, and a steeper faint-end slope. For the whole sample we find M*=-19.7 (for Omega=1, H_0=100 km/sec/Mpc), alpha=-1.3, phi*=0.017. For class 1 (`early-type') we find M*=-19.6, alpha=-0.7, while for class 5 (`late-type') we find M*=-19.0, alpha=-1.7. The derived 2dF luminosity functions agree well with other recent luminosity function estimates.

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arXiv1998

Baryonic Signatures in Large-Scale Structure

We investigate the consequences of a non-negligible baryon fraction for models of structure formation in Cold Dark Matter dominated cosmologies, emphasizing in particular the existence of oscillations in the present-day matter power spectrum. These oscillations are the remnants of acoustic oscillations in the photon-baryon fluid before last scattering. For acceptable values of the cosmological and baryon densities, the oscillations modulate the power by up to 10%, with a `period' in spatial wavenumber which is close to Delta k approximately 0.05/ Mpc. We study the effects of nonlinear evolution on these features, and show that they are erased for k > 0.2 h/ Mpc. At larger scales, the features evolve as expected from second-order perturbation theory: the visibility of the oscillations is affected only weakly by nonlinear evolution. No realistic CDM parameter combination is able to account for the claimed feature near k = 0.1 h/ Mpc in the APM power spectrum, or the excess power at 100 Mpc/h wavelengths quoted by several recent surveys. Thus baryonic oscillations are not predicted to dominate existing measurements of clustering. We examine several effects which may mask the features which are predicted, and conclude that future galaxy surveys may be able to detect the oscillatory features in the power spectrum provided baryons comprise more than 15% of the total density, but that it will be a technically challenging achievement.

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arXiv1998

Models for large-scale structure

This paper reviews selected aspects of the growth of cosmological structure, covering the following general areas: (1) expected characteristics of linear density perturbations according to various candidate theories for the origin of structure; (2) low-order theory for statistical measures of fluctuations; (3) formation of nonlinear structures and nonlinear evolution of the mass distribution; (4) the relation between the density field and the galaxy distribution; (5) constraints on cosmological models from galaxy clustering and its evolution.

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arXiv1998

The evolution of clustering and bias in the galaxy distribution

This paper reviews the measurements of galaxy correlations at high redshifts, and discusses how these may be understood in models of hierarchical gravitational collapse. The clustering of galaxies at redshift one is much weaker than at present, and this is consistent with the rate of growth of structure expected in an open universe. If $Ω=1$, this observation would imply that bias increases at high redshift, in conflict with observed $M/L$ values for known high-$z$ clusters. At redshift 3, the population of Lyman-limit galaxies displays clustering which is of similar amplitude to that seen today. This is most naturally understood if the Lyman-limit population is a set of rare recently-formed objects. Knowing both the clustering and the abundance of these objects, it is possible to deduce empirically the fluctuation spectrum required on scales which cannot be measured today owing to gravitational nonlinearities. Of existing physical models for the fluctuation spectrum, the results are most closely matched by a low-density spatially flat universe. This conclusion is reinforced by an empirical analysis of CMB anisotropies, in which the present-day fluctuation spectrum is forced to have the observed form. Open models are strongly disfavoured, leaving $Λ$CDM as the most successful simple model for structure formation.

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arXiv1998

Old high-redshift galaxies and primordial density fluctuation spectra

We have discovered a population of extremely red galaxies at $z\simeq 1.5$ which have apparent stellar ages of $\gs 3$ Gyr, based on detailed spectroscopy in the rest-frame ultraviolet. In order for galaxies to have existed at the high collapse redshifts indicated by these ages, there must be a minimum level of power in the density fluctuation spectrum on galaxy scales. This paper compares the required power with that inferred from other high-redshift populations. If the collapse redshifts for the old red galaxies are in the range $z_c\simeq 6$ -- 8, there is general agreement between the various tracers on the required inhomogeneity on 1-Mpc scales. This level of small-scale power requires the Lyman-limit galaxies to be approximately $ν\simeq 3.0$ fluctuations, implying a very large bias parameter $b\simeq 6$. The high collapse redshifts of the red galaxies as deduced from gravitational collapse provides independent support for the ages estimated from their stellar populations. Such early-forming galaxies are rare, and their contribution to the cosmological stellar density is consistent with an extrapolation to higher redshifts of the star-formation rate measured at $z<5$; there is no evidence for a general era of spheroid formation at extreme redshifts.

