arXiv subjects

J. A. Peacock

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

155 records · Page 9Linked to original sources

arXiv1996

The Virgo consortium: simulations of dark matter and galaxy clustering

We report on work in progress by the Virgo consortium, a collaboration set up to carry out large simulations of the formation of galaxies and large-scale structure exploiting the latest generation of parallel supercomputers. We show results of $256^3$ particle N-body simulations of the clustering evolution of dark matter in four cold dark matter models with different cosmological parameters. The high resolution and large volume of these simulations allows us to determine reliably the mass autocorrelation function for pair separations in the range $40\hkpc$ to $20\hmpc$. Comparison of these with the observed galaxy correlation function shows that for any of these models to be viable, the distribution of galaxies must be biased relative to the distribution of mass in a non-trivial, scale-dependent fashion. In particular, low $Ω_0$ models require the galaxies to be more ``weakly'' clustered than the mass at small and intermediate pair separations. Simulations which include the evolution of gas show that cold gas knots form with approximately the abundance expected on theoretical grounds, although a few excessively massive objects grow near the centres of rich clusters. The locations where these cold gas knots form are, in general, biased relative to the distribution of mass in a scale-dependent way. Some of these biases have the required sign but they are, for the most part, weaker than is necessary for agreement with observations. The antibias present in our low $Ω_0$ N-body/SPH simulation appears to be related to the merging and disruption of galaxies in rich clusters.

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arXiv1996

The evolution of galaxy clustering

This paper investigates whether nonlinear gravitational instability can account for the clustering of galaxies on large and small scales, and for the evolution of clustering with epoch. No CDM-like spectrum is consistent with the shape of the observed nonlinear spectrum. Unbiased low-density models greatly overpredict the small-scale correlations; high-density models would require a bias which does not vary monotonically with scale. The true linear power spectrum contains a primordial feature at $k\simeq 0.1 \hompc$, and must break quite abruptly to an effective slope of $n\ls -2.3$ on smaller scales This empirical fluctuation spectrum also fits the CFRS data on the evolution of clustering, provided the universe is open with $Ω\simeq 0.3$. Only this case explains naturally how the small-scale spectrum can evolve at the observed rate while retaining the same power-law index. An unbiased open model also matches correctly the large-scale COBE data, and offers an attractively simple picture for the phenomenology of galaxy clustering.

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arXiv1996

Power correlations in cosmology: limits on primordial non-Gaussian density fields

We probe the statistical nature of the primordial density field by measuring correlations between the power in adjacent Fourier modes. For certain types of non-Gaussian field, these k-space correlations would be expected to be more extended than for a Gaussian field, providing a useful discriminatory test for Gaussian fields. We apply this test to the combined QDOT and 1.2-Jy IRAS galaxy survey and find the observed density field to be in good agreement with having Gaussian density fluctuations for modes with k < 0.1 h/Mpc. From this result we are able to set quantitative limits on a class of possible non-Gaussian distributions -- the product of a Gaussian field with an independent stochastic field. The maximum sensitivity is to modulations of a Gaussian field with coherence scales of 30 Mpc/h and the rms modulation on this scale cannot greatly exceed unity. We discuss the improvements to this limit likely to be set by future surveys.

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arXiv1996

Measuring the cosmological constant with redshift surveys

It has been proposed that the cosmological constant $Λ$ might be measured from geometric effects on large-scale structure. A positive vacuum density leads to correlation-function contours which are squashed in the radial direction when calculated assuming a matter-dominated model. We show that this effect will be somewhat harder to detect than previous calculations have suggested: the squashing factor is likely to be $<1.3$, given realistic constraints on the matter contribution to $Ω$. Moreover, the geometrical distortion risks being confused with the redshift-space distortions caused by the peculiar velocities associated with the growth of galaxy clustering. These depend on the density and bias parameters via the combination $β\equiv Ω^{0.6}/b$, and we show that the main practical effect of a geometrical flattening factor $F$ is to simulate gravitational instability with $β_{\rm eff}\simeq 0.5(F-1)$. Nevertheless, with datasets of sufficient size it is possible to distinguish the two effects; we discuss in detail how this should be done. New-generation redshift surveys of galaxies and quasars are potentially capable of detecting a non-zero vacuum density, if it exists at a cosmologically interesting level.

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arXiv1996

Nonlinear evolution of cosmological power spectra

Hamilton et al. have suggested an invaluable scaling formula which describes how the power spectra of density fluctuations evolve into the nonlinear regime of hierarchical clustering. This paper presents an extension of their method to low-density universes and universes with nonzero cosmological constant. We pay particular attention to models with large negative spectral indices, and give a spectrum-dependent fitting formula which is of significantly improved accuracy by comparison with an earlier version of this work. The tendency of nonlinear effects to increase power on small scales is stronger for spectra with more negative spectral indices, and for lower densities. However, for low-density models with a cosmological constant, the nonlinear effects are less strong than for an open universe of the same $Ω$.

