1507.01942
An analysis of the evolving comoving number density of galaxies in hydrodynamical simulations
Torrey, ... Ma, Vogelsberger, Springel, Hernquist
The cumulative comoving number-density of galaxies as a function of stellar mass or central velocity dispersion is commonly used to link galaxy populations across different epochs. By assuming that galaxies preserve their number-density in time, one can infer the evolution of their properties, such as masses, sizes, and morphologies. However, this assumption does not hold in the presence of galaxy mergers or when rank ordering is broken owing to variable stellar growth rates. Present an analysis of the evolving comoving number density of galaxy populations found in the Illustris cosmological hydro sim focused on the range 0<z<3. The primary results are as follows: 1) The inferred average stellar mass evolution obtained via a constant comoving number density assumption is systematically biased compared to the merger tree results at the factor of ~2(4) level when tracking galaxies from z0 out to z=2(3); 2) the median number density evolution for galaxy populations tracked forward in time is shallower than for galaxy populations tracked backward in time; 3) A similar evolution in the median number density of tracked galaxy populations is found regardless of whether number density is assigned via stellar mass, stellar velocity dispersion, or DM halo mass; 4) Explicit tracking reveals a large diversity in galaxies' assembly histories that cannot be captured by constant number-density analyses; 5) the significant scatter in galaxy linking methods is only marginally reduced by considering a number of additional physical and observable galaxy properties as realized in the simulation. Provide fits for the forward and backward median evolution in stellar mass and number density and discuss implications of the analysis for interpreting multi-epoch galaxy property observations.
1507.01948
Subhalo abundance matching and assembly bias in the EAGLE simulation
Chaves-Montero, ... Schaye, et al
SHAM is a widely-used method to connect galaxies with DM structures in numerical sims. SHAM predictions agree remarkably well with observations, yet they still lack strong theoretical support. Examine the performance, search for the best implementation, and analyze the key assumptions of SHAM using cosmo sims from EAGLE. Find that V_relax, the highest value of the circular velocity attained by a sub halo while it satisfies a relaxation criterion, is the sub halo property that correlates most strongly with galaxy stellar mass M*. Using this parameter in SHAM, retrieve the real-space clustering of EAGLE to within the statistical uncertainties on scales greater than 2 Mpc for galaxies with 8.77<log10(M*/Msun)<10.77. On the other hand, clustering is overestimated by 30% on scales below 2 Mpc because SHAM slightly over predicts the fraction of satellites in massive haloes. The agreement is even better in redshift space, where the clustering in EAGLE is recovered to within the statistical uncertainties for all masses and separations. Additionally, analyse the dependence of galaxy clustering on properties other than halo mass, i.e., the assembly bias. Demonstrate that assembly bias alters the clustering in EAGLE by 25% and that V_relax captures its effect to within 15%. Trace the small but systematic difference in the predicted clustering of SHAM and EAGLE galaxies to the failure of a fundamental assumption of SHAM: for the same V_relax, central and satellite sub haloes do not host statistically the same galaxies independently of the host halo mass.
1507.01953
Extreme value statistics of weak lensing shear peak counts
Reischke, Matura, Bartelmann
The statistics of peaks in WL maps is a promising technique to constrain cosmo parameters in present and future surveys. Investigate its power when using general extreme value statistics which is very sensitive to the exponential tail of the halo mass function. To this end, use and analytic method to quantify the number of WL peaks caused by galaxy clusters, LSS and observational noise. Doing so, further improve the method in the regime of high S/N ratios dominated by NL structures by accounting for the embedding of those counts into the surrounding shear caused by LSS. Derive the extreme value and order statistics for both over-densities (positive peaks) and under-densities (negative peaks) and provide an optimized criterion to split a wide field survey into sub-fields in order to sample the distribution of extreme values such that the expected objects causing the largest signals are mostly due to galaxy clusters. Find good agreement of the model predictions with a ray-tracing N-body sim. For a Euclid-like survey, find tight constraints on sigma8 and Omega_m with relative uncertainties of ~1e-3. In contrast, the equation of state parameter w_0 can be constrained only with a 10% level, and w_a is out of reach even z info is included.
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