1609.02924
The effects of physically unrelated near neighbors on the galaxy-galaxy lensing signal
Brainerd
The effects of near neighbors on the gg lensing signal are investigated using a suite of Monte Carlo sims. The redshifts, luminosities, and relative coordinates for the simulated lenses were obtained from a set of galaxies with known spectroscopic redshifts and known luminosities. As expected, when all lenses are assigned a single, fixed redshift, the mean tangential shear is identically equal to the excess surface mass density, scaled by the critical surface mass density: gamma_T=DeltaSigma / Sigma_c. When the lenses are assigned their observed redshifts and Sigma_c is taken to be the critical surface mass density of the central lens, the relationship gamma_T=DeltaSigma/Sigma_c is violated because >~90% of the near neighbors are located at redshifts significantly different from the central lenses. For a given central lens, physically unrelated near neighbors give rise to a ratio of gamma_T to DelaSigma/Sigma_c that spans a wide range of ~0.5 to ~1.5 at projected distances rp~1 Mpc. The magnitude and sense of the discrepancy between gamma_T and DeltaSigma/Sigma_c are functions of both rp and the velocity dispersions of the central lenses, sigma_nu. At large rp, the difference between gamma_T and DeltaSigma/Sigma_c is, on average, much greater for low-sigma_nu central lenses that it is for high-sigma_nu central lenses.
1609.03281
A study of the sensitivity of shape measurements to the input parameters of weak lensing image simulations
Hoeksta, Viola, Herbonnet
Improvements in the accuracy of shape measurements are essential to exploit the statistical power of planned imaging surveys that aim to constrain cosmo parameters using WL by LSS. Although a range of tests can be performed using the measurements, the performance of the algorithm can only be quantified using simulated images. This yields, however, only meaningful results if the simulated images resemble the real observations sufficiently well. In this paper, explore the sensitivity of the multiplicative bias to the input parameters of Euclid-like image sims. Find that algorithms will need to account for the local density of sources. In particular the impact of galaxies below the detection limit warrants further study, because magnification changes their number density, resulting in correlations between the lensing signal and multiplicative bias. Although achieving sub-percent accuracy will require further study, estimate that sufficient archival HST data are available to create realistic population of galaxies.
1609.03388
Halo histories vs. galaxy properties at z=0, I: the quenching of star formation
Tinker, Wentzel, Conroy, Mao
Test whether halo age and galaxy age are correlated at fixed halo and galaxy mass. The formation histories, and thus ages, of DM haloes correlate with their large-scale density rho, an effect known as assembly bias. Test whether this correlation extends to galaxies by measuring the dependence of galaxy stellar age on rho. To clarify the comparison between theory and observation, and to remove the strong environmental effects on satellites, use galaxy group catalogs to identify central galaxies and measure their quenched fraction, f_Q, as a function of LS environment. Models that match halo age to central galaxy age predict a strong positive correlation between f_Q and rho. However, show that the amplitude of this effect depends on the definition of halo age: assembly bias is significantly reduced when removing the effects of splash back haloes --- those halos that are central but have passed through a larger halo or experienced strong tidal encounters. Defining age using halo mass at its peak value rather than current mass removes these effects. In SDSS data, at M_gal>~1e10.0 Msun/h^2, there is a ~5% increase in f_Q from low to high densities, which is in agreement with predictions of DM haloes using peak halo mass. At lower stellar mass there is little to no correlation of f_Q with rho. For these galaxies, age-matching is inconsistent with the data across the wide range the halo formation metrics that we tested. This implies that halo formation history has a small but statistically significant impact on quenching of star formation at high masses, while the quenching process in low-mass central galaxies is uncorrelated with halo formation history.
No comments:
Post a Comment