Day 815

Tuesday, Wednesday.

1501.02798
Assembly bias & redshift-space distortions: impact on cluster dynamics tests of general relativity
Hearin

The RSD of galaxies surrounding massive clusters is emerging as a promising testbed for theories of modified gravity.  Conventional applications of this method rely upon the assumption that the velocity field in the cluster environment is uniquely determined by the cluster mass profile.  Yet, dark matter haloes in N-body simulations are known to violate the assumption that viral mass determines the configuration space distribution, an effect known as assembly bias.  In this letter, show that assembly bias in simulated DM haloes also manifests in velocity space.  In the 1-10 Mpc environment surrounding a cluster, a high-concentration "tracer" haloes exhibit a 10-20% larger pairwise-velocity dispersion profile relative to low-concentration tracer haloes of the same mass.  This difference is comparable to the size of the RSD signal predicted by f(R) models designed to account for the cosmic acceleration.  Use the age matching technique to study how color-selection effects may influence the cluster RSD signal, finding a ~10% effect due to refer satellites preferentially occupying higher mass haloes, and a ~5% effect due to assembly-biased colors of centrals.  In order to use cluster RSD measurements to robustly constrain modified gravity, likely will need to develop empirical galaxy formation models more sophisticated than any in the current literature.

1501.02802
Characterization and correction of charge-induced pixel shifts in DECam
Gruen, Bernstein, Jarvis, Rowe, Vikram, Plazas, Seitz

Interaction of charges in CCDs with the already accumulated charge distribution causes both a flux dependence o the PSF (an increase of observed size with flux, also known as the brighter/fatter effect) and pixel-to-pixel correlations of the noise in flat fields.  Describe these effects in the DECam with charge dependent shifts of effective pixel borders, i.e., the Antilogus+2014 model, which is then fit to measurements of flat-field noise correlations.  The latter fall off approximately as a power-law r^-2.5 with pixel separation r, are isotropic except for an asymmetry in the direct neighbors along rows and columns, are stable in time, and are weakly dependent on wavelength.  They show variations from chip to chip at the 20% level that correlate with the Si resistivity.  The charge shifts predicted by the model cause biased shape measurements, primarily due to their effect on bright stars, at levels exceeding WL science requirements.  Measure the flux dependence of star images and show that the effect can be mitigated by applying the reverse charge shifts at the pixel level during image processing.  Differences in stellar size, however, remain significant due to residuals at larger distance from the centroid.

1501.03014
On the origin of intrinsic alignment in cosmic shear measurements: an analytic argument
Camelio, Lombardi

Galaxy IA can be a severe source of error in WL studies.  The problem has been widely studied by numerical simulations and with heuristic models, but without a clear theoretical justification of its origin and amplitude.  In particular, it is still unclear whether intrinsic alignment of galaxies is dominated by formation and accretion processes or by the effects of the instantaneous tidal field acting upon them.  Investigate this question by developing a simple model of IA for elliptical galaxies, based on the instantaneous tidal field.  Making use of the galaxy stellar distribution function, estimate the IA signal and find that although it has the expected dependence on the tidal field, it is too weak to account for the observed signal.  This is an indirect validation of the standard view that IA is caused by formation and/or accretion processes.

1501.03119
Constraints and tensions in testing general relativity from Planck and CFHTLenS including intrinsic alignment systematics
Dossett, Ishak, Parkinson, Davis

Present constraints on GR at cosmo scales and the impact of IA in CFHTLenS on those constraints.  Consider CMB T data from Planck, galaxy PS from WiggleZ, WL tomography from CFHTLenS, ISW-galaxy X-corrs, and BAO data from 6dF, SDSS DR7, and BOSS DR9.  Use a parameterization of the MG that is binned in z and scale, a parameterization that evolves monotonically in scale but is binned in z, and a functional parameterization that evolves only in z.  Present the results in terms of the MG parameters Q and Sigma.  Employ an IA model with an amplitude A_CHFTLenS that is included in the parameter analysis.  Find an improvement in the constraints on the MG parameters corresponding to 40-53% increase on the figure of merit compared to previous studies, and GR is found consistent with the data at the 95% CL.  The bounds found on A_CFHTLenS are sensitive to whether the MG parameterization is scale dependent, and the correlations between A_CFHTLenS and MG parameters are found weak to moderate.  A_CFHTLENS is found consistent with zero for the 3 MG parmeterizations and when the whole lensing sample is used.  A significantly non-zero A_CFHTLenS for GR and the scale-independent MG parameterization is found when the optimized early-type galaxy sample of Heymans+2013 is used.  Find that the tensions observed in previous studies persist, and there is an indication that CMB data and lensing data prefer different values for MG parameters, particularly for the parameter Sigma.  The analysis of the confidence contours and probability distributions suggest that the bimodality found follows that of the known tension in the sigma_8 parameter.