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arXiv1996

Inflationary cosmology and structure formation

These lectures cover the basics of inflationary models for the early universe, concentrating particularly on the generation of density fluctuations from scalar-field dynamics. The subsequent gravitational dynamics of these fluctuations in dark matter in a Friedmann model are described, leading to a review of the current situation in confronting inflationary models with the latest data on the clustering of galaxies and other measures of large-scale structure.

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arXiv1995

The small-scale clustering power spectrum and relativistic decays

We present constraints on decaying-particle models in which an enhanced relativistic density allows an $Ω=1$ Cold Dark Matter universe to be reconciled with acceptable values for the Hubble constant. Such models may contain extra small-scale power, which can have important consequences for enhanced object formation at high redshifts. Small-scale galaxy clustering and abundances of high-redshift damped Lyman-$α$ absorption clouds give a preferred range for the mass of any such decaying particle of 2 to 30 keV and a lifetime of 0.5 to 100 years for models with a high Hubble constant ($h>0.75$). A lower Hubble constant, $h \simeq 0.5$, weakens the constraint to $0.5< m < 30$ keV, $0.2 < τ< 500$ years. In permitted versions of the model, reionization occurs at redshifts $\sim 10-200$, and this feature may be of importance in understanding degree-scale CMB anisotropies.

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arXiv1995

AN IMAGING K-BAND SURVEY - II: THE REDSHIFT SURVEY AND GALAXY EVOLUTION IN THE INFRARED

We present a redshift survey of 124 galaxies, from an imaging $K$-band survey complete to $K\simeq 17.3$. The optical-to-infrared colours are consistent with the range expected from synthetic galaxy spectra, although there are some cases of very red nuclei. Our data show no evidence for evolution of the $K$-band luminosity function at $z<0.5$, and the results are well described by a Schechter function with $M_K^*=-22.75\pm0.13+5\log_{10}h$ and $ϕ^*=0.026\pm0.003 h^3 {\rm Mpc^{-3}}$. This is a somewhat higher normalization than has been found by previous workers, and it removes much of the excess in faint $K$ and $B$ counts with respect to a no-evolution model. However, we do find evidence for evolution at $z>0.5$: $M_K^*$ is approximately 0.75 mag. brighter at $z=1$. This luminosity evolution is balanced by a reduced normalization at high redshift. The overall evolution is thus opposite to that expected in simple merger-dominated models.

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arXiv1994

Mapping Cluster Mass Distributions via Gravitational Lensing of Background Galaxies

We present a new method for measuring the projected mass distributions of galaxy clusters. The gravitational amplification is measured by comparing the joint distribution in redshift and magnitude of galaxies behind the cluster with that of field galaxies. We show that the total amplification is directly related to the surface mass density in the weak field limit, and so it is possible to map the mass distribution of the cluster. The method is shown to be limited by discreteness noise and galaxy clustering behind the lens. Galaxy clustering sets a lower limit to the error along the redshift direction, but a clustering independent lensing signature may be obtained from the magnitude distribution at fixed redshift. Statistical techniques are developed for estimating the surface mass density of the cluster. We extend these methods to account for any obscuration by cluster halo dust, which may be mapped independently of the dark matter. We apply the method to a series of numerical simulations and show the feasibility of the approach. We consider approximate redshift information, and show how the mass estimates are degraded.

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arXiv1993

Reconstructing the linear power spectrum of cosmological mass fluctuations

We describe an attempt to reconstruct the initial conditions for the formation of cosmological large-scale structure. The power spectrum of the primordial fluctuations is affected by bias, nonlinear evolution and redshift-space distortions, but we show how these effects can be corrected for analytically. Using eight independent datasets, we obtain excellent agreement in the estimated linear power spectra given the following conditions. First, the relative bias factors for Abell clusters, radio galaxies, optical galaxies and IRAS galaxies must be in the ratios 4.5:1.9:1.3:1. Second, the data require redshift-space distortion: $Ω^{0.6}/b_{ßI} = 1.0 \pm 0.2$. Third, low values of $Ω$ and bias are disfavoured. The shape of the spectrum is extremely well described by a CDM transfer function with an apparent value of the fitting parameter $Ωh =0.25$. Tilted models predict too little power at 100 Mpc wavelengths.

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arXiv1993

An imaging K-band survey - I: The catalogue, star and galaxy counts

We present results from a large area (552\,\sqamin) imaging $K$-band survey to a 5$σ$ limit of $K\simeq 17.3$. We have optical-infrared colours of almost all the objects in the sample. Star-galaxy discrimination is performed and the results used to derive the infrared star and galaxy counts. $K$-band ``no-evolution'' galaxy-count models are constructed and compared with the observed data. In the infrared, there is no counterpart for the large excess of faint galaxies over the no-evolution model seen in optical counts. However, we show that the $K$ counts can be remarkably insensitive to evolution under certain reasonable assumptions. Finally, model predictions for $K$-selected redshift surveys are derived.

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