J. D. Cohn

Cluster galaxy dynamics and the effects of large-scale environment

Advances in observational capabilities have ushered in a new era of multi-wavelength, multi-physics probes of galaxy clusters and ambitious surveys are compiling large samples of cluster candidates selected in different ways. We use a high-resolution N-body simulation to study how the influence of large-scale structure in and around clusters causes correlated signals in different physical probes and discuss some implications this has for multi-physics probes of clusters (e.g. richness, lensing, Compton distortion and velocity dispersion). n n n nWe pay particular attention to velocity dispersions, matching galaxies to subhaloes which are explicitly tracked in the simulation. We find that not only do haloes persist as subhaloes when they fall into a larger host, but groups of subhaloes retain their identity for long periods within larger host haloes. The highly anisotropic nature of infall into massive clusters, and their triaxiality, translates into an anisotropic velocity ellipsoid: line-of-sight galaxy velocity dispersions for any individual halo show large variance depending on viewing angle. The orientation of the velocity ellipsoid is correlated with the large-scale structure, and thus velocity outliers correlate with outliers caused by projection in other probes. We quantify this orientation uncertainty and give illustrative examples. Such a large variance suggests that velocity dispersion estimators will work better in an ensemble sense than for any individual cluster, which may inform strategies for obtaining redshifts of cluster members. We similarly find that the ability of substructure indicators to find kinematic substructures is highly viewing angle dependent. While groups of subhaloes which merge with a larger host halo can retain their identity for many Gyr, they are only sporadically picked up by substructure indicators. n n n nWe discuss the effects of correlated scatter on scaling relations estimated through stacking, both analytically and in the simulations, showing that the strong correlation of measures with mass and the large scatter in mass at fixed observable mitigate line-of-sight projections.

Covariant quantization of supersymmetric string theories: The spinor field of the Ramond-Neveu-Schwarz model

Abstract A brief review of superconformal field theory and superstrings is presented. The spacetime spinor contribution to the fermion vertex operator is constructed and the four-fermion amplitude is calculated using the differential equation method in the SO(1, 9) current algebra. The spinor field is also described as a vertex operator in the bosonization of the Ramond-Neveu-Schwarz fermions, and the two-cocycle for the fermion vertex is given.

The Clustering of Massive Halos

The clustering properties of dark matter halos are a firm prediction of modern theories of structure formation. We use two large-volume, high-resolution N-body simulations to study how the correlation function of massive dark matter halos depends on their mass, assembly history, and recent merger activity. We find that halos with the lowest concentrations are currently more clustered than those of higher concentration, the size of the effect increasing with halo mass; this agrees with trends found in studies of lower mass halos. The clustering dependence on other characterizations of the full mass accretion history appears weaker than the effect with concentration. Using the integrated correlation function, marked correlation functions, and a power-law fit to the correlation function, we find evidence that halos that have recently undergone a major merger or a large mass gain have slightly enhanced clustering relative to a randomly chosen population with the same mass distribution.

Constraints on Galaxy Density Profiles from Strong Gravitational Lensing: The Case of B1933+503

We consider a wide range of parametric mass models for B1933+503, a 10-image radio lens, and identify shared properties of the models with the best fits. The approximate rotation curves vary by less than 8.5% from the average value between the innermost and the outermost image (1.5-4.1 h-1 kpc) for models within 1 σ of the best fit, and the radial dependence of the shear strength and angle also have common behavior for the best models. The time delay between images 1 and 6, the longest delay between the radio cores, is Δt = 10.6 h-1 days (Ω0 = 0.3, λ0 = 0.7), including all the modeling uncertainties. Deeper infrared observations, to more precisely register the lens galaxy with the radio images and to measure the properties of the Einstein ring image of the radio sources host galaxy, would significantly improve the model constraints and further reduce the uncertainties in the mass distribution and time delay.

Reconstructing baryon oscillations: A Lagrangian theory perspective

Recently Eisenstein and collaborators introduced a method to reconstruct the linear power spectrum from a nonlinearly evolved galaxy distribution in order to improve precision in measurements of baryon acoustic oscillations. We reformulate this method within the Lagrangian picture of structure formation, to better understand what such a method does, and what the resulting power spectra are. We show that reconstruction does not reproduce the linear density field, at second order. We however show that it does reduce the damping of the oscillations due to nonlinear structure formation, explaining the improvements seen in simulations. Our results suggest that the reconstructed power spectrum is potentially better modeled as the sum of three different power spectra, each dominating over different wavelength ranges and with different nonlinear damping terms. Finally, we also show that reconstruction reduces the mode-coupling term in the power spectrum, explaining why miscalibrations of the acoustic scale are reduced when one considers the reconstructed power spectrum.

