1511.01096
Precision measurement of the local bias of dark matter halos
Lazeyras, Wagner, Baldauf, Schmidt
Present accurate measurements of the linear, quadratic, and cubic local bias of DM haloes, using curved "separate universe" N-body sims which effectively incorporate an infinite-wavelength overdensity. This can be seen as an exact implementation of the peak-background split argument. Compare the results with the linear and quadratic bias measured from the halo-matter power spectrum and bispectrum and find good agreement. On the other hand, the standard peak-background split applied to the Sheth+Tormen (1999) and TInker+(2008) halo mass functions matches the measured linear bias parameter only at the level of 10%. The prediction from the excursion set-peaks approach performs much better, which can be attributed to the stochastic moving barrier employed in the excursion set-peaks prediction. Also provide convenient fitting formulas for the nonlinear bias parameters b2(b1) and b3(b1).
1511.01097
Light versus dark in strong-lens galaxies: dark matter haloes that are rounder than their stars
Bruderer, et al
Measure the projected density profile, shape and alignment of the stellar and DM mass distribution in 11 SL galaxies. Find that the projected DM density profile - under the assumption of a Chabrier stellar IMF - shows significant variation from galaxy to galaxy. Those with an outermost image beyond ~10 kpc are very well fit by a projected NFW profile; those with images within 10 kpc appear to be more concentrated than NFW, as expected if their dark haloes contract due to baryonic cooling. Find that over several half-light radii, the DM haloes of these lenses are rounder than their stellar mass distributions. While the haloes are never more elliptical than e_dm=0.2, their stars can extend to e*>0.2. Galaxies with high DM ellipticity and weak external shear show strong alignment between light and dark; those with strong shear (gamma>0.1) can be highly misaligned. This is reassuring since isolated misaligned galaxies are expected to be unstable. The results provide a new constraint on galaxy formation models For a given cosmology, these must explain the origin of both very round DM haloes and misaligned strong-lens systems.
1511.01102
Improving Cosmological distance measurements using twin type Ia supernovae
Fakhouri, et al
A novel type of SNIa standardization. Begin with a well-measured set of SNe, find pairs whose spectra match well across the entire optical window, and then test whether this leads to a smaller dispersion in their absolute brightnesses. This analysis is completed in a blinded faschon, ensuring that decisions made in implementing the method do not inadvertently bias the result. Find that pairs of SNe with more closely matched spectra indeed have reduced brightness dispersion. Able to standardize this initial set of SNe to 0.083±0.012 mags, implying a dispersion of 0.072±0.010 magnitudes in the absence of peculiar velocities. Estimate that with larger number of comparison SNe, e.g., using the final SNFactory spectrophotometric dataset as a reference, this method will be capable of standardizing high-z supernovae to within 0.06±0.07 mags. These results imply that at least 3/4 of the variance in Hubble residuals in current supernova cosmology analyses is due to previously unaccounted-for astrophysics differences among the SNe.
1511.01352
Observational biases in flux magnification measurements
Hildebrandt
Flux magnification is an interesting complement to shear-based lensing measurements, especially at high z where sources are harder to resolve. One measures either changes in the source density (magnification bias) or in the shape of the flux distribution (e.g., magnitude-shift). The interpretation of these measurements relies on theoretical estimate of how the observables change under magnification. Present simulations to create multi-band photometric mock catalogues of Lyman-break galaxies in a CFHTLenS-like survey that include several observational effects that can change these relations, making simple theoretical estimates unusable. In particular, show how the magnification bias can be affected by photometric noise, color selection, and dust extinction. Find that a simple measurement of the slope of the number-counts is not sufficient for the precise interpretation of virtually all observations of magnification bias. Also explore how sensitive the shift in the mean magnitude of a source sample in different photometric bands is to magnification including the same observational effects. Again, find significant deviation from simple analytical estimates. Also discover a wavelength-dependence of the magnitude-shift effect when applied to a color-selected noise source sample. Such an effect can mimic the reddening by dust in the lens. It has to be disentangled from the dust extinction before the magnitude-shift/color-excess can be used to measure the distribution of either dark matter or extragalactic dust. Using simulations like the ones presented here these observational effects can be studied and eventually removed from observations making precise measurement of flux magnification possible.
1511.01465
Linear response to long wavelength fluctuations using curvature simulations
Baldauf, Seljak, Senatore, Zaldarriaga
Study the local response to long wavelength fluctuations in cosmo N-body sims, focusing on the matter and halo power spectra, haloes abundance and NL transformations of the density field. The long wavelength mode is implemented using an effective curved cosmology and a mapping of time and distances. The method provides an alternative, most probably more precise, way to measure the isotropic halo biases. Limiting to the linear case, find a generally good agreement between the biases obtained from the curvature method and the traditional power spectrum method at the level of a few precent. Also study the response of halo counts to changes in the variance of the field and find that the slope of the relation between the responses to density and variance differed form the naive derivation assuming a universal mass function by 18%. This has implications for measurements of the amplitude of local non-Gaussianity using scale dependent bias. Also analyze the halo PS and halo-DM cross-spectrum response to long wavelength fluctuations and derive second order halo bias from it, as well as the super-sample variance contribution to the galaxy PS covariance matrix.
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