Day 1181

Thursday.

1611.00366
On the level of cluster assembly bias in SDSS
Zu, Mandelbaum, Simet, Rozo, Rykoff

Recently, several studies have discovered a strong discrepancy between the large-scale clustering biases of two subsamples of galaxy clusters at the same halo mass, spilt by their average projected membership distances R_mem.  The level of this discrepancy significantly exceeds the maximum halo assembly bias signal predicted by LCDM.  In this study, explore whether some of the clustering bias differences could be caused by biases in R_mem due to projection effects from other systems along the LoS.  Throughly investigate the halo assembly bias of the photometrically-detected redMaPPer clusters in SDSS, by defining a new variant of the average membership distance estimator R_mcm-tilde that is more robust against projection effects in the cluster membership identification.    Using the angular mark correlation functions of clusters, show that the large-scale bias differences when splitting by R_mem can be largely attributed to such projection effects.  After splitting by R_mem-tilde, the anomalously large signal is reduced, giving a ratio of 1.02±0.14 between the two clustering biases as measured from WL.  Using a realistic mock cluster catalog, predict that the bias ratio between two R_mem-tilde-split subsamples should be <1.10, which is at least 60% weaker than the maximum halo assembly bias signal (1.24) when split by halo concentration.  Therefore, the results demonstrate that the level of halo assembly bias exhibited by redMaPPer clusters in SDSS is consistent with the LCDM prediction.  With a 10-fold increase in cluster numbers, deeper ongoing surveys will enable a more robust detection of halo assembly bias.  The findings also have important implications for how projection effects and their impact on cluster cosmology can be quantified in photometric cluster catalogs.


1611.00367
The galaxy end sequence
Eales, et al

A common assumption is that galaxies fall in 2 distinct regions on a plot of sSFR versus galaxy stellar mass: a star-forming galaxy main sequence (GMS) and a separate region of 'passive' or 'red and dead galaxies'.  Starting from a volume-limited sample of nearby galaxies designed to contain most of the stellar mass in this volume, and thus being a fair representation of the Universe at the end of 12 billion years of galaxy evolution, investigate the distribution of galaxies in this diagram today.  Show that galaxies follow a strongly curved extended GMS with a steep negative slope at high galaxy stellar masses.  There is a gradual change in the morphologies of the galaxies along this distribution, but there is no clear break between early-type and late-type galaxies.  Examining the other evidence that there are two distinct populations, argue that the 'red sequence' is the result of the colors of galaxies changing very little below a critical value of the sSFR, rather than implying a distinct populations of galaxies, and that Herschel observations, which show at least half of early-type galaxies contain a cool interstellar medium, also imply continuity between early-type and late-type galaxies.  This picture of a unitary population of galaxies requires more gradual evolutionary processes than the rapid quenching processes needed to explain two distinct populations.  Challenge theorists to reproduce the properties of the 'galaxy end sequence'.


1611.00752
Galaxy-galaxy lesning estimators and their covariance properties
Singh, Mandelbaum, Seljak, Solar, Vazquez, Gonzalez

Study the covariance properties of real space correlation function estimators -- primarily galaxy-shear correlations, or galaxy-galaxy lensing -- using SDSS data for both shear catalogs and lenses (specifically the BOSS LOWZ sample).  Using mock catalogs of lenses and sources, disentangle the various contributions to the covariance matrix and compare them wth a simple analytical model.  Show that not subtracting the lensing measurement around random points from the measurement around the lens sample is equivalent to performing the measurement using the density field instead of the over-density field, and that this leads to a significant error increase due to an additional term in the covariance.  Therefore, this subtraction should be performed regardless of its beneficial effects on systematics.  Comparing the error estimates from data and mocks for estimators that involve the overdensity, find that the errors are dominated by the shape noise and lens clustering, that empirically estimated covariances (jackknife and standard deviation across mocks) are consistent with theoretical estimates, and that both the connected parts of the 4-point function and the super-sample covariance can be neglected for the current levels of noise.  While the trade-off between different terms in the covariance depends on the survey configuration (area, source number density), the diagnostics that is used in this work should be useful for future works to test their empirically-determined covariances.