Day 420

Thursday.

1304.6087
The abundance of voids and the excursion set formalism
Jennings, Li, Hu

Measure number density of voids in N-body sims of LCDM.  Define voids as spherical regions of rho_v=0.2 rho_m around density minima in order to relate results to the predicted abundances using the excursion set formalism.  Using a linear under-density of delta_v = -2.7, from a spherical evolution model, find that a volume conserving model (which does not conserve number density in the mapping from the linear to the NL regime) matches the measured abundance to within 16% for a range of void radii 1<r<15.  This model fixes the volume fraction of the universe which is in voids and assumes that voids of a similar size merge as they expand by a factor of 1.7 to achieve a NL density of rho_v = 0.2 rho_m today.  Find that the model of Sheth & van de Weygaert for the number density of voids greatly over-predicts the abundances over the same range of scales.  Find that the volume conserving model works well at matching the number density of voids measured from the simulations at higher redshifts, z=0.5 and 1, as well as correctly predicting the abundances to within 25% in a simulation of a matter dominated Omega_m = 1 universe.  Examine the abundance of voids in the halo distribution and find fewer small, r<10 Mpc/h, voids and many more large, r>10 Mpc/h, voids compared to the DM.  These results indicate that voids identified in the halo or galaxy distribution are related to the underlying void distribution in the DM in a complicated way which merits further study if voids are to be used as a precision probe of cosmology.

1304.6696
Studying Inter-cluster galaxy filaments through stacking GMBCG galaxy cluster pairs
Zhang, Dietrich, McKay, Sheldon, Nguyen

Present method to study the photometric properties of galaxies in filaments by stacking the galaxy populations between pairs of galaxy clusters.  This method can detect the inter-cluster filament galaxy overdensity with a significance of ~5 sigma out to z=0.40.  Using this approach, study the g-r color and luminosity distribution of filament galaxies as a function of redshift.  Consistent with expectation, filament galaxies are bimodal in their color distribution and contain a larger blue galaxy population than cluster galaxies.  More interestingly, the observed filament population shows redshift evolution at 0.12<z<0.40: the blue galaxy fraction increases at higher redshift: a filament "Butcher Oemler Effect".  Test the dependence of the observed filament density on the richness of the cluster pair: richer clusters are connected by higher density filaments.  Also test the spatial dependence of filament galaxy overdensity: This quantity decreases when moving away from the inter-cluster axis between a cluster pair.  This method provides an economical way to probe the photometric properties of filament galaxies and should prove useful for upcoming projects like DES.