Day 1347

Wednesday.  Thursday.

1712.01846
Testing gravity on cosmological scales with cosmic shear, cosmic microwave background anisotropies, and redshift-space distortions
Ferté, Kirk, Liddle, Zuntz

Use a range of cosmological data to constrain phenomenological modifications to GR on cosmological scales, through modifications to the Poisson and lensing equations.  Include CMB anisotropies measurements from the Planck satellite, cosmic shear from CFHTLenS and DES-SV, and z-space distortions from BOSS DR12 and the 6dF galaxy survey.  Find no evidence of departures from GR, with the modified gravity parameters constrained to Sigma=-0.01-0.04+0.05 and m=-0.06±0.18.  Also forecast the sensitivity of the full five-year DES and of an LSST-like experiment to those parameters, showing a substantial expected improvement in the constrain on Sigma.


1712.01989
Using velocity dispersion to estimate halo mass: is the Local Group in tension with $\Lambda$ CDM?
Elahi, Power, Lagos, Poulton, Robotham

Satellite galaxies are commonly used as tracers to measure the line-of-sight velocity dispersion (sigma_LOS) of the DM halo associated with their central galaxy, and thereby to estimate the halo's mass.  Recent observational dispersion estimates of the Local Group, including the MW and M31, suggest sigma~50 km/s, which is surprisingly low when compared to the theoretical expectations of sigma~100s km/s for systems of their mass.  Does this pose a problem for LCDM?  Explore this tension using the SURVS suite of N-body sims, containing over 10k (sub)halos with well tracked orbits.  Test how well a central galaxy's host halo velocity dispersion can be recovered yb sampling sigma_LOS of sub haloes and surrounding haloes.  The results demonstrate that sigma_LOS is a biased mass proxy.  Define an optimal window in v_LOS and projected distance D_p -- 0.5<~ D_p/R_vir <~1.0 and v_LOS <~0.5 V_esc, where R_vir is the viral radius and V_esc is the escape velocity -- such that the scatter in LOS to halo dispersion is minimized - sigma_LOS=0.5±0.1 sigma_v,H.  Argue that this window should be used to measureLoS dispersions as a proxy for mass, as it minimizes scatter in the sigma_LOS-M_vir relation.  This bias also naturally explains the results from McConnachie 2012, who used similar cuts when estimating sigma_LOS,LG.  Conclude that the LG's velocity dispersion does not pose a problem for LCDM and has a mass of logM_LG,M_sun=11.99+0.26-0.63.