arXiv subjects

R. E. Smith

Publications and source records attributed to R. E. Smith.

6 recordsLinked to original sources

arXiv2023

Euclid: Modelling massive neutrinos in cosmology -- a code comparison

The measurement of the absolute neutrino mass scale from cosmological large-scale clustering data is one of the key science goals of the Euclid mission. Such a measurement relies on precise modelling of the impact of neutrinos on structure formation, which can be studied with $N$-body simulations. Here we present the results from a major code comparison effort to establish the maturity and reliability of numerical methods for treating massive neutrinos. The comparison includes eleven full $N$-body implementations (not all of them independent), two $N$-body schemes with approximate time integration, and four additional codes that directly predict or emulate the matter power spectrum. Using a common set of initial data we quantify the relative agreement on the nonlinear power spectrum of cold dark matter and baryons and, for the $N$-body codes, also the relative agreement on the bispectrum, halo mass function, and halo bias. We find that the different numerical implementations produce fully consistent results. We can therefore be confident that we can model the impact of massive neutrinos at the sub-percent level in the most common summary statistics. We also provide a code validation pipeline for future reference.

astro-ph.CO↗

arXiv2005

The impact of halo shapes on the bispectrum in cosmology

(Abridged) We use the triaxial halo model of Smith & Watts to investigate the impact of dark matter halo shapes on the bispectrum. Analytic expressions are derived and evaluated through numerical integration. Two models for the ellipsoidal halo profiles are considered: a toy model designed to isolate the effects of halo shape on the clustering alone; and the realistic model of Jing & Suto. For equilateral k-space triangles, we show that the predictions of the triaxial model are suppressed, relative to the spherical model, by roughly ~7% and ~4% for the two profiles respectively. The reduced bispectrum as a function of triangle configuration is found to be very sensitive to halo shapes on small scales. The generic features are that, relative to the spherical model, the signal is suppressed for k-space triangles that are close to equilateral and boosted for configurations that are colinear. This appears to be a unique signature of halo triaxiality. The galaxy bispectrum is also explored. Two models for the halo occupation distribution (HOD) are considered: the binomial distribution of Scoccimarro et al. and the Poisson satellite model of Kravtsov et al. Our predictions show that the galaxy bispectrum is also sensitive to halo shapes, although relative to the mass, the effects are reduced. The HOD of Kravtsov et al. is found to be more sensitive. This owes to the mean number of galaxies per halo being a steeper function of mass in this model, hence giving stronger weight to the high mass (more triaxial) haloes. Interestingly, the functional form is not strongly sensitive to the details of the HOD, but is mainly determined by the halo shape. However, a combination of measurements made on different scales and for different k-space triangles are sensitive to both halo shape and the HOD.

astro-ph↗

arXiv2004

Triaxial haloes, intrinsic alignments and the dark matter power spectrum

(Abridged) We develop the halo model of large-scale structure to include triaxial dark matter haloes and their intrinsic alignments. As a direct application we derive general expressions for the power spectrum. This we calculate for two different triaxial density profiles. The first is a toy-model that allows us to isolate the dependence of clustering on halo shape alone and the second is the more realistic model of Jing & Suto (2002). In both cases, we find that the effects of triaxiality are manifest as a suppression of power at the level of ~5% on scales k~1-10 h/Mpc. When considered by mass, we find that for the first model the power is suppressed and that this is more significant for high mass haloes. For the Jing & Suto model, we find a similar suppression on large scales, but followed by a sharp amplification on small scales at the level of ~10-15%, for the highest mass haloes. One of the important features of our formalism is that it allows for the self-consistent inclusion of the intrinsic alignments of haloes. The alignments are specified through the correlation function of halo seeds. We develop a useful toy model for this and make estimates of the alignment contribution to the power spectrum. Further, through consideration of the case where all haloes are perfectly aligned, we calculate the maximum contribution to the clustering and find the hard limit of <10%.

astro-ph↗

arXiv2004

Spectro-photometric and Weak Lensing Survey of a Supercluster and Typical Field Region -- I. Spectroscopic Redshift Measurements

We present results from a spectroscopic study of ~4000 galaxies in a 6.2 square degree field in the direction of the Aquarius supercluster and a smaller typical field region in Cetus, down to R<19.5. Galaxy redshifts were measured using the Two Degree Field system on the Anglo-Australian Telescope, and form part of our wider efforts to conduct a spectro-photometric and weak gravitational lensing study of these regions. At the magnitude limit of the survey, we are capable of probing L_* galaxies out to z~0.4. We construct median spectra as a function of various survey parameters as a diagnostic of the quality of the sample. We use the redshift data to identify galaxy clusters and groups within the survey volume. In the Aquarius region, we find a total of 48 clusters and groups, of which 26 are previously unknown systems, and in Cetus we find 14 clusters and groups, of which 12 are new. We estimate centroid redshifts and velocity dispersions for all these systems. In the Aquarius region, we see a superposition of two strong superclusters at z=0.08 and z=0.11, which both have estimated masses and overdensities similar to the Corona Borealis supercluster.

astro-ph↗

arXiv2003

Stable clustering, the halo model and nonlinear cosmological power spectra

We present the results of a large library of cosmological N-body simulations, using power-law initial spectra. The nonlinear evolution of the matter power spectra is compared with the predictions of existing analytic scaling formulae based on the work of Hamilton et al. The scaling approach has assumed that highly nonlinear structures obey `stable clustering' and are frozen in proper coordinates. Our results show that, when transformed under the self-similarity scaling, the scale-free spectra define a nonlinear locus that is clearly shallower than would be required under stable clustering. Furthermore, the small-scale nonlinear power increases as both the power-spectrum index n and the density parameter Omega decrease, and this evolution is not well accounted for by the previous scaling formulae. This breakdown of stable clustering can be understood as resulting from the modification of dark-matter haloes by continuing mergers. These effects are naturally included in the analytic `halo model' for nonlinear structure; using this approach we are able to fit both our scale-free results and also our previous CDM data. This approach is more accurate than the commonly-used Peacock--Dodds formula and should be applicable to more general power spectra. Code to evaluate nonlinear power spectra using this method is available from http://as1.chem.nottingham.ac.uk/~res/software.html Following publication, we will make the power-law simulation data available through the Virgo website http://www.mpa-garching.mpg.de/Virgo

astro-ph↗

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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