Day 704

Monday.

1407.4811
Velocity bias from the small scale clustering of SDSS-III BOSS galaxies
Guo, Zheng, Zehavi, Dawson, Skibba, Tinker, Weinberg, White, Schneider

Present the measurements and molding of the projected z-space clustering of CMASS galaxies in the SDSS-III BOSS DR11.  For a volume-limited LRG sample in 0.48<z<0.55, perform HOD molding of the small- and intermediate-scale (0.1-60 Mpc/h) projected and z-space 2PCF, with an accurate model built on high resolution N-body simulations.  To interpret the measured z-space distortions, the distribution of galaxy velocities must differ from that of the DM inside haloes of ~1e13-14 Msun/h, i.e., the data require the existence of galaxy velocity bias.  Most notably, central galaxies on average are not at rest wrt the core of their host haloes (defined by the inner 25% of particles around the halo potential minimum), but rather move around it with a 1d velocity dispersion of 0.22 pm0.03 times that of the DM, implying a spatial offset from the center at the level of <1% of the halo virial radius.  The luminous satellite galaxies move more slowly than the DM, with velocities 0.86pm0.08 times those of DM, which suggests that the velocity and spatial distributions of these satellites cannot both be unbiased.  The constraints mainly arise from the FoG effect at NL scales and the smoothing to the Kaiser effect in the translinear regime; the robustness of the results is demonstrated by a variety of tests.  In addition, no clear evidence is found for a strong luminosity dependence of the velocity bias.  Discuss the implications of the existence of galaxy velocity bias for investigations of galaxy formation and cosmology.  

1407.4813
Intrinsic alignments of group and cluster galaxies in photometric surveys
Chisari, Mandelbaum, Strauss, Huff, Bahcall

IA have been shown to contaminate WL observable on linear scales, r>10 Mpc/h, but studies of alignments in the NL regime have thus far been inconclusive.  Present an estimator for extracting the IA signal of galaxies around stacked clusters of galaxies from multi band imaging data.  The estimator removes the contamination caused by galaxies that are gravitationally lensed by the clusters and scattered in z-space due to photometric redshift uncertainties.  It uses posterior probability distributions for the redshifts of the galaxies in the sample and it is easily extended to obtain the WL signal while removing the IA contamination.  Apply this algorithm to groups and clusters of galaxies identified in SDSS Stripe 82 coadded imaging data over 150 sq deg.  Find that the IA signal around stacked clusters in 0.1<z<0.4 is consistent with zero.  In terms of the tidal alignment model of Catelan+2001, set joint constraints on the strength of the alignment and the bias of the lensing groups and clusters on scales between 0.1 and 10 Mpc/h, b_L * C_l * rho_crit = -2e-4 pm 14e-4.  This constrains the contamination fraction of alignment to lensing signal to the range between [-18,23] % below scales of 1 Mpc/h at 95% CL, and this result depends on the photometric redshift quality and selection criteria used to identify background galaxies.  Results are robust to the choice of photometric band in which the shapes are measured (i and r) and to centering on the BCG or on the geometrical center of the clusters.