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arXiv1998

Evolution of structure in cold dark matter universes

We present an analysis of the clustering evolution of dark matter in four cold dark matter (CDM) cosmologies. We use a suite of high resolution, 17-million particle, N-body simulations which sample volumes large enough to give clustering statistics with unprecedented accuracy. We investigate both a flat and an open model with Omega_0=0.3, and two models with Omega=1, one with the standard CDM power spectrum and the other with the same power spectrum as the Omega_0=0.3 models. The amplitude of primordial fluctuations is set so that the models reproduce the observed abundance of rich galaxy clusters by the present day. The mass 2-point correlation function and power spectrum of all the simulations differ significantly from those of the observed galaxy distribution, in both shape and amplitude. Thus, for any of these models to provide an acceptable representation of reality, the distribution of galaxies must be biased relative to the mass in a non-trivial, scale-dependent, fashion. In the Omega=1 models the required bias is always greater than unity, but in the Omega_0=0.3 models an "antibias" is required on scales smaller than \sim 5\hmpc. The mass correlation functions in the simulations are well fit by recently published analytic models. The velocity fields are remarkably similar in all the models, whether they be characterised as bulk flows, 1-particle or pairwise velocity dispersions. This similarity is a direct consequence of our adopted normalisation. The small-scale pairwise velocity dispersion of the dark matter is somewhat larger than recent determinations from galaxy redshift surveys, but the bulk-flows predicted by our models are broadly in agreement with most available data.

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arXiv1997

Radio galaxies and structure formation

This review discusses three ways in which radio galaxies and other high-redshift objects can give us information on the nature and statistics of cosmological inhomogeneities, and how they have evolved between high redshift and the present: (1) The present-day spatial distribution and clustering of radio galaxies; (2) The evolution of radio-galaxy clustering and biased clustering at high redshift; (3) Measuring density perturbation spectra from the abundances of high-redshift galaxies.

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arXiv1997

Genus Statistics of the Virgo N-body simulations and the 1.2-Jy Redshift Survey

We study the topology of the Virgo N-body simulations and compare it to the 1.2-Jy redshift survey of IRAS galaxies by means of the genus statistic. Four high-resolution simulations of variants of the CDM cosmology are considered: a flat standard model (SCDM), a variant of it with more large-scale power (tCDM), and two low density universes, one open (OCDM) and one flat (LCDM). The fully sampled N-body simulations are examined down to strongly nonlinear scales, both with spatially fixed smoothing, and with an adaptive smoothing technique. While the tCDM, LCDM, and OCDM simulations have very similar genus statistics in the regime accessible to fixed smoothing, they can be separated with adaptive smoothing at small mass scales. In order to compare the N-body models with the 1.2-Jy survey, we extract large ensembles of mock catalogues from the simulations. These mock surveys are used to test for systematic effects in the genus analysis and to establish the distribution of errors of the genus curve. We find that a simple multivariate analysis of the genus measurements is compromised both by non-Gaussian distributed errors and by noise that dominates the covariance matrix. We therefore introduce a principal components analysis of the genus curve. With a likelihood ratio test we find that the 1.2-Jy data favours the LCDM, tCDM and OCDM models compared to SCDM. When genus measurements for different smoothing scales are combined, the SCDM model can be excluded at a 99% confidence level, while the other three models fit the 1.2-Jy data well. (abridged)

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arXiv1997

The structure of galaxy clusters in different cosmologies

We investigate the internal structure of clusters of galaxies in high-resolution N-body simulations of 4 different cosmologies. There is a higher proportion of disordered clusters in critical-density than in low-density universes, although the structure of relaxed clusters is very similar in each. Crude measures of substructure, such as the shift in the position of the centre-of-mass as the density threshold is varied, can distinguish the two in a sample of just 20 or so clusters; it is harder to differentiate between clusters in open and flat models with the same density parameter. Most clusters are in a quasi-steady state within the virial radius and are well-described by the density profile of Navarro, Frenk & White (1995).

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arXiv1997

The Virgo Consortium: The evolution and formation of galaxy clusters

We report on work done by the Virgo consortium, an international collaboration set up in order to study the formation and evolution of Large Scale Structure using N-body simulations on the latest generation of parallel supercomputers. We show results of 256^3 particle simulations of the formation of clusters in four Dark Matter models with different cosmological parameters. Normalizing the models such that one obtains the correct abundance of rich clusters yields an interesting result: The peculiar velocities of the clusters are almost independent of Ω, and depend only weakly on Γ, the shape parameter of the power spectrum. Thus, it is nearly impossible to distinguish between high and low Ωmodels on the basis of the peculiar velocities.

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