Day 670

Wednesday.

1406.0510
The distribution of alpha elements in Andromeda dwarf galaxies
Vargas, Geha, Tollerud

Present alpha to Fe abundance ratios for 226 individual red giant branch stars in 9 dwarf galaxies in M31 satellite system.  The abundances are measured from the combined signal of Mg, Si, Ca and Ti lines in Keck/DEIMOS medium-resolution spectra.  This constitutes the first large sample of alpha abundance ratios measured in the M31 satellite system.  The dwarf galaxies in the sample exhibit a variety of alpha abundance ratios, with the average values in each galaxy ranging from approximately solar ([alpha/Fe] ~ +0.0) to alpha-enhanced (~+0.5).  These variations do not show a correlation with internal kinematics, environment, or stellar density.  Confirm radial gradients in the Fe abundance of two galaxies out of the 5 with sufficient data (NGC 185 and And II).  There is only tentative evidence for an alpha abundance radial gradient in NGC 185.  Homogeneously compare results to the MW classical dSphs, finding evidence for wider variation in average alpha abundance.  In the absence of chemical abundances for the M 31 stellar halo, compare to the MW stellar halo.  A stellar halo comprised of disrupted M31 satellites is too metal-rich and inconsistent with the MW halo alpha abundance distribution even if considering only satellites with predominantly old stellar populations.  The M 31 satellite population provides a second system in which to study chemical abundances of dwarf galaxies and reveals a wider variety of abundance patterns than the MW.  

1406.0543
Simulations of weak gravitational lensing - II : including finite support effects in cosmic shear covariance matrices
Harnois-Deraps, van Waerbeke

Investigate and quantify the impact of unite simulation volume on WL two- and four-point statistics.  These finite support (FS) effects are modeled for several estimators, simulation box sizes and source redshifts, and validated against a new large suite of 500 N-body simulations. The comparison reveals that the theoretical model is accurate to better than 5% for the shear correlation function xi+(theta) and its error.  Find that the most important quantities for FS modeling is the ratio between the measured angle theta and the angular size of the simulation box at the source redshift, the shear correlation function xi_+ is suppressed by 5, 10, 20 and 25% for L_box=1000, 500, 250 and 147 Mpc/h respectively.  When it [what?  ratio?] reaches 0.2, the suppression exceeds 25 % even for the largest box.  The same effect is observed in xi_-(theta) bust at much larger angles.  This has important consequences for cosmological analyses using N-body simulations to calibrate the impact of non-linear gravitational clustering or to estimate errors and systematics effects, and should not be overlooked.  Propose simple semi-analytic solutions to correct for these finite box effects with and without the presence of survey masks, and the method can be generalized to any WL estimator.  This offers a graceful solution to the important problem of estimating accurate covariance matrices for WL studies: there is no need to run extra large simulation volumes, as long as the box effects are corrected.