Simulating subhaloes at high redshift: merger rates, counts and types

Galaxies are believed to be in one-to-one correspondence with simulated dark matter subhaloes. We use high-resolutionN-body simulations of cosmological volumes to calculate the statistical properties of subhalo (galaxy) major mergers at high redshift (z = 0.6–5). We measure the evolution of the galaxy merger rate, finding that it is much shallower than the merger rate of dark matter host haloes at z> 2.5, but roughly parallels that of haloes at z< 1.6. We also track the detailed merger histories of individual galaxies and measure the likelihood of multiple mergers per halo or subhalo. We examine satellite merger statistics in detail: 15–35 per cent of all recently merged galaxies are satellites, and satellites are twice as likely as centrals to have had a recent major merger. Finally, we show how the differing evolution of the merger rates of haloes and galaxies leads to the evolution of the average satellite occupation per halo, noting that for a fixed halo mass, the satellite halo occupation peaks at z ∼ 2.5.

N=2 super-Riemann surfaces

Abstract Super-Riemann surfaces are generalized to N ⩾ 2. Basis differentials and tensors are defined. For even N , a distinction between general O( N ) surfaces and SO( N ) (untwisted) surfaces arises. This is treated in detail for N = 2, and some examples are given. There is a short discussion of the N = 2 extension of SL 2 and Beltrami differentials.

Dark matter halo abundances, clustering and assembly histories at high redshift

We use a suite of high-resolution N-body simulations to study the properties, abundance and clustering of high-mass haloes at high redshift, including their mass assembly histories and mergers. We find that the analytic form which best fits the abundance of haloes depends sensitively on the assumed definition of halo mass, with common definitions of halo mass differing by a factor of 2 for these low-concentration, massive haloes. A significant number of massive haloes are undergoing rapid mass accretion, with major merger activity being common. We compare the mergers and mass accretion histories to the extended Press–Schechter formalism. n n n nWe consider how major merger induced star formation or black hole accretion may change the distribution of photon production from collapsed haloes, and hence reionization, using some simplified examples. In all of these, the photon distribution for a halo of a given mass acquires a large scatter. If rare, high-mass haloes contribute significantly to the photon-production rates, the scatter in photon-production rate can translate into additional scatter in the sizes of ionized bubbles.

The formation histories of galaxy clusters

Abstract A sample of hundreds of simulated galaxy clusters is used to study the statistical properties of galaxy cluster formation. Individual assembly histories are discussed, the degree of virialization is demonstrated and various commonly used formation times are measured and inter-compared. In addition, the fraction of clusters which have “recently” undergone a major merger or significant mass jump is calculated as a function of lookback time and interval. The fraction of three- and four-body mergers is also studied.

Photometric redshift requirements for lens galaxies in galaxy–galaxy lensing analyses

Weak gravitational lensing is a valuable probe of galaxy formation and cosmology. Here we quantify the effects of using photometric redshifts (photo-z) in galaxy–galaxy lensing, for both sources and lenses, both for the immediate goal of using galaxies with photo-z as lenses in the Sloan Digital Sky Survey (SDSS) and as a demonstration of methodology for large, upcoming weak lensing surveys that will by necessity be dominated by lens samples with photo-z. We calculate the bias in the lensing mass calibration as well as consequences for absolute magnitude (i.e. k-corrections) and stellar mass estimates for a large sample of SDSS Data Release 8 (DR8) galaxies. The redshifts are obtained with the template-based photo-z code zebra on the SDSS DR8 ugriz photometry. We assemble and characterize the calibration samples (∼9000 spectroscopic redshifts from four surveys) to obtain photometric redshift errors and lensing biases corresponding to our full SDSS DR8 lens and source catalogues. Our tests of the calibration sample also highlight the impact of observing conditions in the imaging survey when the spectroscopic calibration covers a small fraction of its footprint; atypical imaging conditions in calibration fields can lead to incorrect conclusions regarding the photo-z of the full survey. n n n nFor the SDSS DR8 catalogue, we find σΔz/(1+z)= 0.096 and 0.113 for the lens and source catalogues, with flux limits of r= 21 and 21.8, respectively. The photo-z bias and scatter is a function of photo-z and template types, which we exploit to apply photo-z quality cuts. By using photo-z rather than spectroscopy for lenses, dim blue galaxies and L* galaxies up to z∼ 0.4 can be used as lenses, thus expanding into unexplored areas of parameter space. We also explore the systematic uncertainty in the lensing signal calibration when using source photo-z, and both lens and source photo-z; given the size of existing training samples, we can constrain the lensing signal calibration (and therefore the normalization of the surface mass density) to within 2 and 4xa0per cent, respectively.