Day 447

Wednesday.

1306.3985
The anisotropic imprint of long-wavelength tensor perturbations on cosmic structure
Dai, Jeong (DH!), Kamionkowski

Inflationary models predict a correlation between primordial density perturbations (scalar metric perturbations) and gravitational waves (tensor metric perturbations) in the form of a scalar-scalar-tensor 3-pt correlation, or bispectrum in Fourier space.  The squeezed limit of this bispectrum implies a quadrupolar asymmetry in the observed local PS for matter and galaxies.  Show (like others before) that an IR divergence in the amplitude of this power asymmetry predicted in single-field slow-roll models is canceled by projection effects when considering the observed PS.  Then further evaluate the non-zero, but finite, residual quadrupolar power asymmetry that remains after the divergences are canceled.  While the quadrupolar power asymmetry is small, it is conceptually important [why/how?].  Calculation moreover clarifies how the predictions for this power asymmetry may change in models with different scalar-scalar-tensor bispectra, and shows that convincing detection of the quadrupolar power asymmetry would role out the single-field slow-roll models of inflation.  [is it detectable?]

1306.3990
Stellar mass-gap as a probe of halo assembly history and concentration: youth hidden among old fossils
Deason, Conroy, Wetzel, Tinker

Investigate the use of the mass-gap statistic (defined as the log difference in mass between the host halo and its most massive satellite sub halo) as a probe of halo age and concentration.  A cosmological N-body sim is used to study N~25k group/cluster sized haloes in the mass range 1e12.5<M_halo/Msun<1e14.5.  In agreement with previous work, find that mass-gap is related to halo formation time and concentration.  On average, older and more highly concentrated haloes have larger mass-gaps, and this trend is stronger than the mass-concentration relation over a similar dynamic range.  However, there is a large amount of scatter owing to the transitory nature of the satellite population, which limits the use of the mass-gap statistic on an object-by-object basis.  For example, find that 20% of very large mass-gap systems (akin to "fossil groups") are young, and have likely experienced a recent merger between a massive satellite and the central galaxy.  Relate halo mass-gap to the observable stellar mass-gap via abundance matching,  Using a galaxy group catalog constructed from SDSS DR7, find that the star formation and structural properties of galaxies at fixed mass show no trend with stellar mass-gap.  This is despite a variation in halo-age of ~3 Gyr over ~1.2 dex in stellar mass-gap.  Find no evidence to suggest that the halo formation history significantly affects galaxy properties.  

1306.3993
Weighing galaxy clusters with gas. I. on the methods of computing hydrostatic mass bias
Lau, Nagai, Nelson

Mass estimates of galaxy clusters from X-ray and SZ assume the intracluster gas is in hydrostatic equilibrium with their gravitational potential.  However, since galaxy clusters are dynamically active objects whose dynamical states can deviate significantly from the equilibrium configuration, the departure from the hydrostatic equilibrium assumption is one of the largest sources of systematic uncertainties in cluster cosmology.  In the literature there has been 2 methods for computing the hydrostatic mass bias based on the Euler and the modified Jeans equations, respectively, and there has been some confusion about the validity of these two methods.  The word "Jeans" was a misnomer, which incorrectly implies that the gas is collisionless.  To avoid further confusion, instead refer these methods as "summation" and "averaging" methods, respectively.  In this work, show that these two methods for computing the hydrostatic mass bias are equivalent by demonstrating that the equation used in the second method can be derived from taking spatial averages of the Euler equation.  Specifically, identify the correspondences of individual terms in these two methods mathematically and show that these correspondences are valid to within a few percent level using hydrodynamical simulations of galaxy cluster formation.  In addition, compute the mass bias associated with the acceleration of gas and show that its contribution is small in the virialized regions in the interior of galaxy cluster, but becomes non-negligible in the cluster outskirts of massive clusters where materials are being actively accreted.  Discuss implications for understanding the origin of hydrostatic mass biases and for characterizing them using hydrodynamical simulations as well as observations.